Aquaculture HelperTM

Aquaculture is one of the fastest-growing food production sectors in the world and now provides more than half of all aquatic animal products consumed by humans. Global production continues to increase as wild fisheries have reached biological and practical limits, making aquaculture essential for future food security. According to the FAO, aquaculture has surpassed capture fisheries as the primary source of aquatic animals for human consumption and is expected to play an even larger role in supplying protein to a growing global population.

Asia dominates production volumes, accounting for the vast majority of global aquaculture output, with China, India, Indonesia, Vietnam and Bangladesh among the leading producers. Europe and North America, while representing a smaller share of total volume, lead in high-value species such as Atlantic salmon, Arctic char, trout and marine finfish. These regions are also global centres for advanced aquaculture technology, including recirculating aquaculture systems (RAS), water treatment solutions, oxygenation systems, digital monitoring, automation and artificial intelligence.

Industry profitability is driven by a combination of biological performance and operational efficiency. Key factors include feed conversion ratio (FCR), energy costs, fish survival rates, growth performance, market pricing, financing conditions and regulatory permitting processes. As modern facilities become increasingly technology-intensive, operational excellence and resource efficiency are becoming major competitive advantages.

A rapidly growing segment of the aquaculture value chain is the technology supplier ecosystem. Companies providing water treatment, oxygen generation, ozone systems, UV disinfection, filtration, sensors, automation, digital twins and AI-driven decision support are experiencing strong demand as producers seek higher productivity, lower environmental impact and improved regulatory compliance. Technology exports have therefore become a strategic industry in their own right, supporting aquaculture development globally - including from the Nordic countries.

Long-term growth is expected to be driven by increasing demand for sustainable protein, pressure on wild fish stocks, urbanization, stricter environmental regulations and continuing advances in land-based farming and precision aquaculture technologies. These trends are creating opportunities not only for fish producers but also for engineering, water treatment and digital infrastructure companies serving the sector.

Production trend (FAO FishStat)

Capture fisheries and aquaculture in the statistics

After the Second World War, population and demand for protein rose. From the 1960s to the 1980s industrial capture fisheries expanded with larger vessels, more efficient gear and greater range. FAO time series then show global capture volumes of aquatic animals largely levelling off, while aquaculture continues to grow. According to SOFIA 2024, aquaculture first surpassed capture fisheries in aquatic-animal production in 2022. That plateau is commonly interpreted in research and FAO reports as many fisheries approaching biological and management limits while demand for aquatic protein keeps rising.

In many fisheries the composition of the catch has changed over time - including use of more species and size classes - while management, quotas and technology have evolved. Aquaculture complements capture fisheries by adding farmed volume under controlled conditions. It does not replace responsible stock management, but can support food supply when wild catch can no longer grow at the same pace. Land-based and semi-land-based systems are also part of EU and national strategies for a sustainable blue economy, alongside sustainable fisheries.

RAS - a fast-growing technology segment

Recirculating Aquaculture Systems (RAS) are among the fastest-growing segments of the industry. By continuously treating and reusing water, RAS facilities can reduce freshwater consumption by more than 95%, enable production closer to consumers, improve biosecurity and support sustainable growth in regions where traditional net-pen farming is limited. This has created substantial demand for advanced filtration, oxygenation, monitoring and water treatment technologies - areas where DABCE and similar technology suppliers operate. See also the RAS section and our DABCE FlowFarm model for concept sizing.

Modern aquaculture can be divided into two main production models: land-based systems and sea/lake-based systems. Each approach has distinct advantages, challenges and environmental considerations. The choice depends on species, location, regulations, water availability, logistics and investment strategy.

Land-based aquaculture

Land-based facilities raise fish in tanks located indoors or outdoors on land. Most modern facilities use Recirculating Aquaculture Systems (RAS), where water is continuously filtered, treated and reused.

• High control of temperature, oxygen, water quality and production conditions.
• Excellent biosecurity with reduced exposure to parasites and external disease vectors.
• Year-round production independent of storms, seasonal changes and ocean conditions.
• Can be located close to consumers, reducing transport costs and carbon footprint.
• Nutrient-rich sludge can be collected and treated or reused.
• Advanced monitoring, automation and AI systems can optimise operations.
• Requires higher initial investment (CAPEX).
• Energy demand is significantly higher than traditional open-water farming.
• Technical failures can have rapid consequences if redundancy is inadequate.

Typical species include Atlantic salmon, rainbow trout, Arctic char, sturgeon, shrimp, eel and various hatchery-produced marine species.

Sea and lake-based aquaculture (net pens)

Net-pen farming uses cages installed in oceans, fjords, bays or freshwater lakes. Fish grow in natural water bodies while operators provide feed, monitoring and husbandry.

• Lower CAPEX per tonne of fish produced.
• Large natural water volumes provide dilution and temperature buffering.
• Generally lower energy consumption than intensive land-based facilities.
• Well-established technology with decades of commercial experience.
• Exposure to parasites, pathogens, predators and harmful algal blooms.
• Risk of fish escapes and interaction with wild populations.
• Environmental concerns related to waste discharge and ecosystem impacts.
• Production affected by weather, storms and changing environmental conditions.
• Permits, stakeholder acceptance and social licence are often critical.

Net-pen systems are widely used for salmon, trout, sea bass, sea bream and many other marine species.

Pond culture (land/freshwater)

Earthen and lined ponds are a third, very widespread form - often on land but at lower technical intensity than RAS. They dominate volume in parts of Asia, Africa and Latin America, while Europe and North America more often use raceways, net pens and land-based systems.

Key comparison

• Biosecurity: land-based systems generally provide higher disease control.
• Environmental control: RAS facilities offer much greater control over water quality and production conditions.
• Investment cost: sea-based systems usually require lower upfront capital investment.
• Operating cost: land-based systems typically have higher energy and technical operating costs.
• Environmental management: land-based farms can capture and treat solids and nutrient streams more effectively.
• Scalability: sea-based farms can often scale production more easily when permits are available.
• Market proximity: land-based facilities can be located near consumers and processing facilities.

Industry trend

Land-based RAS facilities are expanding rapidly in regions where environmental regulations, biosecurity requirements, freshwater scarcity or market proximity favour controlled production. However, sea-based net-pen farming still accounts for the majority of global salmon production due to its lower production cost and large-scale operational maturity. Hybrid and semi-flow-through solutions (e.g. land-based start with strong treatment) often bridge the two models. See also the RAS section and our DABCE FlowFarm model for concept sizing.

The future of aquaculture is shaped by food demand, environmental requirements, technology and geography. Wild fisheries often cannot grow at the same pace as demand; at the same time societies want more traceable, local and controlled production. That drives investment in land-based systems, better water treatment, digital operations and circular flows - not least in Europe and the Nordic countries.

Land-based, semi-flow and precision

Land-based RAS and semi-flow-through grow where permits, biosecurity or market proximity favour controlled culture. Precision tools - sensors, automation, AI for feeding and biomass, digital twins - reduce risk and improve FCR and energy use. Hybrid models (hatchery and juveniles on land, on-growing in cages or the reverse) remain important where they deliver the best economics and lower environmental risk.

Circular economy and sustainability

• More land-based farms near consumers and power grids.
• AI for feeding, biomass and early disease warning.
• Circular sludge handling and nutrient recovery (fertiliser, HTC, digestion).
• Genetics, triploidy and improved welfare.
• Integration with renewables and waste heat.
• Stricter discharge rules that favour treatment technology and traceability.

Feed remains the large environmental and cost factor. Future feeds mix more alternative proteins and oils, while culture of herbivorous and omnivorous species can reduce dependence on fishmeal. Technology export - water treatment, oxygen, UV/ozone, monitoring - becomes a growth engine beside fish production itself.

Circular food ecosystems

The future of aquaculture may not consist of isolated farms producing a single species. Instead, future systems could become highly integrated biological ecosystems where nutrients, carbon and energy are continuously recycled between multiple organisms.

Microalgae and macroalgae (seaweed/kelp) are expected to play a central role. Using sunlight or renewable energy, algae can convert carbon dioxide and nutrients into valuable biomass rich in proteins, oils, vitamins and minerals. This biomass can then serve as feed for the next level of the food chain.

Crustaceans such as shrimp, freshwater prawns and certain filter-feeding species can utilise algae-derived feeds, while herbivorous and omnivorous fish species such as carp, tilapia and some catfish can efficiently convert plant-based ingredients into high-quality animal protein. Because these species are lower on the food chain, they generally require less feed input than carnivorous fish.

Predatory fish species such as salmon, trout, sea bass and tuna occupy higher trophic levels and require greater amounts of protein and energy. Future feed technologies may reduce reliance on wild fish resources by using ingredients derived from algae, insects, microbial protein and circular agriculture systems.

In the most advanced concepts, waste streams become resources. Fish waste and uneaten feed can be processed into fertiliser, biogas or nutrient concentrates. These nutrients can then support algae cultivation, hydroponic crops, aquatic plants and kelp production, creating a circular industrial ecosystem with minimal waste.

Food chain in a circular system

Sunlight / renewable energy
⬇️
Algae & kelp
⬇️
Crustaceans & insects
⬇️
Herbivorous & omnivorous fish (carp, tilapia)
⬇️
Carnivorous fish (salmon, trout, sea bass)
⬇️
Nutrients, sludge & organic residues
⬇️
Algae & kelp (recirculation)

Looking forward: Future aquaculture may increasingly resemble natural ecosystems, where nutrients continuously cycle through algae, plants, crustaceans, fish and microbial communities. Such systems could maximise food production while minimising freshwater use, waste generation and environmental impact. On Earth these closed-loop systems may improve sustainability and resource efficiency. In the future, similar concepts could support permanent human settlements on the Moon, Mars and beyond, where every litre of water and every gram of nutrients must be reused.

DABCE's long-term view - aquaculture in space

It may sound like science fiction, but aquaculture could become an important part of future food production in space. As humanity moves toward long-duration missions, lunar bases, and eventually settlements on Mars, astronauts will need reliable sources of fresh food that can be produced using limited water, energy and resources.

The first step will likely be algae cultivation. Microalgae grow quickly, require little space, consume carbon dioxide, and can produce proteins, vitamins, healthy fats and even oxygen. In many proposed life-support systems, algae are expected to play a key role in recycling nutrients and supporting human habitats.

As space agriculture becomes more advanced, aquaculture could expand beyond algae. Herbivorous and omnivorous fish such as carp may become attractive candidates because they efficiently convert plant-based feeds into high-quality protein. Unlike carnivorous fish that require feed made from other fish, carp can thrive on algae, aquatic plants, insects and other renewable food sources that could potentially be produced within a closed-loop space ecosystem.

Future space farms may combine hydroponics, algae cultivation, insect production and fish farming into a single integrated food system. Nutrients from fish waste could fertilise crops, while plants and algae help clean and recycle water. Such circular systems could dramatically reduce the need for resupply from Earth.

Perhaps one day, recirculating aquaculture systems (RAS) will not only produce fish on Earth but also support human settlements millions of kilometres away. The future of aquaculture may be truly out of this world - from FlowFarm on Earth to closed loops in space. The technology we develop now - water treatment, gas balance, monitoring and circular flows - is the same set of building blocks.

Aquaculture Readiness Level (ARL)

Not an official standard, just DABCE having a little fun. But there is a serious point behind it: whether supporting fish on Earth, a habitat on Mars, or a future station beyond the Kuiper Belt, the challenge is fundamentally the same. Build systems that recycle water, recover nutrients, minimise waste and keep life thriving in a closed loop. ♻️

ARL = Aquaculture Readiness Level. Earth: serious status. Space: vision - with carp as the favourite "space fish" candidate.

Many people imagine fish farming as cages in the ocean or ponds in the countryside. In reality, some of the world's most advanced fish farms look more like water treatment plants, food factories, or even spacecraft life-support systems.

At its core, a Recirculating Aquaculture System (RAS) is surprisingly easy to understand. Think about a home aquarium. Fish swim in the tank and produce waste. Uneaten feed and feces accumulate. A filter removes particles and beneficial bacteria convert toxic ammonia into less harmful compounds. Air stones add oxygen, and from time to time water is replaced.

A RAS facility follows exactly the same principles, just on a much larger scale. Instead of a 200-litre aquarium, a commercial farm may contain millions of litres of water and thousands of tonnes of fish. Massive filtration systems, biofilters, oxygen generators and control systems continuously maintain water quality so the same water can be reused over and over again. In a traditional fish farm, water is used once and discharged. In a RAS facility, the water becomes a valuable resource that is treated, recovered and reused.

Why reuse water?

Fish do not consume water. They live in it. The challenge is keeping the water clean enough for the fish to thrive. Every kilogram of feed added to a fish tank eventually becomes: fish growth, carbon dioxide, ammonia, suspended solids and dissolved nutrients. A RAS system continuously removes or transforms these waste products so the water can return to the fish tanks. The result is that modern RAS farms may reuse more than 95% and in some cases more than 99% of their water while maintaining excellent environmental control.

Typical water flow

Fish tanks
↓
Mechanical filtration
(drum filter / disc filter)
↓
Biofilter
(MBBR, fixed bed, biosand filter,
MBR, algae-producing unit / photobioreactor)
↓
CO2 removal
(degasser)
↓
Oxygen addition
(oxygen cone / LHO, aerators,
trickling filters, etc.)
↓
Microbiological control / sterilisation
(UV, ozone, etc.)
↓
Pump
↓
Back to fish tanks
+
Small make-up water stream
&
effluent treatment

Easy RAS

Not all RAS systems are the same. The simplest systems are often used for hatcheries, research facilities or small farms. These systems may exchange 10-30% of the water each day and rely heavily on incoming freshwater.

• Relatively simple operation
• Lower investment cost
• High safety margin
• Easier management
• Higher water consumption

Many small trout hatcheries and fish research stations operate in this way. Think of it as a large aquarium with a generous flow-through component.

Moderate RAS

This is where much of today's commercial RAS industry operates. Water reuse typically ranges from 90-98%. These farms include drum filters, biofilters, degassers, oxygen systems and advanced monitoring.

• Greatly reduced water use
• Improved biosecurity
• Better environmental control
• Commercial production scalability

Most modern land-based salmon, trout, Arctic char, eel and sturgeon facilities fall into this category. For many species and locations, this balance provides the best combination of economics, performance and operational reliability.

Advanced RAS

As water reuse increases, engineering becomes more demanding. At reuse levels above roughly 99%, almost everything that accumulates in water becomes important: carbon dioxide, fine particles, nitrogen compounds, dissolved organics, salts, microbial populations and trace elements. At this stage, the system behaves less like a fish farm and more like an industrial ecosystem.

Additional technologies may include ozone treatment, foam fractionation, membrane filtration, advanced oxidation, nutrient recovery, CO2 management systems and automated water chemistry control. These facilities can produce very large amounts of fish while using surprisingly little freshwater.

Towards closed-loop aquaculture

The future may push RAS even further. Instead of viewing waste as a problem, future systems may treat every output as a resource - for example feed for algae, crustaceans and omnivorous fish, while nutrients return into the system. Seaweed and kelp may become part of the cycle. Such systems begin to resemble natural ecosystems where nutrients flow through multiple organisms rather than being discarded. See also circular food ecosystems under future of aquaculture.

Fish
⬇
Nutrients & organic matter
⬇
Algae production
⬇
Shrimp, prawns & crustaceans
⬇
Omnivorous fish (carp, tilapia)
⬇
Carnivorous fish (salmon, trout)
⬇
Nutrients recovered back into the system

Why biosecurity matters

A major advantage of land-based RAS is biosecurity. In sea cages, fish are exposed to parasites, disease organisms, harmful algae, storms, predators and escapes. In a RAS facility, water conditions can be tightly controlled and external biological threats greatly reduced. This level of control is one reason why land-based production continues to grow globally.

Energy in RAS

The trade-off for all this control is energy. Energy is typically used for: pumping water, oxygen generation, heating or cooling, ventilation, UV systems, ozone generation, monitoring and automation. In cold climates, heating can dominate energy use. In warm climates, cooling may be equally important. As facilities become larger, even small improvements in pumping efficiency, oxygen transfer efficiency or hydraulic design can save substantial operating costs. Maximising air-side oxygen uptake in degassers and choosing the right total ΔDO cuts both flow and power - see our DABCE FlowFarm model for concept sizing.

Thinking bigger

A home aquarium might recycle a few hundred litres of water. A commercial RAS facility may recycle millions of litres every day. The underlying principle, however, is the same: keep the fish comfortable, keep the water clean, and continuously recycle valuable resources. That simple idea has evolved from the household aquarium into one of the most important technologies for future sustainable seafood production.

Advantages and limitations at a glance

• Advantages: site flexibility, high biosecurity, stable temperature, low freshwater use, controlled discharges.
• Limitations: high CAPEX and electricity, need for skill and redundancy, sludge and salts must be managed.

Flow-through is the classic land-based farming model: fresh water is taken from a source (river, lake, spring or well), passed through culture units - often raceways, concrete channels, ponds or tanks - and returned to the receiving water after more or less treatment. Unlike full RAS, only a small share of the water is reused (or none at all); oxygen and water quality in the farm are driven mainly by the incoming flow and how long the water stays with the fish.

The principle is simple and robust. Where water rights, source quality and the receiving water allow, flow-through has long been the standard for e.g. rainbow trout, smolt and other salmonids in cool freshwater. The site does not need the heavy pumping, filtration and oxygen infrastructure of a high-reuse RAS. The trade-off is high freshwater use, and nutrient, solids and sometimes pathogen discharges that must fit permits and the capacity of the receiving water.

In many parts of the world where aquaculture has grown faster than environmental regulation, this has led to significant local and regional environmental challenges. Eutrophication, oxygen depletion, sediment impacts and competition for water resources are examples of effects observed around intensive production areas. In water-stressed regions, large abstractions can also reduce water available to downstream users, agriculture and natural ecosystems.

As water becomes an ever more valuable resource and environmental requirements tighten, interest grows in technologies that cut both water use and discharges. This is a major driver behind modern RAS facilities and other high-reuse concepts. By treating and reusing water, production can be decoupled from large abstractions while nutrients and organic matter can be collected and treated in a more controlled way.

Typical water flow

Water source
(river / lake / spring / well)
↓
Intake & coarse screening
(screens, trash racks, sometimes settling)
↓
Culture units
(raceways / tanks / ponds)
↓
Limited treatment
(settling, solids removal,
sometimes biofilter / polish)
↓
Discharge to receiving water
+
Optional partial return pumping
(semi-flow-through / PRAS)

What drives the design?

Sizing starts from biomass, feed load and the water quality you need in the tank - above all dissolved oxygen, temperature, ammonia/TAN and solids. In pure flow-through, flow is the main lever: more water means more oxygen and more dilution of waste. That makes the farm sensitive to source flow, temperature and seasonal variation. In drought, ice runs or heat waves, capacity can fall quickly unless there is buffer volume, oxygen addition or scope to cut feed and biomass.

• Water rights and permitted abstraction from the source.
• Source quality: temperature, oxygen, turbidity, pathogens, seasonal swings.
• Receiving-water permits and capacity for nutrients and solids.
• Topography: gravity feed cuts energy cost a lot.
• Species and density - salmonids in raceways are most common in cool regions.

Advantages

• Low energy cost for recirculation - often gravity and simple intake/outlet.
• Simple, proven technology with lower CAPEX than full RAS.
• Good robustness where the source is stable and the receiver can take the load.
• Natural temperature and mineral profile from the source (pro or con depending on species).
• Simpler operation than tightly controlled closed loops - fewer critical process steps.

Limitations and risks

• High freshwater use per tonne of fish versus RAS.
• Nutrient (N, P) and solids discharge must be sized against permits and the receiver.
• Limited independence from the environment - drought, heat, ice, turbidity and source pathogens hit the farm directly.
• Lower biosecurity than closed land-based systems if intake protection is weak.
• Growth and density are often limited by available flow rather than by technology ceilings.

Treatment before discharge

Even "simple" flow-through often needs solids removal and sometimes nutrient reduction under modern permits. Common steps are settling basins, microscreens, sludge thickening and in some cases biofilters or wetlands on the effluent. The tighter the permit and the more sensitive the receiver, the more the farm looks like semi-flow-through: more treatment, more reuse, more control - but still with a clear make-up stream from nature.

From pure flow-through to semi-flow-through

Many farms sit on a spectrum. Pure flow-through: almost all water passes once. Partial reuse (PRAS) / semi-flow-through: part of the flow is treated and returned, while a make-up stream brings in fresh water and a matching stream goes to the receiver after polish. That cuts freshwater need and discharge load without the extreme reuse rate and energy profile of full RAS. Raceways, hatcheries and inland salmonid farms often use that kind of hybrid.

The FlowFarm concept (DABCE™)

The DABCE FlowFarm concept is built on the same core principle: produce more fish with less water, better control of resource flows and significantly lower impact on surrounding aquatic environments. The more efficiently water, oxygen, energy and nutrients are recirculated, the closer the facility comes to a truly circular production system. ♻️

DABCE FlowFarm™ is a real, buildable semi-flow-through concept: water is taken in (e.g. via subsea pump and pipeline), protected with a bio-barrier and coarse filtration/UV, aerated, passed through fish units, then sludge separation and an integrated biostage/particle filter suited to Nordic conditions (e.g. DynaSand), with controllable recirculation back to culture and a return pipeline to the receiving water. Intake, culture, treatment, sludge and return form one coherent system - without the heavy up-front investment that full RAS often requires.

The schematic includes, among other things: subsea pump and intake pipeline; bio-barrier with 50 µm filter and UV; aerator; fish units; sludge separation with separate sludge handling; integrated biostage and particle filter; controllable recirculation pump; return pipeline back to the river. These are the same building blocks as in many modern inland systems, arranged as a clear process chain.

See also the sections on RAS, raceways, hybrid systems and water quality.

Open FlowFarm model

Raceways are long, narrow and relatively shallow culture channels where water flows from one end to the other. They are usually built in concrete, FRP (glass-fibre reinforced plastic) or as earthen channels with liner or other reinforcement. A raceway is primarily a culture unit (tank geometry and hydraulics) - not a production system in the same sense as "flow-through" or "RAS". In practice, raceways most often sit in flow-through, semi-flow-through / partial reuse or hybrid setups where part of the water is treated and returned.

The directed current carries away faeces and feed fines, mixes the water and gives many species a swimming stimulus. Oxygen is partly supplied by the incoming flow, but at higher density and feed load that is rarely enough on its own: modern raceways are often supplemented with pure oxygen (cone, LHO), diffusers or surface aeration. Raceways are among the earliest forms of intensive land-based farming and are still used where there is a stable supply of clean water from springs, rivers, lakes or wells.

Use in Europe and the Nordics

In Europe, classic raceways are mainly linked to cold-water salmonids and related culture: rainbow trout, brown trout, salmon smolt, Arctic char and to some extent whitefish (Coregonus). Sturgeon for meat and caviar is also grown in channels or raceway-like units on some sites. Many Nordic hatcheries and smolt facilities use the raceway principle in parts of the chain - often together with troughs, circular tanks and more closed steps - thanks to simple handling, good overview and proven hydraulics.

Global use

Internationally the spectrum is wider. In North America, raceways still dominate much of rainbow trout and hatchery salmonid production. In warmer regions, channels and raceway-like units are used among other things for carp, tilapia and African catfish (Clarias), where the focus is often simple operation, low CAPEX and high volume rather than "natural" current simulation for salmonids.

Some marine and specialty species (e.g. European sea bass, gilthead seabream, eel, barramundi) are sometimes grown in long tanks or channels that resemble raceways, but often with different materials, salinities and flow regimes. Abalone and some other invertebrates have their own raceway-style systems - related in geometry, not the same as a classic freshwater trout raceway. The point is that "raceway" describes a hydraulic form more than a single species list.

Why do raceways work?

Species such as salmon, brown trout, rainbow trout and Arctic char are adapted to flowing water. Raceways mimic parts of that hydraulics: directed flow, swimming stimulus and quick transport of solids toward the outlet. Continuous swimming can support condition and more even growth - provided density, flow and oxygen are sized correctly. For robust species such as carp, catfish and tilapia, the raceway form is more a practical production unit with easy harvest and overview than a "natural river habitat".

Design and operation in brief

Sizing starts from biomass, feed load, species oxygen demand and the water quality you need along the channel - especially dissolved oxygen and solids at the outlet. Pure flow-through is driven mainly by flow; the higher the density, the more critical flow, oxygen addition and solids removal become. Water rights, source quality and receiving-water permits set the outer frame - see also the flow-through section.

• Series vs parallel: series raceways save space/flow but worsen water quality and raise pathogen risk downstream; parallel gives more equal units.
• Gravity and head: natural head cuts energy cost; pumping raises OPEX and operational dependence.
• Hydraulics: even cross-section flow reduces dead zones; screens, stop-boards and emergency outlets are standard.
• Solids and discharge: settling, microscreen or other treatment before the receiver is often required under modern permits.
• Handling: grading, harvest and daily checks are easy in channels - a practical advantage in hatchery and grow-out.
• Biosecurity: intake protection, zoning, dedicated equipment and controlled egg/fish intake limit pathogens - critical because the system is more open than full RAS.

Advantages

• Simple, proven technology with low complexity compared with full RAS.
• Good hydraulics: solids transport, mixing and swimming stimulus for many salmonids.
• Easy monitoring, grading and harvest.
• Lower CAPEX and simpler operation than tightly controlled closed loops, where source and receiver allow.
• High production capacity when flow, temperature and oxygen are stable.

Limitations

• High freshwater use in pure flow-through; capacity is limited by available flow.
• Source quality (temperature, turbidity, pathogens, upstream impacts) hits the farm directly.
• Nutrients and organic matter must fit discharge permits and the receiving water's capacity.
• Lower biosecurity than closed land-based systems if intake and pathogen control are weak.
• Exposure to seasonal swings, drought, heat, ice runs and sudden water-quality changes.
• Oxygen risk at peak feed if flow and oxygen addition are undersized - flow alone is not a guarantee.

Raceways as a bridge to semi-flow and RAS

Historically, raceways were a central part of early intensive land-based farming, alongside ponds and hatcheries. Today they often act as an intermediate form: the same channel geometry can sit in pure flow-through or be linked to treatment and partial reuse.

Many modern facilities use a chain where raceway volume is combined with process-water treatment:

Raceway (culture unit)
↓
Mechanical filtration
↓
Biofilter
↓
Oxygenation / degassing
↓
Partially returned water
+ make-up from source
(partial reuse / hybrid-RAS)

Such partial-reuse or hybrid solutions keep the handling simplicity of the raceway unit while cutting freshwater need and discharge load versus pure flow-through - without requiring the extreme reuse rate of full RAS.

Raceways can therefore be seen as a historical and practical step between pond culture and more closed systems: full RAS on one side, and semi-flow-through / partial reuse (e.g. DABCE FlowFarm) on the other - where FlowFarm is a buildable semi-flow concept rather than full recirculation.

Design considerations - raceways in a larger puzzle

From an optimisation and engineering perspective, raceways are rarely "the whole answer". They are one piece among several culture units and process steps. A well designed farm often combines different units in a chain where each step matches life stage, risk, cost and site conditions - rather than forcing all production into a single tank form.

Typical building blocks that can be mixed in the same project or value chain:

• Small tanks and troughs - hatchery, first feed, quarantine, high control and biosecurity.
• Raceways - juveniles and grow-out where flow, swimming stimulus and easy handling pay off.
• Circular or octagonal tanks - RAS and semi-flow with even hydraulics and compact footprint.
• Ponds - low CAPEX and volume in suitable climates, often lower density.
• Cages in lake or sea - large grow-out volume when environment, permits and logistics allow.
• Water treatment and gas balance - filters, biofilters, oxygen, degassing, UV/ozone linking the units.

When several units and life stages sit in the same facility or value chain, flow and biosecurity design become critical. The ideal is to separate water flows between zones - e.g. hatchery/quarantine, juveniles and grow-out - so that a pathogen is not carried "backwards" in the chain via shared return water, common sumps or uncontrolled reuse. Where full hydraulic separation is not practical (shared source, partial reuse, same receiving water), carefully sized bio-barriers are needed at critical points instead: solids removal before disinfection, then e.g. UV and/or ozone (often in series), sometimes with residual-ozone handling, on make-up, transfer water or between zones. A barrier is only as strong as its weakest link - turbid water, wrong UV dose, bypass or shared equipment undermines the effect.

Example chains: hatchery in small tanks → raceway or semi-flow on land → finish in cages; or pond-based start in warmer climates → intensification in channels; or a fully land-based chain with raceways in flow-through where the source is strong, and more closed steps where water or discharge is the bottleneck. Optimal design depends on species, life stage, water supply and temperature, receiving water and permits, energy and labour, market and logistics, and risk profile (disease, escapes, storms, ice).

The point is to assign the right unit to the right step - and to see the farm as an engineering puzzle where raceways, small tanks, ponds, cages and treatment can be combined into a whole that fits the site. There is rarely a universal "best" raceway farm; there are solutions that are proportionate to local conditions.

Ponds are one of the oldest and still most important production forms in aquaculture. Despite growing attention on RAS and high-tech land-based systems, a very large share of world aquaculture production still takes place in various pond types - especially in Asia, Africa and Latin America.

From satellite imagery, some regions show vast areas covered by aquaculture ponds. Along coasts and river deltas in e.g. India, Bangladesh, Vietnam, Thailand, Indonesia, China and Ecuador, pond systems sometimes form continuous landscapes over tens or hundreds of square kilometres. In several of these areas, pond culture is among the dominant food industries.

Construction

Ponds are built in many ways depending on climate, soil, materials and production goals:

• Earthen ponds and clay ponds - embanked land where water is held by soil or clay dikes.
• Lined ponds (HDPE or other geomembranes) - cut seepage, ease cleaning and improve control.
• Concrete ponds - more permanent, higher CAPEX, easier to drain and disinfect.
• Partly lined ponds with earthen dikes, or natural ponds with reinforced banks.

The simplest ponds are embanked land with earthen dikes. In more intensive production, plastic liners are often used to reduce seepage and sediment issues and stabilise water quality. Area per pond can range from a few hundred square metres to several hectares; depth is often about 0.8-2.5 m depending on species, climate and strategy.

Global importance and species

Pond culture is used for a very wide group of organisms. In freshwater, common examples include carp (common, grass, silver, bighead), tilapia, pangasius, Clarias catfish, channel catfish, sturgeon, gourami, tambaqui/pacu and regional freshwater species. In brackish and marine water, mullet, milkfish, barramundi and in some cases sea bass/seabream are grown in lagoon- or pond-like units. Shellfish in ponds is globally huge: whiteleg shrimp (Litopenaeus vannamei), black tiger (Penaeus monodon), other penaeids and freshwater prawns (Macrobrachium). Add crabs, sea cucumbers, algae/macroalgae and shallow lagoon culture of mussels/oysters in some systems.

In Europe, ponds are often associated with carp culture, sturgeon for meat/caviar, recreational fisheries and more extensive freshwater farming. In Asia, by contrast, pond culture is the backbone of the aquaculture sector and accounts for enormous volumes of world fish and shellfish production.

Shrimp and "mud ponds"

When many people think of aquaculture they picture salmon cages or modern RAS. Globally, however, the classic shrimp pond is among the most widespread intensive production forms. A large share of the world's farmed penaeid shrimp is still produced in earthen / mud ponds, especially in India, Vietnam, Thailand, Indonesia, Bangladesh, China and Ecuador. Ponds can be several hectares; whole coastal landscapes in parts of Southeast Asia and Latin America are dominated by shrimp culture.

Aeration and oxygenation

In extensive ponds, photosynthesis and moderate water exchange can supply enough oxygen. In intensive production, mechanical aeration is almost always used. The most iconic technology is the paddlewheel aerator - floating units with rotating paddles that throw water into the air.

• Add oxygen and drive circulation in the pond.
• Reduce low-oxygen zones and break temperature/oxygen stratification.
• Help move sludge toward collection areas and even out water quality.

On modern shrimp farms, long rows of paddlewheels may run around the clock during the cycle. Other common techniques: diffusers, aspirators, propeller aerators, fountain aerators, pure oxygen injection, and micro- and nanobubble systems on more advanced sites.

Natural productivity

One of pond culture's strengths is that the pond acts as an ecosystem. Besides added feed, it often produces phytoplankton, algae, zooplankton, bacteria, benthos and biofilms. Many species - especially carp, tilapia and shrimp - can use part of that natural production. Pond systems can therefore yield large biomass with relatively low technical complexity, provided nutrient balance, oxygen and sediment are managed.

Advantages

• Low investment cost (CAPEX) per area compared with tanks/RAS.
• Simple, robust technology with a large global experience base.
• Natural biological production that can complement feed.
• Suitable for many species and easy to scale to large areas.
• Often lower energy use than intensive closed land-based loops (depending on aeration and pumping).

Limitations

• Limited control of temperature and day-night oxygen swings.
• High water use and exchange in many systems.
• Exposure to weather, rain, drought and climate variation.
• Harder biosecurity than closed systems - risk of disease, predators and escapes.
• Algal blooms, night-time oxygen crashes and sediment build-up.
• Nutrient leakage to the surroundings can be substantial without treatment and good operation.

Development and place in the puzzle

Where water is scarce or environmental rules tighten, there is a gradual shift toward more controlled solutions: biofloc, partial reuse, hybrid facilities, lined ponds with stronger aeration and in some cases links to land-based treatment. Ponds can also be one step in a production chain - e.g. juveniles in more controlled units then grow-out in ponds, or the reverse - together with raceways, cages and RAS. See also design considerations under Raceways.

At the same time, ponds are likely to remain one of the world's most important production forms for the foreseeable future thanks to simplicity, low cost and the ability to produce large amounts of food where climate and land allow. Advanced concepts such as full RAS and semi-flow (e.g. DABCE FlowFarm) are complements and alternatives where site, species and rules require higher control - not an immediate replacement for global pond volume.

Net-pen (cage) farming means fish grow in floating or moored net volumes in the sea, fjord, bay or a freshwater lake. The farmer supplies feed, nets, mooring, monitoring and harvest - but the water is the surroundings: temperature, oxygen, current, ice, algae and pathogens arrive "free" and uncontrolled. This is the model that made Atlantic salmon a global industry, and it remains the backbone of Norwegian, Chilean, Scottish, Faroese and Canadian salmon production.

How a pen works

A typical marine pen is a circular float collar (often HDPE) with a hanging net bag, bottom, weights and sometimes bird or predator nets. Several pens sit in a frame or in rows and are moored to the seabed with chain, rope and anchors. Feed comes from a barge or shore station via hoses; cameras and sensors control the ration. Nets are changed and cleaned; biofouling raises drag and cuts through-flow if it is not managed.

• Circular HDPE collars - most common in modern salmon farming; scalable diameter and depth.
• Steel and square pens - older or special sites; more deck area, different load paths.
• Freshwater lake pens - often simpler construction; ice, stratification and under-ice oxygen dominate.
• Submersible or more closed units - attempts to avoid lice, algae, ice or storms in the surface layer.

Sea and lake - two different worlds

Marine cage farming (salmon, sea bass, sea bream, some tropical species) uses large water volume, salt and often good exchange. The price is lice (on salmon), storms, escapes toward wild stocks, and local benthic load under the pen. Freshwater cages (rainbow trout, Arctic char, sometimes other inland species in the Nordics and elsewhere) avoid sea lice but meet ice, winter oxygen risk, algal blooms and often tighter local lake rules. An inland pen is not "the same farm as in Norway, just in a lake".

Operations in practice

• Daily feed control against appetite, temperature and oxygen - overfeeding hits both economics and the seabed.
• Net inspection, change-out and washing; holes are escape risk and a predator entry.
• Temperature and DO profiles - especially in summer stratification, algal blooms and ice.
• Lice and health protocols in marine salmon (counts, treatment, coordination between sites).
• Contingency: storm, ice, oxygen crash, predator attack, escape, algal warning.

Environment and permits

Nutrients and solids from feed and faeces enter the water column and the seabed under and around the pen. Modern permits often require environmental monitoring, fallowing, biomass caps and sometimes relocation or technology. Social licence - neighbours, capture fisheries, tourism, wild salmon stocks - is often as decisive as the hardware. The pen is cheap per tonne when it works; it is politically and environmentally sensitive when it does not.

Advantages

• Lower CAPEX per tonne than land-based RAS - an established large-scale model.
• Low energy use for water exchange compared with pumped recirculation.
• Large natural volume gives dilution and temperature buffering.
• Mature supply chain for pens, feed, vessels and service (especially marine salmon).
• Fast scale-up when permits and a site exist.

Limitations

• Lice, pathogens, predators and harmful algal blooms reach the fish directly.
• Escapes and genetic/pathogen interaction with wild stocks.
• Local benthic and nutrient impact; fallowing and site access cap capacity.
• Weather, ice, current and storm - production stops and insurance cost.
• Limited control of temperature and oxygen; inland pens are vulnerable under ice.
• Permits and social acceptance can stop expansion even when the technology works.

Place in the puzzle

The pen is not the opposite of land-based - they are often coupled. Common: hatchery and smolt/post-smolt on land (higher biosecurity), then on-growing in cages. Conversely, land-based food fish can replace cages where lice, ice, permits or market proximity make open water unsuitable. See land vs sea, offshore, hybrid systems and biosecurity. For inland salmonids where cages are not an option, semi-flow / FlowFarm and RAS are the usual alternatives.

Offshore (exposed ocean farming) means sites further out and more open than sheltered fjords and bays: more wave height, more current, greater depth, longer to port. The driver is often the same - more space, less coastal conflict, hope of better water exchange and in some cases lower lice and benthic impact near shore. The price is structure, vessels, weather windows, crewing and insurance in an environment closer to offshore energy than to classic cage farming.

There is no sharp legal line that every country calls "offshore". In practice it is a spectrum: from more exposed coastal sites to truly open ocean. The further out, the more the pen, mooring and logistics must be sized for extreme loads - and the fewer days it is safe to work on the site.

What has to take more

• Mooring and buoyancy against wave, current and fatigue - not only against average wind.
• Larger or submersible units so fish and structure survive storms.
• Vessels, cranes and weather windows for feed, harvest, net change and repair.
• Communications, emergencies and evacuation further from shore.
• Insurance, classing and documentation closer to offshore industry.

Construction: steel and concrete

Exposed units are in practice built with the same structural thinking as the rest of the offshore industry. Structural steel - welded jackets, hulls and decks as in oil and gas and marine - is the usual choice: known classing, yard capacity, maintenance and fatigue design against wave and current. Large concrete structures can also be viable, as for some oil platforms and floating concrete hulls: mass, stiffness and corrosion resistance against a heavier formwork and transport job. The choice is not "fish farm vs platform" but load, depth, mooring, service life and which yard or casting site can deliver.

Both routes imply significant CAPEX: classed structure, mooring, offshore installation and spare capacity. OPEX follows the same logic as oil and gas and marine service - not a sheltered coastal pen. Marine supply vessels, weather windows, crew, bunkering, crane lifts, inspection and maintenance at sea are recurring cost items. The further out and the heavier the unit, the more the economics are driven by vessel time and uptime, not only by feed and growth.

Opportunities

Further out, water exchange can be higher and conflict with near-shore use lower. Some concepts aim at larger single units and more automated operation to offset the fewer working days. Offshore is also discussed as a way to move production away from sensitive fjords and wild salmon rivers. It is a direction - not a proven replacement for the coastal pen everywhere.

Limitations and honesty

• Significant CAPEX for a steel or concrete unit plus high OPEX for supply vessels, weather windows and maintenance at sea - until scale and uptime are proven.
• Lice, algae and escapes do not automatically disappear just because the site is more exposed.
• Occupational safety and rescue at sea are governing - not only fish biology.
• Permits, shipping, defence and offshore environmental monitoring are processes of their own.
• Many projects are still demonstration or early commercial operation.

Closed and semi-closed units at sea

Alongside "a larger pen further out", more closed floating units are being developed: walls or liners that separate culture water from the surroundings, with pumping, filtration and sometimes lice exclusion. They aim at biosecurity and discharge control but are heavier, costlier and more energy-hungry than an open net bag. They belong in the same family as hybrid and land-based solutions more than classic cages.

When offshore is (and is not) the answer

Offshore is interesting where the coast is full, conflict is high and capital can fund marine infrastructure. It is rarely the first step for inland salmonids, small projects or sites without a port and service fleet. For Nordic inland sites, land-based semi-flow, raceway or RAS is usually the relevant path - not an exposed ocean pen. See net pens, land vs sea, hybrid systems and FlowFarm.

Most real farms are hybrids - not pure textbook cases. Water is partly reused, juveniles start on land and grow on in cages, or effluent is treated harder than in classic flow-through. Hybrid thinking is often what makes a project permit- and economically possible: enough control where risk is high, simple enough technology where nature already does the work.

Common hybrid forms

• Partial reuse (PRAS) / semi-flow-through: part of the flow is treated and returned; make-up from the source and polished return to the receiver.
• Land-based start + cage finish: hatchery, first feed and sometimes post-smolt in controlled tanks; food fish in net pens.
• Cage or pond + land-based treatment of side-streams / sludge - capture solids and nutrients without closing the whole loop.
• Raceways or tanks with biofilter and drum filter but still a clear river/lake intake.
• Seasonal hybrid: more reuse and oxygen in summer when the source is warm or scarce; more flow-through when cold and flow suffice.

Why hybrid?

Full RAS gives maximum control but high CAPEX, electricity and skill demand. Pure flow-through or an open pen is cheaper but vulnerable to permits, lice, drought and discharge. Hybrid puts treatment and reuse where they pay most: solids and biosecurity early in life, effluent polish toward the receiver, temperature and oxygen buffer when the source fails. It is also how many existing inland salmonid farms evolve - stepwise, not as a jump to a closed loop.

Design questions

• What share of flow should be reused in normal operation and in drought/heat?
• Which steps are mandatory: screen, drum filter, biofilter, degasser, UV, sludge?
• Where does biosecurity sit - intake, compartments, people flow, cage vs land?
• What happens on pump failure: gravity, emergency outlet, oxygen reserve?
• How are discharge and abstraction documented against the permit?

The FlowFarm concept (DABCE™)

DABCE FlowFarm™ is a buildable semi-flow-through example: intake with protection and filtration/UV, aeration, fish units, sludge separation and an integrated biostage/particle filter with controllable recirculation and return to the receiving water. It sits between pure flow-through and full RAS - more control and lower discharge than a classic raceway, without the heaviest RAS investment. See also flow-through for the process chain and schematic.

Other hybrids (e.g. post-smolt on land + marine cage) solve a different problem: lice and early seawater mortality, not inland water balance. Choose the hybrid for the problem you actually have - not after a generic "more technology is better" rule.

Open FlowFarm model

Temperature is one of the most important water-quality parameters in aquaculture. It directly influences fish metabolism, appetite, growth rate, oxygen consumption, immunity, digestion, reproduction and overall welfare. Unlike mammals, fish are ectothermic ("cold-blooded"): body temperature closely follows the surrounding water. Even small temperature changes can therefore have significant effects on performance and health.

As water temperature rises, fish metabolism accelerates. Fish eat more, grow faster and become more active, but they also consume more oxygen and produce more waste (ammonia and carbon dioxide). At the same time, warmer water holds less dissolved oxygen. This combination is one of aquaculture's core challenges: oxygen demand increases while oxygen availability decreases.

Temperature therefore influences nearly every part of the farm: growth and feed conversion (FCR), oxygen demand, biofilter performance, ammonia toxicity, disease pressure, energy for heating and cooling, and production planning and harvest timing. See also the sections on dissolved oxygen, carbon dioxide and RAS.

Species have different temperature preferences

Different species have evolved for different thermal environments. A temperature that produces rapid growth in one species may cause severe stress in another.

Cold-water species - e.g. Arctic char (Salvelinus alpinus), Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) - thrive in cool, oxygen-rich water and are often farmed in northern Europe, North America and mountain regions. Warm-water species - e.g. tilapia, carp, catfish, pangasius and kingfish (Seriola lalandi) - often grow best in warmer water (roughly 18-30 °C depending on species) and are farmed in temperate to tropical regions as well as in heated RAS. The same logic applies to shrimp and algae: tropical shrimp and many red seaweeds want warm water, while sugar kelp (Saccharina) is a cold-water species.

Typical culture windows - exact values depend on genetics, life stage, salinity, dissolved oxygen and husbandry. Algae and shrimp are not fish; ranges are culture windows, not a species protocol.

Temperature ranges by species

Growth slows (cold) Preferred range Above optimum Increased risk (warm)

Cold-water fish

Arctic char

Rainbow trout

Atlantic salmon

Warm-water fish

Common carp

Tilapia

African catfish

Kingfish (yellowtail)

Shrimps

Whiteleg shrimp

Black tiger shrimp

Kelp and algae

Sugar kelp

Tropical red algae

Microalgae (typical)

0° 10° 20° 30° 38°

© 2026 DABCE AB

Same typical culture windows as the table. Orientation - not a species protocol.

Temperature and oxygen

The relationship between temperature and oxygen is fundamental. Cold water can hold much more dissolved oxygen than warm water. At the same time, fish require more oxygen as temperature rises.

Approximate saturation in freshwater at sea level (1 atm). Salinity, pressure and actual percent saturation change the field value.

A trout farm at 20 °C therefore has roughly 20% less oxygen available than the same farm at 10 °C, while the fish simultaneously need more oxygen. In intensive RAS and flow-through this often becomes the primary production bottleneck. Curves for air and pure O₂ at different pressures are under dissolved oxygen (DO).

Temperature and growth

Growth generally follows a bell-shaped curve. As temperature increases, growth accelerates until an optimum is reached. Beyond that point feed conversion deteriorates, stress increases and growth declines. Operating outside the optimal thermal range often means reduced feed intake, poorer FCR, slower growth, higher mortality and greater disease susceptibility.

Rainbow trout can grow very well in RAS around about 13 °C, provided oxygen and nutrition are maintained. That is a common target range in controlled salmonid facilities - not a universal recipe. Arctic char generally needs cooler water; tilapia and carp the opposite.

Temperature and disease

Disease outbreaks often become more likely under thermal stress. Rapid temperature swings can weaken immunity. Many pathogens also reproduce faster at elevated temperatures.

• Increased bacterial disease activity during warm periods.
• Higher parasite loads in warm water.
• Greater mortality during heat waves.
• Stress-related outbreaks after sudden temperature changes.

That is why a target value is not enough - stability matters as much as the level itself.

Temperature in RAS

RAS makes it possible to hold a more stable temperature than ponds or cages: year-round growth, more predictable harvest, often better FCR and less seasonal swing. The price is energy. Heating, cooling, pumps, heat exchangers and oxygen must all be designed together with temperature control.

Many modern facilities recover energy via heat exchangers, waste heat, ground-source systems, district heating or heat pumps. In cold inland sites (e.g. Arctic char), summer cooling can be as governing as winter heating. See RAS, heat pumps and chillers under equipment.

Seasonal considerations in open systems

Outdoor systems follow nature. Summer: lower oxygen, more algal blooms, higher fish oxygen demand, heat-stress risk. Winter: slower growth, lower feeding, sometimes ice and under-ice oxygen risk in lakes/pens; on land-based sites heating demand may rise. Production planning - especially ponds and cages - often revolves around these swings. See net pens, ponds and dissolved oxygen.

Monitoring and management

Good temperature management relies on continuous monitoring, not occasional spot checks.

• Continuous logging with calibrated sensors.
• High and low temperature alarms.
• Several measuring points in the system (intake, tank, return - not a single thermometer).
• Seasonal trends and history.
• Integration with feeding and oxygen control.

In digital farms, temperature is often a key input to automated feeding, biomass models, oxygen dosing and predictive analytics. See monitoring systems and AI.

Why temperature matters

Temperature is much more than a number on a sensor. It determines how fast fish grow, how much oxygen they need, how efficiently feed becomes biomass, and how resistant they are to disease. A few degrees can separate exceptional production from severe biological stress - whether farming trout in a Nordic RAS, tilapia in tropical ponds or salmon in net pens. Understanding and controlling temperature is therefore one of the foundations of successful aquaculture.

For farmers, investors or anyone who wants the basics: like people, fish need oxygen to live. The difference is that we breathe air while fish take up oxygen dissolved in water across the gills. If dissolved oxygen falls too low, fish struggle to respire - roughly like a human in a very thin atmosphere.

The air we breathe is roughly 78% nitrogen, 21% oxygen gas and about 1% other gases. When water is aerated with ordinary air, only about one fifth of the gas is actually oxygen. Many modern aquaculture facilities therefore use pure oxygen (100% O₂) instead of air alone. That makes higher density possible and keeps oxygen supply stable even at high feed rates and biomass.

What does oxygen do in the fish?

Oxygen drives fish metabolism: swimming, growth, digestion, immunity and reproduction. The more the fish eat and grow, the more oxygen they need.

When oxygen starts to fall you often see increased stress, poorer growth, higher feed conversion ratio (FCR), greater disease susceptibility and fish crowding toward inlets or aeration zones - in the worst case mass mortality. Oxygen is therefore often described as aquaculture's most important water-quality parameter.

For the chemist

Dissolved oxygen (DO) is the concentration of oxygen dissolved in water, usually in mg/L (roughly the same as ppm in water) or as percent saturation. How much oxygen can dissolve at maximum depends mainly on temperature, salinity, air pressure and elevation. Cold water can hold much more oxygen than warm water.

Theoretical oxygen solubility versus temperature

For many people outside water chemistry it is surprising that water in equilibrium with air only holds about 7-13 mg/L oxygen depending on temperature. That is because air is only about 21% oxygen gas; nitrogen does not supply the fish. With pure oxygen the partial pressure of O₂ rises about fivefold, so much more oxygen can dissolve. If oxygen is added under pressure (e.g. oxygen cone or LHO at about 5 bar absolute), theoretical solubility rises further - so inlet water can be produced well above ordinary air saturation. That is a main reason modern intensive culture often uses pure oxygen systems beyond or instead of aeration alone.

But more dissolved oxygen is not always better. When water is saturated (or supersaturated) under pressure in pipes and oxygen units and then released into culture tanks that are open to the air at about 1 atm, a "soda-bottle effect" can appear: gas wants to leave solution as pressure drops. The result can be bubbles, foam, local supersaturation and uneven gas balance right at the inlet and in jets. Excessively high dissolved oxygen and/or sudden pressure relief can stress fish - including gill irritation, behaviour changes and in serious cases gas bubble disease (especially when total gas pressure and nitrogen are also high). So it is not enough to "max out" DO in the pipe; you must understand how water spreads in the tank - inlet direction, velocity and mixing, dead zones vs short-circuiting, and where the fish actually live. Good hydraulics and controlled inlet mixing distribute oxygen evenly and avoid both local hypoxia and harmful gas pockets. Measure DO where it matters (in the tank, not only in the pressurised line), and see also the section on saturation, supersaturation and N₂.

Oxygen measurement

Without reliable measurement, the oxygen strategy is guesswork. In practice three paths dominate: handheld checks (routine, troubleshooting, calibration control), fixed process sensors linked to PLC/SCADA (continuous trend, oxygen/aeration control and alarms), and sometimes portable loggers or temporary points at commissioning and performance tests. Match technology to risk and density: ponds and low intensity may manage with spot checks; raceways, semi-flow and RAS at high biomass usually need continuous DO where the fish live - often with redundancy on critical units.

Two main types of oxygen sensor are used on farms:

• Optical probes (luminescence / fluorescence). The usual choice on new farms. Often need less maintenance, longer intervals between calibrations, and they do not consume oxygen while measuring.
• Electrochemical probes (Clark-type: galvanic or polarographic membrane probes). Often cheaper to buy, but membrane, electrolyte and zero need checking more often.

Both types usually show mg/L and % saturation, if temperature (and sometimes salinity) is compensated correctly. Check two things: that the sensor has proper temperature compensation, and that % sat is calculated against the right reference - fresh or salt water, plus elevation/air pressure.

Capacity and performance often specified: a range covering both low DO (emergency) and supersaturation at oxygenated inlets (e.g. 0-20+ mg/L or 0-200% sat depending on model), response time suitable for process alarms (seconds to tens of seconds - not minutes if oxygen drives automation), stability in flowing, particle-laden water, and outputs that fit the plant (4-20 mA, Modbus, digital alarms). Placement is as important as sensor type: avoid dead corners, avoid sitting straight in a pure oxygen jet if you want a tank average, and avoid sucking in air bubbles. For oxygen dosing control one often measures both "critical low" in the tank and sometimes inlet after the oxygen unit - knowing that pipe and tank do not share the same DO.

Problems with oxygen probes (regardless of principle) are common and underestimated: fouling by biofilm, feed oils and particles that make readings sluggish or falsely low/high; drying out or damaged membrane/optical face; wrong or forgotten calibration; temperature errors that hit both mg/L and % sat; cable faults and moisture in connectors; air bubbles or stagnant pockets at the sensor; and "zombie values" that look stable in SCADA while the real tank has already dropped. Probes age and drift - compare regularly against a freshly calibrated handheld reference, keep a spare sensor or probe, and treat alarm limits with common sense (fish behaviour, feed response). Good oxygen measurement is maintenance and placement as much as hardware choice. See also monitoring systems and sensors under equipment / AI & monitoring.

Oxygen consumption in biological systems

In biological systems oxygen is consumed mainly by fish respiration, biofilters (nitrification), other microbes and breakdown of organic matter. In a RAS, a substantial share of oxygen use can occur in the biofilter rather than in the fish themselves. Nitrification NH₄⁺ → NO₂⁻ → NO₃⁻ consumes large amounts of oxygen and must be included in the oxygen balance.

There is also a biological paradox: warmer water holds less oxygen, while fish need more oxygen as temperature rises. That makes summer operation and high feed rates the most critical periods on many farms.

For the process engineer

On a commercial farm, oxygen is in practice a mass-balance question. Every kilogram of feed drives fish respiration, biofilter nitrification and oxidation of organics. Oxygen demand can therefore often be estimated from daily feed - but it is not the average that sizes the system. It is the peak load: peak feed.

Peak feed sizes oxygen tanks, LOX, oxygen cones, LHOs, degassers, emergency power, pumps and emergency systems. Always size oxygen and related hydraulics for maximum daily feed (plus safety margin), not for average seasonal operation.

Biology and chemistry: from feed to emission profile

Feed is the dominant mass input on an intensive farm. The fish convert part of the carbon, nitrogen, phosphorus and energy into growth; the rest leaves as respiratory gases, urine/gill excretion, faeces and feed waste. For salmonids (here exemplified with Atlantic salmon / salmonids in grow-out on modern dry feed), process design can therefore be tied to daily feed in kg/day - but the figures are order-of-magnitude for teaching and early sizing, not bankable design. Feed recipe (protein, fat, digestibility), FCR, temperature, size and waste change the profile strongly.

Simplified biology/chemistry: organic carbon in feed is oxidised to CO₂ (respiration) or leaves as particulate organics (faeces/waste); protein nitrogen not built into biomass is excreted mainly as ammonia/ammonium (TAN) across the gills; phosphorus largely follows faeces and dissolved P; solids load filters and the receiving water. In RAS, TAN is also nitrified in the biofilter (extra O₂ demand and alkalinity use) and solids must be removed before they break down into more dissolved COD/BOD.

Key figures per kg feed (salmonids - example)

Approximate design rules of thumb for 1 kg dry feed to Atlantic salmon/salmonids in good growth (illustrative ranges from established RAS/water-quality practice). Always use feed analysis, measured FCR and site data in hard engineering.

From 1 kg feed — simplified mass-flow picture

1 kg feed in dry feed, salmonids

O₂ ≈ 200–300 g fish respiration / kg feed

↓

Weight gain about 0.8–0.9 kg fish at FCR ≈ 1.1–1.2 (good growth)

↓

Solids (sludge) Faeces + feed waste ≈ 200–400 g TSS
N ≈ 10–20 g · P ≈ 4–12 g · C ≈ 80–150 g

Dissolved in the water TAN ≈ 25–45 g N
NO₂⁻-N: normally low (intermediate)
NO₃⁻-N ≈ 25–45 g if TAN is nitrified
Dissolved P ≈ 1–4 g

↓

Biofilter (RAS) TAN → nitrate. Extra O₂ ≈ 140–200 g if all TAN is nitrified (≈ 4.57 g O₂ / g TAN). System total often ≈ 400–700 g O₂/kg feed.

Teaching sketch in the spirit of older Swedish EPA (Naturvårdsverket) nutrient-flow figures for fish farms. Not a permit calculation — use feed analysis, measured FCR and current guidance in licensing.

© 2026 DABCE AB

Teaching approximations for salmonids. Not a substitute for mass balance against actual feed and life stage.

Example (worked illustration, not a fixed answer): peak feed 1,000 kg/day × 0.25 kg O₂/kg feed ≈ 250 kg O₂/day for fish metabolism alone. Add nitrification (e.g. 35 g TAN/kg feed × 4.57 ≈ 160 g O₂/kg feed → ~160 kg O₂/day) and system demand quickly sits around 400+ kg O₂/day before efficiency, losses and safety margin - which is why LOX/PSA, cones/LHO and peak-feed thinking are central in commercial salmonid RAS.

Full emission profile (simplified)

Per kg feed in, one can think of a complete "emission and residue profile" that the farm must handle - either in water treatment (RAS/semi-flow) or via the receiving water (flow-through/cages) within permit limits:

• Gases: O₂ is consumed; CO₂ is produced (respiration). N₂ issues relate more to source water/total gas than to feed - but tank gas balance is still affected.
• Nitrogen: TAN from gills; then NO₂⁻/NO₃⁻ if nitrification occurs; faecal organic N.
• Solids: faeces + feed waste (TSS) - mechanical removal, sludge, receiving water.
• Dissolved/fine organics: BOD/COD, turbidity, fouling on gills and UV.
• Phosphorus and other nutrients: dissolved + particulate - important for discharge permits and eutrophication.
• Heat: metabolism and equipment - relevant in closed halls.
• Biomass out: harvested fish (the goal) - FCR links feed to sellable growth.

1 kg feed in
↓
Growth (depends on FCR)
+
O₂ consumed (fish + biofilter)
+
CO₂ out (respiration)
+
TAN → (NO₂⁻) → NO₃⁻
+
TSS / sludge (faeces + waste)
+
P and nutrients
+
Heat / residual organics

The design point: do not only count "how much oxygen". Count the whole chain - oxygen, CO₂ stripping, solids, nitrogen and sludge - against the same peak feed. See also carbon dioxide (CO₂), ammonia, sludge handling, FCR and FlowFarm for concept sizing.

Delta DO (ΔDO)

One of the most important design parameters is the difference between oxygen into and out of the fish unit. Example: inlet 10 mg/L, outlet 7 mg/L gives ΔDO = 3 mg/L. Larger allowed ΔDO → less water flow needed; smaller ΔDO → more flow. That directly affects pump size, energy use, pipe sizes, tank hydraulics and CAPEX. In many modern RAS facilities, optimising ΔDO (together with oxygen transfer and CO₂ stripping) is among the main levers to cut energy cost.

Common oxygenation techniques

• Low-pressure aeration, diffusers, paddlewheels (especially ponds).
• Oxygen cones and LHOs (Low Head Oxygenators) with pure oxygen.
• Venturi, microbubbles and nanobubbles in more specialised systems.

Pure oxygen is often used at high biomass because transfer efficiency is much higher than with ordinary air - see also the sections on oxygen cones, LHO, aeration and oxygen demand under equipment and oxygen & gas.

Oxygen and CO₂ must be seen together

Oxygen can never be viewed in isolation. Intensive systems must also manage oxygen supply, carbon dioxide stripping, alkalinity, pH, ammonia load and hydraulics. Many high-intensity facilities are limited by carbon dioxide long before they are limited by oxygen supply. A farm can show good DO values while fish performance is limited by elevated CO₂. See the carbon dioxide (CO₂) section.

Design rules

• Keep oxygen near saturation or the desired supersaturation on inlet to culture units.
• Avoid chronically low levels in the tanks - measure where the fish actually are.
• Size the oxygen system for maximum daily feed (peak feed), not the average.
• Build in redundancy for power loss, oxygen failure and pump trips.
• Optimise both oxygen transfer and CO₂ stripping - and think ΔDO and flow together.

Oxygen is in practice the fish farm's equivalent of fuel in an engine. Without enough oxygen, neither the fish, the biofilter nor the production system works as intended. For many modern facilities, oxygenation is therefore one of the most important technical, biological and economic design parameters.

For farmers, designers or anyone understanding intensive systems: carbon dioxide (CO₂) forms when fish and microbes respire - roughly the mirror image of oxygen use. Fish take up O₂ and release CO₂ into the water. In open ponds and cages, CO₂ is often diluted by large water volumes and gas exchange with air. In RAS, semi-flow and other high-density, low-exchange systems, dissolved CO₂ can rise until it limits growth and welfare - sometimes while oxygen still looks "fine" on the display.

That is why CO₂ is one of the parameters that separate "enough oxygen" from "fish that actually perform". Modern intensive culture must design for both oxygen supply and carbon dioxide stripping - see also the dissolved oxygen (DO) section.

What does CO₂ do to the fish?

Dissolved CO₂ affects blood and respiration. High levels lower blood pH (respiratory acidosis), make it harder to load and unload oxygen via haemoglobin systems, and cause stress even when DO in mg/L looks acceptable. Fish may increase ventilation rate, reduce feed intake, lose growth and become more disease-prone. At strongly elevated CO₂ you see lethargy, crowding at the surface or inlet, and in the worst case mortality - often combined with other stressors (heat, low alkalinity, handling).

Tolerance depends on species, life stage, acclimation and how fast CO₂ rises. Salmonids in intensive land-based culture are often held to fairly low targets in literature and practice (many farms aim on the order of single-digit to low tens of mg/L free CO₂ depending on species and policy) - exact limits should be set with species knowledge and veterinary/production standards, not web rules of thumb alone.

Chemistry: CO₂, pH and alkalinity

In water, CO₂ is part of the carbonate system: dissolved CO₂ (roughly H₂CO₃*) ↔ HCO₃⁻ ↔ CO₃²⁻. When more CO₂ dissolves, the equilibrium shifts, pH falls and the fraction of "free CO₂" rises. Alkalinity (buffer, often measured as HCO₃⁻ equivalents) dampens pH swings: at low alkalinity the same CO₂ addition causes a larger pH drop and more stress. So CO₂, pH and alkalinity belong together - see also the pH and alkalinity sections. In freshwater RAS, bicarbonate/lime make-up may be needed both for the biofilter and to hold buffer when CO₂ is stripped and nitrification consumes alkalinity.

Important: pH alone is not the same as CO₂. The same pH can mean different free CO₂ depending on alkalinity and temperature. So it is rarely enough to "control on pH" if you want to know whether fish are breathing in a CO₂-loaded environment - you need either measured/calculated free CO₂ or a calibrated model against alkalinity and pH.

Where does CO₂ come from?

• Fish respiration - rises with feed, biomass, activity and temperature.
• Microbes and biofilters - heterotrophic and nitrifying activity also produce CO₂.
• Breakdown of organics (feed fines, faeces) in systems with high organic load.
• In ponds: night-time respiration from plankton and sediment can raise CO₂ while DO falls.

As with oxygen, peak feed is the sizing load: more feed → more O₂ demand and more CO₂ production at roughly the same time. Summer operation (warmer water, higher metabolism) is often critical for both gases. For salmonids, early design often uses roughly 250-400 g CO₂ per kg feed as an order of magnitude (same uncertainty as for O₂) - see mass balance and emission profile under dissolved oxygen (DO).

Why CO₂ is often the bottleneck before oxygen in RAS

Pure oxygen and cones/LHOs can raise inlet DO quickly. Removing CO₂, by contrast, needs gas exchange against a gas phase with low CO₂ partial pressure - typically air in a degasser, cascade or packed column - plus enough contact time and area. If you only "pump in oxygen" without stripping CO₂, you can create a farm with high DO and still reduced feed intake and growth. Many high-intensity systems are therefore CO₂-limited: degasser size, air flow, head and hydraulics set the ceiling, not oxygen gas flow alone.

Stripping and technology choices

• Degasser / CO₂ stripper - cascade, packed bed or similar; air against water, often after the biofilter.
• Aeration in tank or sump - simpler, lower efficiency; more common in semi-flow and ponds (paddlewheels etc.).
• Trickling filters / aerated biofilters - can provide both nitrification and some gas exchange.
• Water exchange / make-up - dilutes CO₂ but costs water and heat; complements, rarely replaces a degasser in high-reuse RAS.

Best practice in many RAS designs: maximise air-side gas exchange in the degasser (CO₂ out, O₂ in from air), then add pure oxygen to reach the desired total ΔDO - do not replace all aeration with pressurised O₂ alone. That often cuts energy need and the risk of forgetting CO₂. See degassers, LHO, oxygen cones and oxygen demand under equipment and oxygen & gas.

Measuring CO₂

CO₂ is harder to "just put a probe on" than temperature. Common paths: (1) calculate free CO₂ from pH, alkalinity, temperature and salinity with carbonate equilibria - needs reliable alkalinity and pH measurement; (2) direct CO₂ sensors / membrane or NDIR solutions in process water where available and maintained; (3) lab or field kits as spot checks. Online pH is often used as a fast indicator, but as noted above pH is not identical to CO₂ without known buffer.

Practical problems: wrong alkalinity → wrong calculated CO₂; pH electrodes that drift or foul; slow response; measurement in a sump that does not represent the tank; and staff trusting DO alarms while CO₂ quietly rises at peak feed. On high-intensity farms, CO₂ (measured or calculated) should be part of the operating picture together with DO, pH and feed - see monitoring systems.

Design rules

• Design oxygen and CO₂ together - size degasser/air for peak feed, not oxygen gas alone.
• Track alkalinity and pH; low buffer amplifies CO₂ problems.
• Avoid "fixing" low DO with more O₂ alone if fish show CO₂ stress (poor appetite, high ventilation).
• Ensure air flow, head and maintenance on the degasser - fouling and bad air cause quiet CO₂ rise.
• Measure or calculate free CO₂ as needed; do not rely only on green DO.

In short: oxygen is the fuel, but CO₂ is the exhaust. An engine that only gets fuel without an exhaust outlet runs poorly - the same logic applies to intensive fish farming. Good gas balance is O₂ in, CO₂ out, stable buffer and hydraulics that mix the tank evenly.

pH is one of the most important water-quality parameters in aquaculture. It influences fish physiology, oxygen transport, ammonia toxicity, biofilter performance, nutrient availability and overall system stability. Even when oxygen, temperature and filtration look acceptable, poor pH control can cut growth and raise stress.

pH measures how acidic or alkaline the water is on a scale from 0 to 14: 7 is neutral, below 7 acidic, above 7 alkaline. The scale is logarithmic - a change from 7 to 6 is a tenfold increase in acidity. Small pH steps are therefore large chemical changes.

Why pH matters for fish

Fish continuously exchange ions and gases across the gills. Water that is too acidic or too alkaline can damage gill tissue, disrupt osmoregulation, raise stress and reduce growth. Sudden swings are usually worse than a slow move. In many cases stability matters more than a precise target.

• Reduced appetite and slower growth.
• Higher feed conversion (FCR) and weaker immunity.
• Reproductive problems and, at worst, higher mortality.

Typical pH ranges

Freshwater orientation. Most freshwater farms run best around pH 6.8–8.0. Do not apply this table unchanged to brackish or seawater.

Brackish water and seawater have a different background chemistry. Open sea is often around pH 7.8–8.3 (typically near 8.0–8.2) with higher alkalinity than most fresh waters. Brackish water sits in between and follows the mix of fresh and salt, algal production and CO₂. Marine and brackish RAS can still be pulled down by fish CO₂ - then gas control, not "an acidic source", is usually the fix. At pH 8+ the toxic NH₃ fraction is already higher than in near-neutral freshwater: the same TAN is more serious. See salinity, carbon dioxide and ammonia.

Orientation. Natural seawater is more strongly buffered than most fresh waters, but closed marine RAS can still lose pH via CO₂ and nitrification. Species and salinity set the target.

Species differences

Cold-water salmonids (rainbow trout, Arctic char, Atlantic salmon) often do well around pH 6.8–7.8. Warm-water species such as tilapia, carp, catfish and pangasius often tolerate 6.5–8.5. Tilapia are among the more pH-tolerant farmed fish. Kingfish (Seriola) is usually farmed in more marine or brackish water where background pH is often higher than in freshwater - steer to species and salinity, not a freshwater target.

pH and biofiltration

In RAS, pH is tightly linked to nitrification. Bacteria convert ammonia (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). The process generates acid and consumes alkalinity. Without enough buffer, pH falls over time, nitrification slows or stops, and the system becomes unstable. Alkalinity often has to be topped up - see alkalinity and ammonia.

pH and ammonia toxicity

Total ammonia nitrogen (TAN) exists as NH₄⁺ (ammonium, relatively harmless) and NH₃ (unionised ammonia, highly toxic). As pH rises, a larger fraction becomes NH₃. The same TAN level can be relatively safe at pH 7.0 and dangerous at pH 8.5. Temperature also shifts the equilibrium - always read pH, temperature and TAN together. Calculate the NH₃ fraction in the ammonia section.

Open TAN calculator →

Daily pH fluctuations

In ponds, raceways and algae-rich systems, pH often changes through the day. In the morning photosynthesis is low, CO₂ has built up overnight and pH is typically lowest. In the afternoon algae consume CO₂, pH rises and oxygen often peaks. Large daily swings (sometimes more than one full pH unit in productive ponds) can stress fish even when average pH looks fine. See ponds and dissolved oxygen.

pH and carbon dioxide (CO₂)

pH and CO₂ are tightly linked. Dissolved CO₂ forms carbonic acid (CO₂ + H₂O ⇌ H₂CO₃) and lowers pH. When CO₂ is stripped (degassing, aeration or photosynthesis), carbonic acid falls and pH rises. In intensive RAS many "pH problems" are actually poor CO₂ control. See carbon dioxide (CO₂).

Alkalinity and buffer capacity

Alkalinity is the water's ability to neutralize acid and resist a pH drop. In aquaculture it is mainly the carbonate system: bicarbonate (HCO₃⁻), carbonate (CO₃²⁻) and dissolved CO₂. pH shows the condition now; alkalinity shows how much buffer remains. Simplified: CO₂ ⇌ H₂CO₃ ⇌ HCO₃⁻ ⇌ CO₃²⁻. When acid is added, bicarbonate/carbonate take up H⁺ and pH falls more slowly.

In RAS, nitrification consumes alkalinity continuously. Without make-up, pH falls, the biofilter loses performance and a pH crash can come suddenly when the buffer is gone. Two farms at the same pH can therefore behave very differently - one is stable, the other close to collapse. Treat alkalinity as a consumable, much like oxygen. See alkalinity.

Typical freshwater/RAS bands. Targets depend on species, feed load, nitrification, source and strategy.

Common ways to hold alkalinity: sodium bicarbonate (NaHCO₃), calcium carbonate (limestone), dolomite, calcium hydroxide, sodium carbonate. Sodium bicarbonate is widely used in commercial RAS - it replenishes HCO₃⁻ and is relatively easy to handle. Degassing removes CO₂ and often raises pH; more bicarbonate mainly strengthens resistance to future pH decline, not necessarily a dramatic pH jump.

Monitoring and alarms

• Continuous pH in RAS; calibrate probes and check by hand.
• Regular alkalinity tests - not pH alone.
• Read pH together with CO₂, TAN, nitrite and temperature.
• Alarms for both low and high pH. Examples (not universal): low about 6.5, high about 8.5 - adjust to species and strategy.

Signs of pH problems

Reduced feeding, flashing/rubbing, increased gill movement, surface aggregation, erratic swimming, poor growth and more disease. The same picture can be oxygen, ammonia or CO₂ - measure, do not guess. See monitoring systems.

Rule of thumb

Temperature determines how fast biology works. Oxygen determines whether fish can breathe. pH determines whether water chemistry stays stable enough for everything else to function. Alkalinity tells you how resistant the system is to becoming acidic tomorrow.

Alkalinity is one of the most important but often overlooked water-quality parameters in aquaculture. pH says how acidic or alkaline the water is right now. Alkalinity says how much the water can take before pH collapses.

In most freshwater farms the buffer is the carbonate system: CO₂ ⇌ H₂CO₃ ⇌ HCO₃⁻ ⇌ CO₃²⁻. Bicarbonate and carbonate take up hydrogen ions (H⁺) from biology and slow a pH drop.

A simple rule: pH shows where chemistry stands today. Alkalinity shows how resistant the system is to becoming more acidic tomorrow. See pH and carbon dioxide (CO₂).

Why alkalinity matters

Alkalinity supports both fish health and the biofilter. In RAS, nitrifiers continuously convert ammonia (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). The process releases H⁺ and therefore consumes alkalinity. Without enough buffer, pH becomes unstable, nitrification slows, feed capacity may be limited, and a sudden pH crash becomes possible. Treat alkalinity as a consumable — much like oxygen. See Ammonia / TAN and dissolved oxygen.

Alkalinity and biofilter stability

With enough alkalinity, pH changes slowly, nitrifiers work steadily and TAN removal stays predictable. When the buffer is gone, small acid additions can swing pH a lot, the biofilter can drop off and nitrite can rise. Two farms at the same pH can therefore behave very differently: strong buffer is stable; low alkalinity may be days from a crash.

Typical alkalinity targets

Typical freshwater/RAS bands. Targets depend on species, feed load, source, density and biofilter loading.

Alkalinity, CO₂ and pH

Read the three together. More CO₂ lowers pH. Degassing removes CO₂ and usually raises pH. Adding alkalinity increases buffer capacity — it does not always raise pH dramatically, but it makes future pH drops less likely. Many "pH problems" in intensive culture are actually too little alkalinity or poor CO₂ control.

Maintaining alkalinity

Freshwater RAS often needs continual top-up to replace what nitrification consumes. Common additions: sodium bicarbonate (NaHCO₃), calcium carbonate (limestone), dolomite, calcium hydroxide, sodium carbonate. Sodium bicarbonate is common in commercial RAS — it replenishes HCO₃⁻ and is relatively easy to dose. No dose here: calculate against feed load and measure.

Differences between systems

• Ponds: natural alkalinity from soils and the catchment. Low buffer + photosynthesis means large daily pH swings.
• Raceways / flow-through: make-up often replenishes buffer; low-alkalinity sources may need treatment.
• Marine: seawater usually has high natural alkalinity, though intensive production can still affect local carbonate chemistry.
• RAS: alkalinity is a process parameter — routine measurement and top-up are normal.

Measurement

Alkalinity is usually reported as mg/L CaCO₃ (calcium carbonate equivalent), sometimes meq/L. It changes more slowly than DO or pH, so daily measurement is often unnecessary — but regular checks are good practice in intensive farming. Especially when raising feed load, expanding biomass, starting a biofilter, seeing unexplained pH drops, or adjusting bicarbonate dose.

Practical rule of thumb

pH tells you how the water behaves today. Alkalinity tells you how resilient the system will be tomorrow. A stable pH with low alkalinity may mean a crash is approaching. Strong buffer helps keep biofilter performance, fish health and water chemistry predictable through the cycle.

Hardness is a measure of dissolved divalent minerals in water, mainly calcium (Ca²⁺) and magnesium (Mg²⁺). They matter for osmoregulation, skeleton, reproduction and stable water chemistry. Hardness and alkalinity are both often reported as mg/L CaCO₃ — but they measure different things. Hardness is mineral content. Alkalinity is the ability to resist acidification. See alkalinity.

Fish are in constant contact with the water and exchange ions across gills, skin and gut. Dissolved minerals are therefore not only "chemistry on paper" — they are part of the fish's internal balance.

Why hardness matters

• Osmoregulation and ion balance, gill function, muscle and nerve.
• Skeleton, enzyme activity, egg quality, fertilisation and larval survival.

Very soft water forces the fish to spend more energy on salt balance. That can cut growth, raise disease sensitivity and hurt reproduction. Hardness can be raised by mineralisation treatment: let the water trickle over a bed of crushed limestone or similar (example from another industry: common on ships, where evaporator or RO water is almost mineral-free and must be post-treated before use).

Hardness versus alkalinity

Hardness comes mainly from Ca²⁺ and Mg²⁺. Alkalinity comes mainly from HCO₃⁻ and CO₃²⁻. A source can therefore have high hardness and low alkalinity — or the reverse — or high or low values of both. Both matter, for different reasons.

Eggs, hatcheries and early life stages

Calcium supports egg membranes, fertilisation, water regulation in the egg, the embryo and hatching. Extremely soft water can mean poorer hatch, fragile eggs, more deformities and weak larvae. Many hatcheries therefore watch hardness, calcium, alkalinity and conductivity as closely as oxygen and temperature.

More than calcium and magnesium

Two sources with the same hardness can behave differently if the ion mix differs. Advanced hatcheries and RAS often measure conductivity and individual ions, not only total hardness.

Groundwater, iron and manganese

In groundwater, Fe²⁺ and Mn²⁺ are often more important in practice than hardness itself. When the water meets oxygen the metals oxidise and precipitate: Fe²⁺ → Fe³⁺ → Fe(OH)₃ and Mn²⁺ → MnO₂(s). The particles can irritate gills, foul drum filters, biofilters, nozzles and sensors, and increase sludge.

Common pretreatment: aeration, degassing/cascade, sand filters, oxidation and catalytic media. Fe and Mn technically count toward hardness as divalent ions, but on a farm the issue is mostly fish health and fouling. For many groundwater sites, Fe/Mn is a bigger job than hardness. See also filtration and aeration under equipment.

Typical hardness ranges

Many freshwater farms run well around 50–250 mg/L CaCO₃. The ideal depends on species, life stage and strategy. Orientation, not a protocol.

Put hardness and alkalinity on the same plot and a rough target region appears for most freshwater farms. The box is orientation, not a requirement — and it does not replace species- or source-specific analysis.

Green box: hardness about 50–250 mg/L and alkalinity about 80–200 mg/L (as CaCO₃). Orientation — species, life stage and source govern. Not a permit value.

© 2026 DABCE AB

Hardness and toxicity

Adequate calcium can reduce sensitivity to nitrite, aluminium, some metals and other stress. Fish in soft water are often more sensitive to disturbances than fish in harder water. See nitrite.

Hardness in RAS

In RAS, mineral content can drift with water exchange, solids removal, source chemistry, treatment and biology. Rainwater, RO water and very soft groundwater may need remineralisation. Low hardness can mean poorer hatchery results, poorer welfare, a less stable biofilter and more physiological stress.

Remineralisation

Common additions: calcium carbonate (limestone), dolomite, calcium chloride, magnesium sulphate (Epsom salt) and commercial blends. The aim is rarely "as hard as possible" but enough minerals and stable operation. No dose here — calculate against analysis and species.

Measurement

Total hardness (TH) as mg/L CaCO₃, by kit, lab or online. In hatcheries and advanced RAS it is often worth measuring Ca, Mg, Fe, Mn, conductivity and alkalinity separately.

Differences between systems

• Ponds: driven by geology and the catchment. Liming is often used when both hardness and alkalinity are low.
• Flow-through: hardness reflects the source. Groundwater often has elevated Fe and Mn.
• RAS: mineral management is part of routine operation. Hardness, alkalinity, calcium and iron may all need active control.
• Marine: hardness is naturally high and rarely limiting. Salinity and trace elements matter more.

Practical rule of thumb

Alkalinity stabilises the chemistry. Hardness provides the minerals. Iron and manganese decide how much pretreatment a groundwater source may need before it is fit for aquaculture. For osmoregulation, individual ions and conductivity, see mineral and ion management.

Why ions matter

Fish do not simply live in water. They live in a complex chemical environment containing dissolved gases, minerals, nutrients, salts, and organic compounds. Every second, water passes across the gills where fish absorb oxygen, release carbon dioxide, excrete ammonia, and continuously exchange ions with their surroundings.

The most important dissolved ions include:

• Calcium (Ca²⁺)
• Magnesium (Mg²⁺)
• Sodium (Na⁺)
• Potassium (K⁺)
• Chloride (Cl⁻)
• Bicarbonate (HCO₃⁻)

Together, these ions influence nearly every aspect of fish biology, including respiration, growth, stress tolerance, reproduction, immunity, and energy use.

Unlike terrestrial animals, fish are constantly exposed to the water around them. Their internal body fluids have a very different ionic composition than the surrounding environment, meaning they must continuously regulate water and salt movement across the gills, skin, and digestive tract.

This process is known as osmoregulation and is one of the most important biological functions in aquatic animals.

A fish living in poor water chemistry may spend a significant amount of its energy maintaining ionic balance instead of converting feed into growth. Even when oxygen, temperature, and nutrition are optimal, unsuitable ion concentrations can reduce growth performance, increase stress, impair reproduction, and make fish more vulnerable to disease.

For fish farmers, understanding ion chemistry helps explain why parameters such as hardness, alkalinity, conductivity, salinity, calcium, magnesium, sodium, and chloride matter at all. These parameters are not isolated measurements; they describe the chemical environment that fish experience every moment of their lives.

See also

• Hardness
• Alkalinity
• pH
• Conductivity
• Salinity
• Ammonia (TAN)
• Nitrite

Osmoregulation

Fish maintain an internal salt concentration that differs from the surrounding water. The mechanism depends on whether the water is fresher or saltier than the blood. Although the mechanisms differ, both freshwater and marine fish depend on stable ionic conditions.

Osmoregulation — freshwater versus seawater

Freshwater fish

The water is dilute. The fish constantly gains water and loses salts.

↓ H₂O in (osmosis)

↓ Salts out

• Excrete excess water
• Actively absorb ions through the gills
• Retain salts in the kidneys

Marine fish

The water is saltier. The fish constantly loses water and gains salts.

↑ H₂O out (osmosis)

↑ Salts in

• Drink seawater
• Excrete excess salts through the gills
• Regulate internal osmotic pressure

The mechanisms differ, but both depend on stable ionic conditions. Teaching sketch — not a physiological model.

© 2026 DABCE AB

Freshwater fish

Freshwater fish constantly gain water and lose salts. They must:

• Excrete excess water
• Actively absorb ions through the gills
• Retain salts through the kidneys

Marine fish

Marine fish constantly lose water and gain salts. They must:

• Drink seawater
• Excrete excess salts through the gills
• Regulate internal osmotic pressure

The most important ions

The same ions reappear under hardness, alkalinity and nitrite. Two waters with the same hardness can behave differently if the rest of the ion mix differs.

Chloride competes directly with nitrite at the gill surface. That is why salt is often used during nitrite events — chloride reduces nitrite uptake.

Conductivity and salinity

Conductivity is the fast overview of how many ions are present. Salinity describes how salty the water is — in practice mainly Na⁺ and Cl⁻ in brackish water and the sea. Neither gives the full ion mix. See conductivity and salinity.

Groundwater and trace elements

Groundwater often contains significant concentrations of iron, manganese, carbon dioxide and dissolved gases. While iron and manganese contribute little to hardness, they are frequently among the most important water-treatment challenges in groundwater-fed aquaculture.

Both can:

• Foul biofilters
• Coat gills
• Block equipment
• Increase maintenance requirements

As a result, many hatcheries remove iron and manganese before water enters production units. See hardness.

When remineralisation is needed

Some waters are naturally very soft. Examples include:

• Rainwater
• Reverse osmosis water
• Mountain streams
• Certain groundwater sources

These waters may require supplementation of calcium, magnesium, alkalinity and trace minerals before optimal fish production can be achieved. A simple way to raise hardness is to let water trickle over crushed limestone (example from another industry: common on ships after evaporator or RO). See hardness and alkalinity. No dose here — calculate against analysis and species.

Water chemistry as a system

Successful aquaculture is rarely about a single parameter. The most productive facilities manage an interconnected system:

• Temperature → metabolism
• Dissolved oxygen → respiration
• pH → water chemistry
• Alkalinity → buffer capacity
• Hardness → mineral availability
• TAN → nitrogen loading
• Nitrite → intermediate toxicity
• Nitrate → long-term accumulation
• Ions → osmoregulation

All parameters influence one another. Understanding these interactions is often the difference between a system that merely functions and one that achieves consistently high growth, survival and feed efficiency.

Practical rule of thumb

Fish do not live in H₂O. They live in a solution of water, minerals, salts, gases and nutrients. Understanding the ionic environment is just as important as understanding oxygen or temperature.

Conductivity (often EC) measures how well the water conducts electricity. The current is carried by dissolved ions. Conductivity is therefore usually the fastest way to estimate total dissolved salts — without saying which ions are present.

It is an overview parameter, a bit like temperature for metabolism: one number that quickly shows whether the ionic environment is low, typical or unusually high. For which ions are present you need hardness, alkalinity, individual analyses or salinity. See mineral and ion management.

Units

Common units are µS/cm (microsiemens per centimetre) in freshwater and mS/cm in brackish water and the sea (1 mS/cm = 1 000 µS/cm). Some instruments show TDS (total dissolved solids) in mg/L — that is usually a conversion from conductivity, not a separate measurement. Temperature affects the reading; use temperature-compensated conductivity if the instrument has it.

What low and high conductivity mean

Low conductivity usually indicates:

• Very soft water
• Low mineral content
• Possible need to remineralise (rain, RO, mountain water)

High conductivity may indicate:

• Elevated salinity
• High dissolved solids from the source or from feed/chemicals
• Concentration by evaporation, especially in RAS

Approximate orders of magnitude

Orders of magnitude, not operating targets. The same conductivity can come from different ions.

What conductivity does not say

Two waters with the same EC can behave differently. One may be soft with a lot of sodium chloride, the other hard with calcium and bicarbonate. Fe and Mn barely show in conductivity but can be the largest pretreatment job. EC is therefore complemented by hardness, alkalinity, salinity and — when needed — individual ions.

Use on the farm

• Hatchery and RAS: daily or continuous overview of the ionic environment.
• Source change or make-up: see whether new water is softer or saltier than the system.
• Evaporation in RAS: a slow rise can mean salts are concentrating.
• Remineralisation: EC rises when you add minerals — but also measure hardness and alkalinity.

Differences between systems

• Ponds and flow-through: EC reflects the source and the catchment.
• RAS: EC can drift with feed, chemicals, evaporation and water exchange.
• Marine: EC is high and stable in open sea; in closed marine RAS it can still change.

Practical rule of thumb

Conductivity counts the ions. It does not name them. A sudden change is often more important than the level itself — something has happened with source, salt, evaporation or dosing. See salinity, hardness and mineral and ion management.

Salinity describes the concentration of dissolved salts in water and is one of the most important environmental parameters in aquaculture. While freshwater systems often focus on hardness, alkalinity and conductivity, marine and brackish-water systems are largely defined by salinity.

In seawater, dissolved salts are dominated by sodium (Na⁺) and chloride (Cl⁻), but also include significant amounts of sulphate (SO₄²⁻), magnesium (Mg²⁺), calcium (Ca²⁺), potassium (K⁺) and bicarbonate (HCO₃⁻). Together these ions create the osmotic environment in which fish must survive and grow.

Salinity influences:

• Osmoregulation
• Growth and feed conversion
• Stress tolerance
• Oxygen solubility
• Water density
• Ammonia toxicity
• Smoltification and life-stage transitions
• Species distribution and suitability

For many marine and brackish-water farms, salinity is as important as temperature and dissolved oxygen.

See also

• Mineral and ion management
• Conductivity
• Dissolved oxygen
• pH
• Ammonia (TAN)

Why salinity matters

Fish do not passively adapt to the surrounding water. They continuously spend energy maintaining an internal salt concentration that differs from their environment. Salinity therefore directly affects the amount of energy required for osmoregulation.

• Fish maintain an internal salt concentration different from the surrounding water.
• Species differ greatly in salinity tolerance.
• Life stages often have different requirements.
• Sudden salinity changes are usually more stressful than gradual transitions.
• Higher salinity reduces oxygen solubility.
• Salinity affects ammonia chemistry, pH behaviour and overall water chemistry.

A fish experiencing osmotic stress may eat less, grow more slowly and become more susceptible to disease even when oxygen, temperature and feeding are otherwise optimal.

Salinity and osmoregulation

Water naturally moves across biological membranes from lower salt concentrations toward higher salt concentrations. Fish must continuously compensate for this process.

Osmoregulation — freshwater versus seawater

Freshwater fish

The water is dilute. The fish constantly gains water and loses salts.

↓ H₂O in (osmosis)

↓ Salts out

• Excrete excess water
• Actively absorb ions through the gills
• Retain salts in the kidneys

Marine fish

The water is saltier. The fish constantly loses water and gains salts.

↑ H₂O out (osmosis)

↑ Salts in

• Drink seawater
• Excrete excess salts through the gills
• Regulate internal osmotic pressure

The mechanisms differ, but both depend on stable ionic conditions. Teaching sketch — not a physiological model.

© 2026 DABCE AB

Freshwater fish

Freshwater contains far fewer dissolved salts than fish blood. As a result, water tends to enter the fish and salts tend to leave. Fish actively absorb ions through specialised gill cells and remove excess water as dilute urine. Examples: rainbow trout, Arctic char, carp and most freshwater species.

Marine fish

Seawater contains far more dissolved salts than fish blood. As a result, water tends to leave the fish and salts tend to enter. Marine fish drink seawater and actively excrete excess salts through the gills. Examples: sea bass, sea bream, many marine flatfish and numerous marine aquaculture species.

Species and life stage matter

Not all fish respond to salinity in the same way.

Atlantic salmon are anadromous fish that migrate from freshwater to seawater. Before entering the sea they undergo smoltification, a major physiological transformation that prepares the gills, kidneys and endocrine system for a marine environment. Moving fish to seawater before smoltification is complete can cause severe osmotic stress and mortality.

Rainbow trout are remarkably adaptable and may be farmed in freshwater, brackish water and some marine environments. However, large salinity changes still require careful acclimation.

Many tilapia strains tolerate a surprisingly wide salinity range and are often cultured in brackish environments where traditional freshwater species perform poorly.

Many marine fish cannot tolerate prolonged exposure to freshwater because they are unable to maintain osmotic balance outside a saline environment. For this reason, salinity targets should always be based on species and life stage rather than generic recommendations.

Normal culture salinity by species (orientation)

The tables below focus on normal culture ranges — the band farmers usually target — not theoretical maximum survival. “Typical salinity” is what is common in commercial operation. “Extreme tolerance” is an approximate upper limit where stress and mortality rise quickly; it is not a target.

Orientation for knowledge and early project work — not a species culture protocol. Strain, life stage and acclimation govern real limits.

Shrimp and other aquaculture species

At low salinity for L. vannamei, ion balance (Ca, Mg, K, Na) is often as important as the ppt figure itself. See shrimp and mineral/ion management.

Use the species tables when matching a species to a given water source or when setting operating targets in RAS, ponds or cages. “Extreme tolerance” is useful for risk assessment (e.g. leaks, dosing errors, seasonal swings), but always size against the normal culture range — that is where growth, FCR and health are usually acceptable.

Practical rule of thumb by species group

When you quickly want to place a species or concept in the right “salinity world”, the grouping below is often enough. It does not replace the species table above, but helps with first orientation between freshwater, lightly salted RAS, brackish water and marine production.

Orientation. Overlap exists — acclimation, life stage and local practice govern. See also conductivity and mineral/ion management.

Salinity and dissolved oxygen

Salinity influences how much oxygen water can physically hold. As salinity increases, oxygen solubility decreases. At the same temperature and atmospheric pressure, freshwater can therefore contain more dissolved oxygen than seawater.

This effect occurs because dissolved salt ions occupy space within the water and reduce the ability of oxygen molecules to dissolve. In practice, seawater has a lower oxygen-carrying capacity than freshwater. At the same temperature, freshwater holds slightly more oxygen than brackish water, and brackish water slightly more than seawater. The difference becomes increasingly important as biomass and feeding intensity increase.

While the reduction may appear modest on paper, it becomes significant in commercial aquaculture where every milligram of dissolved oxygen matters. A marine farm may therefore have a smaller oxygen reserve available to support fish, biofilters and microbial processes than an equivalent freshwater system.

Why this matters in aquaculture

Higher salinity does not reduce the fish's need for oxygen. In many cases, fish require the same or even greater oxygen availability despite the water holding less oxygen. This creates a narrower operating margin for:

• High stocking densities
• Peak feeding periods
• Warm-water conditions
• Intensive RAS operation
• Live fish transport

The effect is compounded because temperature also reduces oxygen solubility. Warm seawater therefore represents one of the most challenging environments for maintaining high dissolved oxygen concentrations.

Salinity, oxygen and biomass

At low stocking densities, the effect of salinity on oxygen availability is often of limited practical importance. As biomass increases, oxygen consumption, feeding rates and biofilter oxygen demand all rise — while available oxygen reserves decrease. For intensive marine and brackish-water systems, oxygen management often becomes one of the primary production constraints. This is particularly true in marine RAS, sea cages during periods of low water exchange, shrimp farms and warm-water marine production.

Practical example

A dissolved oxygen concentration of 8 mg/L does not necessarily represent the same level of oxygen saturation in freshwater and seawater. For this reason, oxygen measurements are often interpreted together with temperature, salinity and atmospheric pressure. Many farms therefore monitor both dissolved oxygen concentration (mg/L) and oxygen saturation (%) rather than relying on a single value. See dissolved oxygen.

Practical rule of thumb for oxygen and salinity

The warmer and saltier the water becomes, the less oxygen it can hold. For intensive aquaculture this means that oxygen management generally becomes more critical as salinity, temperature, biomass and feeding rates increase.

Salinity and ammonia toxicity

Salinity influences ammonia chemistry both directly and indirectly. The most important effect in practice is that marine and brackish-water systems often operate at a higher pH than freshwater systems. Because the proportion of toxic unionised ammonia (NH₃) increases rapidly as pH rises, the same total ammonia concentration (TAN) can represent very different levels of risk depending on the water chemistry.

Ammonia exists in equilibrium between NH₄⁺ ⇌ NH₃ + H⁺. Ammonium (NH₄⁺) is relatively harmless, while unionised ammonia (NH₃) is highly toxic to fish and can damage gills, reduce growth and increase mortality.

As pH increases, the equilibrium shifts towards NH₃. Higher temperatures have a similar effect. Salinity also influences the equilibrium through changes in ammonia dissociation chemistry, although this effect is generally smaller than the influence of pH and temperature.

For example, a TAN concentration that appears acceptable in a freshwater trout farm operating at pH 7.0 may result in significantly higher NH₃ concentrations in a marine or brackish-water system operating at pH 8.0–8.3.

This is why TAN should never be interpreted on its own. The real toxicity risk depends on:

• TAN concentration
• pH
• Temperature
• Salinity
• Exposure time
• Species sensitivity

As a general rule, fish respond to NH₃, not TAN. Marine and brackish-water operators should therefore pay particular attention to pH control and ammonia monitoring, especially during periods of high feeding, elevated temperatures or reduced biofilter performance. See Ammonia / TAN.

Units and typical ranges

Salinity is commonly expressed as ppt (parts per thousand), ‰ (per mille) or PSU (Practical Salinity Units). For aquaculture purposes these values are usually treated as approximately equivalent.

Common estuarine/oceanographic bands (Venice system). Open ocean is typically close to 35 ppt. Species often occupy only a portion of these ranges. Orientation, not a species protocol.

Salinity and conductivity

Salinity and conductivity are closely related but not identical. Electrical conductivity (EC) is the water's ability to conduct electricity, usually in µS/cm or mS/cm. Dissolved ions carry the current, so EC is a fast estimate of how many ions are present — not which ones. Salinity attempts to estimate the concentration of dissolved salts. Two waters may have the same conductivity but different ionic compositions.

In freshwater aquaculture, conductivity is often more useful than salinity because salinity values are very low and difficult to distinguish accurately. See conductivity.

Salinity in different aquaculture systems

Sea cages: salinity is largely determined by the site. It can change due to river discharge, rainfall, season, water depth, tides and currents. The Baltic Sea, fjords and estuaries often experience substantial salinity gradients.

Freshwater flow-through systems: salinity generally reflects the source water and remains low.

Marine and brackish RAS: salinity becomes an operational process parameter. Operators may need to manage evaporation, make-up water, dilution, salt addition and biological performance.

Shrimp and brackish-water ponds: salinity is often one of the primary control variables alongside temperature and dissolved oxygen.

Salt as a management tool

Salt is occasionally used in freshwater aquaculture to reduce nitrite toxicity, support handling and transport, assist during certain treatments and reduce short-term osmotic stress. The protective effect against nitrite occurs because chloride ions compete with nitrite at the gill surface. Salt is a management tool, not a substitute for proper water-quality management. Species, life stage, existing water chemistry and treatment objectives must always be considered before any application. See nitrite.

Measurement

Salinity can be measured using refractometers, conductivity meters with salinity compensation, multi-parameter probes or laboratory analysis. For freshwater systems, conductivity often provides more useful information than salinity alone. Measurements should be temperature compensated and instruments should be calibrated against known standards.

Practical rule of thumb

Salinity is much more than the amount of salt dissolved in water. It determines the osmotic environment that fish experience every second of their lives. Appropriate salinity supports efficient osmoregulation, reduces stress, improves welfare and helps fish allocate energy toward growth rather than survival.

Keep salinity where the species belongs and change it slowly. Fish usually tolerate a range of salinities, but they rarely tolerate sudden jumps. Temperature controls metabolism, oxygen supports respiration, and salinity determines how hard fish must work simply to maintain their internal balance.

Think of ammonia as the first dissolved nitrogen waste from the fish. The fish eats protein in the feed. Some of that nitrogen becomes new fish (growth). Some is converted in metabolism to ammonia and leaves the body, mostly across the gills. What we measure in the water is called TAN - total ammonia nitrogen. TAN is the sum of two forms: NH₄⁺ (ammonium), which is relatively harmless at ordinary levels, and NH₃ (unionised ammonia), which is highly toxic even at low concentrations.

Why TAN is one of the most important parameters

In most intensive systems ammonia is not just a water-quality number. It shows whether biological treatment is keeping up with fish metabolism. Every gram of feed eventually becomes fish biomass, solid waste or dissolved waste. The most important dissolved nitrogen waste is ammonia.

Unlike many other problems, TAN can rise quickly after feeding, a biofilter disturbance, equipment failure, a power cut or a sudden biomass increase. Fish produce ammonia continuously - there is no practical way to switch the source off without cutting feed or removing fish. Rising TAN is therefore often an early warning, before fish show clear stress.

A steadily rising TAN level may indicate:

• Biofilter overload, insufficient capacity, or reduced nitrification.
• Low alkalinity or low DO in the biofilter.
• Excessive feeding or inadequate water exchange.
• Recent disinfection or medication that hit nitrifiers.

Where does TAN come from?

Start with the feed. The fish eats protein. In the body, protein is broken down into amino acids. Some are used to build growth. Some are used as energy. When amino acids are burned, the nitrogen is left over - it is converted to ammonia (TAN). Some of this TAN stays briefly in the fish, but most is excreted into the water, mainly across the gills. So it is not "unused protein dumped as-is". It is protein the fish actually ate and metabolised - and then released as TAN.

That is why TAN follows the feed: more feed → more metabolised protein → more TAN in the water. For salmonids in good growth a common rule of thumb is about 25–45 g TAN-N per kg dry feed. The figure rises if the feed is protein-rich, if the fish grow poorly (bad FCR), or if more protein goes to energy than to growth. Peak feed means peak TAN. See the mass-flow sketch under dissolved oxygen.

Two forms: the harmless and the toxic

TAN in the water is always a mixture. Think of a seesaw: NH₄⁺ ⇌ NH₃ + H⁺. On one side sits the relatively harmless ammonium ion. On the other the toxic gas form NH₃. Raise pH (fewer hydrogen ions) and the board tips toward NH₃. Raise temperature and it tips the same way, more moderately.

Three things to keep in mind: TAN says how much ammonia is there in total. pH decides how large a share becomes toxic NH₃. Temperature raises that toxic share further. Always read the three together. In freshwater we often use Emerson et al. (1975) - that is what the calculator below does. In seawater the equilibrium is almost the same, but not identical. See pH and temperature.

Pourbaix diagram (Eh–pH)

A Pourbaix diagram shows which chemical form is thermodynamically most stable at a given pH and redox potential. Horizontal is pH. The left vertical axis is Eh in volts vs SHE (the lab reference). The right axis is ORP in mV vs Ag/AgCl — what most farm probes display. Roughly: Eh (V) ≈ ORP (V) + 0.20. High Eh/ORP = oxidising (oxygen present). Low = reducing. Dashed lines are water's own window: above the upper line water would want to make O₂, below the lower line H₂.

For nitrogen, the vertical line at pKa ≈ 9.25 is the same seesaw as the calculator: NH₄⁺ to the left, NH₃ to the right. Up and down is oxidation state: NO₃⁻ at the top (oxic), then a thin NO₂⁻ band, then NH₄⁺/NH₃ at the bottom (reducing). N₂ is not drawn — it would cover almost the whole box, but N≡N is so inert that it is not the form we steer day to day in the tank.

In an oxygenated tank nitrogen "wants" to sit as nitrate. The path TAN → NO₂⁻ → NO₃⁻ is slow without bacteria. The biofilter is that path. The black point is a typical well aerated tank (oxic) — in the nitrate field, left of NH₃. The fish can still have TAN if the filter does not keep up. A nitrite spike is getting stuck in the thin middle band.

Metastable farm diagram, 25 °C. N₂ omitted: thermodynamically huge but kinetically inert — in practice we see NH₄⁺/NH₃, NO₂⁻ and NO₃⁻. Left: Eh (SHE). Right: ORP (mV, Ag/AgCl). Vertical line: pKa ≈ 9.25. The biofilter is the kinetics between fields.

© 2026 DABCE AB

Calculator: TAN → NH₃ (unionised)

TAN (mg/L as N)

pH

Temperature (°C)

NH₃ fraction–

NH₃-N (mg/L)–

NH₄⁺-N (mg/L)–

Inside preferred region

NH₃-N vs preferred region

0 0.012 0.05 0.20 0.25

Method / source Equilibrium formula: Emerson et al. (1975), freshwater. Colour bands match the table below (salmonids): green < 0.012 mg/L NH₃-N. Not a limit value — species and time govern.

© 2026 DABCE AB

Interpreting the calculator

The calculator takes your TAN, pH and temperature and answers: how much is toxic NH₃ right now? That is a more useful question than only "how much ammonia is in the sample?". The same 1 mg/L TAN can be almost harmless in cool, near-neutral water and serious in warm, alkaline water. A common mistake is to look only at the TAN number.

Same TAN = 1.0 mg/L. NH₃-N from Emerson (freshwater), rounded. Try the same values in the calculator.

Orientation - not a limit value

Common orientation bands in freshwater salmonid literature. Warm-water species often tolerate more. Not a regulatory value — species, life stage and exposure time govern. See also the combined rainbow trout diagram under nitrite (TAN, NH₃, NO₂, NO₃).

What if NH₃ is too high?

When unionised ammonia approaches problematic levels, operators generally have a few options:

• Cut production: temporarily less feed, fewer meals or lower biomass - that lowers TAN at the source.
• Improve biological conversion: DO and alkalinity/pH in the biofilter, remove sludge, confirm nitrification capacity. A healthy filter takes TAN → nitrite → nitrate.
• Dilute: more make-up, higher flow-through. Lowers TAN quickly but costs water and often energy.
• Reduce the NH₃ fraction: in some cases a careful pH drop cuts toxicity even if TAN is unchanged. Many intensive RAS plants run a little below natural-water pH - but pH must be balanced against the biofilter and alkalinity. See pH.

TAN as a process-control parameter

In modern RAS, TAN is increasingly treated as a process-control parameter, not only as water quality. Oxygen shows whether fish can breathe; TAN shows whether treatment is keeping pace with feed and growth. Successful farms rarely look at TAN alone. They follow the whole nitrogen chain Feed → TAN → nitrite → nitrate together with pH → alkalinity → CO₂ → dissolved oxygen. Together that is a clearer picture than any single measurement.

In RAS and many semi-flow systems, nitrifiers convert TAN → NO₂⁻ → NO₃⁻. That lowers TAN but consumes oxygen and alkalinity and can give a nitrite spike at start-up or if the biofilter is knocked out. Nitrate accumulates until you exchange water or denitrify. See nitrite, nitrate, pH and dissolved oxygen.

Differences between systems

• Ponds: TAN is diluted and partly taken up by algae; daily pH drives NH₃ risk more than average TAN.
• Flow-through / raceway: flow dilutes TAN - too little water means both oxygen and ammonia risk.
• Net pen: large volume, but local TAN in dense fish and weak current; seawater pH (~8) means a higher NH₃ share.
• RAS: TAN is a process parameter. Biofilter, peak feed and pH/CO₂ matter more than dilution.

Measurement

Spot checks (kit or lab) are common. Online TAN exists but is costlier and needs care. More important than the instrument: measure where the fish live, together with pH and temperature, and after the feed peak - not only in the morning in a sump. Compare with nitrite if the biofilter is young or recently disturbed.

Signs of ammonia stress

Reduced appetite, increased gill ventilation, darker colour, surface behaviour, poorer growth and in the worst case gill damage and death. The picture resembles CO₂, low DO and nitrite - measure TAN, pH, T, NO₂⁻ and DO before changing everything at once.

Practical rule of thumb

Fish produce TAN continuously. Biofilters remove TAN continuously. The operator's job is to keep removal ahead of production. Never interpret TAN without also looking at pH and temperature. The fish care about NH₃, not TAN.

Nitrite (NO₂⁻) is the intermediate in nitrification: TAN (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). A healthy biofilter keeps nitrite low. When it rises, conversion has stalled in the middle — common at start-up, after disinfection, when the filter is short of oxygen, or when feed load has risen faster than the bacteria.

Nitrite is toxic even at low levels, especially for salmonids in soft freshwater. It impairs the blood: nitrite is taken up across the gills and converts haemoglobin to methaemoglobin, which carries oxygen poorly ("brown blood"). The fish can therefore suffocate even when DO in the water looks fine. See Ammonia / TAN and dissolved oxygen.

Why nitrite appears

Ammonia-oxidising bacteria (first step) usually establish faster than nitrite-oxidising bacteria (second step). That is why a nitrite spike often comes after TAN has started to fall — the classic "new biofilter" pattern. The same can happen if the filter is knocked out and then recovers in the wrong order.

• Start-up or restart of the biofilter.
• Too fast an increase in feed or biomass.
• Low DO or low alkalinity in the filter.
• Disinfection, medication or a sudden pH drop that hits nitrifiers.
• Dead matter or fouling that creates oxygen-poor zones.

Chloride protection

Chloride (Cl⁻) competes with nitrite at the gill surface. Higher chloride reduces nitrite uptake — that is why salt is often used in freshwater during a nitrite event. It protects the fish temporarily. It does not remove the nitrite and it does not fix the biofilter. The root cause is always that the second step of nitrification is not keeping up.

Salmonids in soft, low-chloride water are among the most sensitive. In brackish water and the sea there is already a lot of chloride, so the same nitrite is usually less dangerous — but a large spike is still a process fault. No dose here: calculate against species, measured nitrite and existing chloride/salinity. See salinity and mineral and ion management.

Measurement: NO₂⁻ or NO₂-N

Kits and labs may report nitrite as NO₂⁻ or as nitrite-nitrogen (NO₂-N). The numbers are not the same: NO₂⁻ ≈ 3.3 × NO₂-N. Only compare values in the same unit. Measure together with TAN and nitrate so you see where the chain is stuck: high TAN = first step lagging; high nitrite and low TAN = second step lagging; both high = the filter is not keeping up at all.

Orientation — not a limit value

The aim in intensive freshwater culture is in practice "as close to zero as the filter can hold". Salmonids and early stages tolerate the least. Warm-water species often tolerate more. Even moderate levels over time cut growth and stress tolerance. Use species, chloride and exposure time — not a generic number from another farm.

Nitrogen chain — levels for rainbow trout

Below are approximate orientation levels for rainbow trout (Oncorhynchus mykiss) in freshwater grow-out. All values are as nitrogen (mg/L N). The diagram uses the same logarithmic scale for every row (0.01–300 mg/L N) so concentration differences are obvious: NH₃ is in thousandths, nitrate in tens to hundreds. NH₃ depends on TAN, pH and temperature (calculator under Ammonia / TAN). Hatchery and fry often need tighter targets. Not permit values.

In short: keep NH₃-N preferably below about 0.012 mg/L (above about 0.05 risk rises clearly). TAN below about 0.5 mg/L is a common soft target when pH is moderate — but NH₃ governs toxicity. NO₂-N in soft freshwater should sit near zero (below about 0.1 mg/L as an operating target; above about 0.5 is clear risk). NO₃-N below about 50 mg/L is often good long-term; 50–100 can be acceptable on many farms; above about 200 plan more exchange, denitrification or nutrient recovery.

mg/L as nitrogen (N). Rainbow trout, freshwater, grow-out — teaching orientation.

Nitrogen — orientation levels (rainbow trout, freshwater)

mg/L N (same log scale for all)

Target / low Elevated High risk Too high Outside band

NH₃-N (toxic form)

Often the governing target

TAN (total NH₃+NH₄⁺)

pH/temp set toxicity via NH₃

NO₂-N (nitrite)

Target ≈ zero in soft freshwater

NO₃-N (nitrate)

Accumulates in low-exchange RAS

0.01 0.1 1 10 50 100 300

Same scale for all rows: logarithmic 0.01–300 mg/L as nitrogen (N). That makes the gap between NH₃ (thousandths) and NO₃ (tens–hundreds) obvious. Green = common operating target, yellow = elevated, orange = clear risk, red = unacceptable for long-term operation; grey = outside that parameter's typical band. TAN depends strongly on pH/temp. Orientation for grow-out rainbow trout — not hatchery, not a regulatory limit.

© 2026 DABCE AB

What you do when nitrite rises

• Protect the fish: cut feed load, keep DO high, consider temporary chloride protection in freshwater (no dose here).
• Fix the filter: DO and alkalinity in the biofilter, remove sludge, stop hitting the bacteria, do not raise feed until nitrite falls.
• Dilute if you have water: more make-up lowers nitrite quickly but does not fix an undersized filter.
• Follow TAN, nitrite and nitrate together until the chain is whole again.

Differences between systems

• RAS: nitrite is a process parameter. Start-up and load increase are the usual risk windows.
• Flow-through: nitrite is less common if the source is clean; it can appear with reuse or a biofilter stage.
• Ponds: algae and sediment affect nitrogen; nitrite can rise when the bottom goes low in oxygen.
• Marine: more chloride gives some protection, but a nitrite spike still means treatment is lagging.

Signs of nitrite stress

Increased gill ventilation, lethargy, gathering at the surface or inlet, darker colour, reduced appetite. Gills and blood can look brown in severe methaemoglobinaemia. The picture resembles low DO, high CO₂ and ammonia — measure NO₂⁻, TAN, DO, pH and temperature before changing everything at once.

Practical rule of thumb

TAN shows whether the first step is keeping up. Nitrite shows whether the second step is keeping up. Nitrate shows what has accumulated over time. Salt can buy time. Only the biofilter removes the problem. See Ammonia / TAN, alkalinity and nitrate.

Nitrate (NO₃⁻) is the final product of biological nitrification and is generally much less toxic to fish than ammonia (NH₃/NH₄⁺) and nitrite (NO₂⁻). In recirculating aquaculture systems (RAS), nitrate accumulates continuously as biofilters convert ammonia excreted by fish into nitrite and subsequently nitrate.

Unlike ammonia and nitrite, nitrate rarely causes acute toxicity at concentrations typically encountered in aquaculture. However, long-term exposure to elevated nitrate may reduce growth performance, impair feed conversion efficiency, increase physiological stress, and negatively affect fish health and survival. Sensitive life stages such as eggs, larvae and fry are generally more susceptible than larger fish.

Why nitrate rises in RAS

Because nitrate is the end product of aerobic nitrification, conventional biofilters do not remove it. As feed is added and metabolised, nitrate levels gradually increase unless active removal is implemented. In most RAS facilities, nitrate is controlled through water exchange (make-up). More advanced systems may incorporate dedicated denitrification units that convert nitrate into harmless nitrogen gas (N₂). Integrated aquaponics, algae cultivation and other biological nutrient recovery approaches can also contribute to nitrate removal.

Nitrate management becomes increasingly important as water exchange rates decrease. Modern low-exchange or near-zero-exchange RAS can experience substantial nitrate accumulation over time - making routine monitoring essential for fish welfare and environmental discharge requirements. See also ammonia/TAN, nitrite and the mass balance under dissolved oxygen (feed → TAN → nitrite → nitrate).

Key points

• End product of nitrification (TAN → NO₂⁻ → NO₃⁻).
• Far less toxic than ammonia and nitrite.
• Accumulates continuously in RAS unless removed.
• Cannot be removed by conventional nitrification biofilters.
• Controlled through water exchange, denitrification or nutrient recovery systems.
• Important parameter in low-exchange and intensive RAS production.
• Can be seen as a resource: plant nutrient and a link to circular production.

Typical interpretation (orientation)

Values below refer to nitrate-nitrogen (NO₃-N). Units are often mixed: NO₃⁻ (nitrate ion) is about 4.4 × NO₃-N. Only compare values in the same unit. Ranges are teaching orientation - tolerance varies with species, life stage, salinity and management objectives. Many farms set internal targets well below known toxicity thresholds.

For rainbow trout in freshwater (grow-out), the diagram and table under nitrite give a combined view of the whole chain: NH₃-N, TAN, NO₂-N and NO₃-N. On nitrate alone: below about 50 mg/L NO₃-N is often good; 50–100 acceptable for many; above about 200 consider more exchange, denitrification or nutrient recovery. See also "Nitrate as a resource" further below.

Nitrogen — orientation levels (rainbow trout, freshwater)

mg/L N (same log scale for all)

Target / low Elevated High risk Too high Outside band

NH₃-N (toxic form)

Often the governing target

TAN (total NH₃+NH₄⁺)

pH/temp set toxicity via NH₃

NO₂-N (nitrite)

Target ≈ zero in soft freshwater

NO₃-N (nitrate)

Accumulates in low-exchange RAS

0.01 0.1 1 10 50 100 300

Same scale for all rows: logarithmic 0.01–300 mg/L as nitrogen (N). That makes the gap between NH₃ (thousandths) and NO₃ (tens–hundreds) obvious. Green = common operating target, yellow = elevated, orange = clear risk, red = unacceptable for long-term operation; grey = outside that parameter's typical band. TAN depends strongly on pH/temp. Orientation for grow-out rainbow trout — not hatchery, not a regulatory limit.

© 2026 DABCE AB

Orientation, not permit values or species-specific limits. Always check against your standard and discharge requirements.

Control in practice

• Water exchange: simplest and most common - but costs water, heat/cooling and can load the receiving water with nitrate.
• Denitrification: anaerobic/anoxic step taking NO₃⁻ → N₂; needs a carbon source, control and skill.
• Nutrient recovery: plants, algae, aquaponics - take up N but need their own design and light/area.
• Measure TAN, nitrite and nitrate together so you see the whole nitrogen chain - not only "nitrate high".

Nitrate as a resource

While nitrate is often viewed as a water-quality parameter that must be controlled in RAS, it is also a valuable plant nutrient and one of the most widely used forms of nitrogen fertiliser. Rather than treating nitrate solely as a waste product, many modern aquaculture concepts view it as a resource that can be recovered and utilised.

Nitrate-rich water can support the cultivation of algae, aquatic plants, vegetables, herbs and greenhouse crops. This has led to growing interest in integrated production systems such as aquaponics, hydroponics, algae farming and nutrient recovery technologies. In these systems, nitrogen originating from fish feed is converted by the biofilter into nitrate and then reused to produce additional biomass.

Potential products include microalgae, macroalgae (seaweed), leafy greens, tomatoes, herbs, ornamental plants and other horticultural crops. By recovering nutrients instead of discharging them, integrated aquaculture systems can improve resource efficiency, reduce environmental impacts and create additional revenue streams from the same water and feed inputs.

Nitrate is therefore increasingly seen not only as a parameter to manage, but also as a link between aquaculture, agriculture, horticulture and future circular bioeconomy systems. See also circular ecosystems under future of aquaculture, sludge handling and sustainability.

In short: nitrate is a sign that nitrification is working - the nitrogen that must leave the fish loop if you do not exchange water - and at the same time a possible feedstock for plants and algae. The tighter the loop, the more deliberate nitrate management is required, whether the path is exchange, denitrification or recovery.

Turbidity describes how cloudy the water appears, while Total Suspended Solids (TSS) measures the actual concentration of particles suspended in the water. In aquaculture, these particles primarily consist of fish faeces, uneaten feed, biofilm fragments, microorganisms, and fine organic matter generated within the system.

Excessive suspended solids can negatively affect fish health and system performance. Fine particles may irritate or clog the gills, increase physiological stress, reduce growth rates, and create favourable conditions for opportunistic pathogens. Suspended particles can also reduce water clarity, interfere with fish behaviour and feeding, decrease the effectiveness of UV disinfection by shielding microorganisms from ultraviolet light, and increase the biological oxygen demand as organic matter decomposes.

Solids management in RAS

In recirculating aquaculture systems (RAS), solids management is a critical design and operational consideration. The goal is to remove particles as quickly as possible after they are produced, before they break down into dissolved wastes that are more difficult to remove. Effective solids control typically relies on self-cleaning tank hydraulics, central drains, swirl separators, radial flow settlers, drum filters, microscreens, and regular sludge removal. Well-designed hydraulics help transport particles rapidly to treatment units while preventing accumulation in tanks and pipework.

Particle size is also important. Larger particles are generally easier to remove mechanically, whereas fine particles may remain suspended for long periods and require advanced filtration, foam fractionation, or polishing treatment. As particles break apart through pumping and recirculation, removal becomes increasingly difficult, making rapid solids capture a key objective in modern RAS design. See drum filters, sludge handling and the mass balance under dissolved oxygen (TSS per kg feed).

Key points

• Turbidity measures water clarity; TSS measures suspended particle concentration.
• Main sources are faeces, uneaten feed, biofilm, and organic debris.
• High TSS can damage gills, increase stress, and reduce growth.
• Suspended particles can shield bacteria and parasites from UV treatment.
• Organic solids consume oxygen as they decompose.
• Rapid removal improves water quality and biofilter performance.
• Good tank hydraulics and drum filtration are essential for intensive RAS.

Typical interpretation (orientation)

Values below refer to TSS in mg/L. Acceptable levels vary with species, life stage, system design and production intensity. Turbidity (e.g. NTU) often correlates with TSS but is not the same measurement - only compare values in the same unit. Orientation, not a permit value.

Teaching orientation for intensive culture. Salmonids and hatcheries often aim toward the lower end. Always check against your standard and UV requirements (UVT).

Species comparison — who tolerates murky water?

Not all species are equally sensitive to solids. Salmonids (rainbow trout, salmon, Arctic char) are often "primadonnas": they want clear, oxygen-rich water and react early to gill irritation and stress. Carp are more robust in ponds. Catfish and mudfish (e.g. Clarias) cope better with murkier, organic-rich water. Shrimp sit in between — intensive tank/RAS prefer clearer water, while pond culture often runs murkier. Figures are teaching approximations for an upper preferred TSS (mg/L), not culture protocols.

Same linear thinking as the bar chart. Intensive RAS usually aims lower than ponds for the same species. Fry and hatchery tighter than grow-out.

TSS tolerance by species group (teaching orientation)

Approx. upper preferred (mg/L TSS)

Salmonids (trout, salmon, char…)

Primadonnas — want clear

12

Shrimp (intensive tank / RAS)

Better in clearer water

25

Shrimp (pond / semi-intensive)

Tolerates more murk

55

Carp

Robust pond species

70

Catfish / mudfish (e.g. Clarias)

Murky water OK

120

0 25 50 75 100 125 150

Bars show an approximate upper preferred TSS (mg/L) for “good culture water”, same linear scale 0–150. Salmonids want clear water; carp tolerate more; catfish/mudfish and many pond shrimp cope with murkier water. Intensive RAS usually aims lower than ponds. Not a species protocol or permit value.

© 2026 DABCE AB

Teaching point: design solids control for the species — a rainbow trout RAS needs fast, fine filtration; a Clarias pond can live with more murk if oxygen and gills stay under control. "Tolerates murky" does not mean ignore filters and UV when the system needs clear water.

Differences between systems

• RAS / semi-flow: solids are driven by filters and hydraulics; delayed removal means fines, lower UVT and more BOD.
• Flow-through / raceway: source turbidity + faeces; settling and screens before discharge.
• Ponds: algae, clay and sediment — turbidity can be natural but night-time oxygen risk and gill stress remain.
• Net pen: surrounding water dominates; algal blooms and sediment under the pen.

In short: remove particles early, whole and dry - before they become dissolved problems for gills, UV, oxygen and the biofilter. See drum filters, disc filters, sludge and UV.

Oxidation-Reduction Potential (ORP), sometimes called redox potential, is a measurement of the water's ability to oxidise or reduce substances. It is expressed in millivolts (mV) and is widely used in aquaculture as an indirect indicator of water cleanliness, organic loading, and the effectiveness of oxidising treatments such as ozone.

In RAS, ORP is most commonly associated with ozone systems. When ozone is added to water, it oxidises dissolved organic matter, improves water clarity, reduces colour, enhances microsolids removal, and can help lower microbial loads. As ozone dosage increases, ORP generally rises. For this reason, ORP is often used as a control parameter for automated ozone dosing systems.

While elevated ORP can indicate improved water quality, excessively high ORP values may be harmful to fish. Exposure to residual ozone or excessive oxidation can damage gill tissue, irritate skin and eyes, increase stress, and in severe cases cause mortality. For this reason, ozone treatment is normally applied in dedicated contact chambers or side-stream treatment loops, followed by degassing and residual ozone destruction before the water returns to the fish culture tanks.

ORP should not be viewed as a direct measurement of ozone concentration. Many factors influence ORP, including dissolved oxygen, pH, temperature, organic loading, salinity, and other oxidising or reducing compounds present in the water. ORP is therefore best used as a trend indicator and process control tool rather than as an absolute water quality parameter.

Modern intensive RAS facilities commonly use ORP monitoring to optimise ozone dosing while maintaining safe conditions for fish. Proper system design, residual ozone destruction, and continuous monitoring are essential when operating ozone treatment systems. See ozone systems, UV and turbidity/TSS.

Key points

• ORP measures the oxidising strength of the water.
• Commonly used to monitor and control ozone treatment.
• Higher ORP often indicates lower organic loading and clearer water.
• Excessive ORP or residual ozone can damage fish gills.
• ORP is an indirect indicator and not a direct ozone measurement.
• Ozone-treated water should be degassed and residual ozone destroyed before returning to fish tanks.
• Continuous monitoring is recommended whenever ozone is used.

Typical interpretation (orientation)

Values below are approximate mV bands often mentioned in RAS practice. Target values vary with species, system design, salinity and ozone strategy. Always interpret ORP together with residual ozone measurements (where used) and overall water quality — not alone. Orientation, not a permit value.

Reference electrode and calibration affect the reading. Only compare measurements on the same farm and with the same probe/type. Trend over time is more useful than a single number.

Design and operation

• Ozone in side-stream or contact chamber — not free in the fish tank.
• Residual ozone destruction (e.g. UV, carbon, residence time) before return to culture.
• ORP alarms and dose control with upper limits; manual override and emergency stop.
• Calibrate and maintain the ORP probe; fouling and drift give false trends.
• Combine with DO, pH, turbidity/UVT and fish observation.

In short: ORP is a useful steering wheel for ozone and a rough "cleanliness" trend — but it is not an ozone sensor and not harmless when it spikes. Design so fish never meet residual ozone.

Drum filters (microscreens) are the most common primary solids step in modern RAS and semi-flow facilities. A rotating drum with mesh (often about 40–90 µm) removes faeces, feed waste and other suspended particles as water passes outward or inward through the cloth.

When the mesh blinds, backwash starts — usually high-pressure spray nozzles and/or level or pressure control. Backwash water becomes a concentrated sludge stream that continues to settling, thickening or other sludge handling.

Why drum filters are used

• Continuous operation with automatic cleaning.
• Protects biofilters, UV and ozone from solids loading.
• Improves water clarity and oxygen/UV performance.
• Creates a manageable sludge stream instead of letting solids break down in the system.

Mesh size and sizing

Finer mesh clears water more but raises backwash volume, headloss and wash frequency. Coarser mesh lets more fines through to biofilters and UV. Choose micron rating from species, feed, downstream steps and available sludge handling — not “as fine as possible”.

• Hydraulic capacity (m³/h) at the selected mesh and solids load.
• Peak feed and expected TSS — not only averages.
• Backwash volume and where it will go.
• Redundancy: at least two units or a bypass with a planned emergency strategy.

Operation and maintenance

• Check nozzles, pressure and that the mesh is not damaged.
• Replace or repair mesh before tears pass large solids downstream.
• Monitor level/pressure alarms — a stuck filter can empty or flood sumps.
• Log wash frequency: a sudden rise may indicate overfeeding or disease.

Orientation — not a supplier selection. Capacity depends on mesh, load, temperature and water quality.

See also

• Disc filters
• Sand filters
• Sludge handling
• Turbidity / TSS
• RAS
• UV

Disc filters use stacked discs (often grooved or micro-structured) as filter media. Water passes between or through the discs while particles are retained. When the filter blinds, the disc pack is backwashed or opened so solids can be flushed away.

Disc filters are a compact alternative to drum filters for some flows and footprints — especially where floor space is limited or several smaller units fit better than one large drum.

When disc filters can fit

• Limited footprint compared with large drums.
• Modular lines where capacity is built with several units.
• Some effluent or side-stream applications.

Compared with drum filters

• Drum filters: very common in RAS; mature practice for continuous backwash.
• Disc filters: often more compact; check hydraulics, backwash and service access.
• Both need a plan for the sludge stream and emergency/bypass.

Operating points

• Track differential pressure or level — early warning of blinding.
• Maintain seals, actuators and wash sequence per supplier.
• Verify removal against downstream TSS/turbidity, not only that “the filter is running”.

See also

• Drum filters
• Sand filters
• Sludge handling
• RAS

Sand filters are classic deep-bed filtration: water passes through a bed of sand (or similar media) where particles lodge in the pores. The technology is well known in municipal and industrial water treatment; in aquaculture it is often used for polishing, effluent treatment or continuous sand filters (e.g. DynaSand class) rather than as the sole solids step in heavily loaded RAS.

Types in farming contexts

• Batch/pressure sand filters with periodic backwash.
• Continuous sand filters where sand is washed and returned during operation.
• Multimedia filters (sand + anthracite, etc.) for a wider particle range.

Strengths and limitations

• Good polishing and steady solids removal when sized correctly.
• Continuous designs can give stable operation with fewer batch spikes.
• Backwash needs water and energy; poor operation causes channelling or media loss.
• In intensive RAS, drum filters are often first — sand as polishing or on effluent.

Design and operating notes

Size on filtration rate (m/h), raw-water quality and target outlet TSS/turbidity. Monitor headloss, backwash frequency and media level. In FlowFarm-like semi-flow concepts, continuous sand filtration is often used as an integrated biostage/particle step — see flow-through.

See also

• Drum filters
• Flow-through / FlowFarm
• Turbidity / TSS
• Sludge handling
• MBBR

Bead filters use floating plastic beads as both mechanical filter media and biofilm surface. Water is pushed or pumped through the bed; particles are trapped between beads while nitrifying bacteria can grow on the surfaces.

The technology is common in smaller RAS, hatcheries and hobby/research systems where one compact unit should combine solids capture and some biological treatment. In large commercial RAS, separate drum filters plus dedicated biofilters (MBBR, fixed bed, trickling) are more often the standard.

Advantages

• Compact “two-in-one” function (solids + biofilm area).
• Relatively simple installation in smaller systems.
• Backwash/bed expansion releases sludge.

Limitations

• Capacity and hydraulics can bottleneck at high feed rates.
• Biofilm and solids in the same bed can cause oxygen-poor zones if aeration/expansion is inadequate.
• Backwash must be regular — otherwise headloss and TAN risk rise.
• Harder to optimise mechanics and biology separately than with dedicated stages.

Operating practice

• Track headloss and schedule washes before the bed locks up.
• Keep DO adequate in the bed if nitrification is expected.
• Expect large systems often to benefit from splitting solids and biological stages.

See also

• Drum filters
• MBBR
• Fixed-bed filters
• RAS
• Ammonia / TAN

A Moving Bed Biofilm Reactor (MBBR) is one of the most common biological filtration technologies used in modern aquaculture. The process consists of thousands of small plastic carrier elements suspended in a continuously mixed and aerated tank. These carriers provide a large protected surface area where beneficial bacteria form a biofilm.

The primary role of an MBBR is nitrification: ammonia (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). By continuously removing toxic ammonia and nitrite, the biofilter helps maintain a stable environment for fish and enables much higher stocking densities than would otherwise be possible. See Ammonia / TAN and nitrite.

See also

• Fixed bed biofilters
• Trickling filters
• Dissolved oxygen
• Alkalinity
• pH
• Ammonia (TAN)
• Nitrite

How it works

Water containing dissolved waste enters the reactor and flows around freely moving plastic carriers. The biofilm growing on the carriers contains nitrifying bacteria that convert ammonia into nitrite and nitrite into nitrate.

Aeration, mechanical mixing or the water flow itself (pumped circulation through the reactor) keeps the carriers in motion, ensuring:

• Uniform oxygen distribution
• Good contact between water and biofilm
• Reduced risk of clogging
• Continuous self-cleaning of excess biomass

Unlike fixed-media biofilters, MBBRs do not rely on stationary packing material and are generally less prone to channeling and fouling.

Why MBBRs are popular in aquaculture

MBBRs have become a standard component in many RAS and intensive flow-through systems because they combine simplicity with high treatment capacity. Advantages include:

• Robust nitrification
• High treatment capacity per unit volume
• Scalability from small hatcheries to large commercial farms
• Tolerance to fluctuating feed loads
• Relatively low maintenance requirements
• Good operational reliability
• Simple expansion by adding more carrier media

For these reasons, MBBRs are commonly used in facilities producing salmon, trout, char, tilapia, shrimp and many other cultured species.

Oxygen and alkalinity demand

MBBRs do not remove waste for free. Nitrification consumes oxygen and alkalinity. For every amount of ammonia converted, significant quantities of oxygen are required and alkalinity is gradually consumed. As a result, biofilter performance depends on adequate dissolved oxygen, stable pH, sufficient alkalinity, appropriate temperature and healthy bacterial populations. Poor oxygenation or low alkalinity can quickly reduce nitrification efficiency. See dissolved oxygen, alkalinity and pH.

Simple sizing

The calculator below goes from peak feed to TAN load, biofilm area needed, media volume, reactor size, oxygen and mixing air. Pick a scenario (salmon/trout, sturgeon, tilapia, clarias) to set TAN yield and conversion per m² available area — you can change the value. Set mixing air to 0 if pumped circulation moves the carriers. Not a design calculation — media, temperature, salinity and load govern real conversion.

Simple sizing (orientation)

Scenario

Salmon / trout Sturgeon Tilapia Clarias (African catfish)

Typical culture temperature about 10–16 °C. Conservative conversion per m².

Peak feed (kg/d)

TAN yield (g N/kg feed)

Conversion per m² available area (g TAN-N/m²·d)

Area per m³ media (m²/m³)

Fill fraction (%)

Mixing air (Nm³/m³·h)

TAN load–

Biofilm area needed–

Area per m³ media (m²/m³)–

Effective area per m³ reactor–

Media volume–

Reactor volume–

Rec. reactor (incl. 25% margin)–

Oxygen for nitrification–

Alkalinity (as CaCO₃)–

Mixing air–

Method / defaults The scenario sets TAN yield and conversion per m² available carrier area. Salmon/trout 35 g N/kg and 0.45 g/m²·d; sturgeon 35 / 0.60 (temperate); clarias 40 / 0.85 and tilapia 38 / 0.80 (warm). Media 500 m²/m³, 60% fill. O₂ = 4.57 g/g TAN; alkalinity ≈ 7.14 g CaCO₃/g TAN. Air 10 Nm³/m³·h is mixing — set 0 if flow moves the carriers. Not a design calculation.

© 2026 DABCE AB

MBBR start-up

A newly installed MBBR contains little or no nitrifying biomass. The biofilm must develop over time as bacteria colonise the carriers. During start-up, ammonia removal is initially limited, nitrite spikes are common, feeding may need to be restricted and water quality requires close monitoring. Biofilter maturation can take weeks to months depending on temperature, water chemistry, bacterial seeding and loading rates. See nitrite.

MBBR in RAS

In Recirculating Aquaculture Systems (RAS), the MBBR is often regarded as the biological heart of the facility. A typical RAS process train may include:

Fish tanks
↓
Solids removal
↓
Degassing
↓
MBBR
↓
Oxygenation
↓
Back to fish tanks

The order can vary (biofilter and degassing sometimes swap places). The biofilter works continuously and directly determines how much feed the system can process and therefore how much fish biomass the farm can support. Operators often monitor TAN, nitrite, nitrate, pH, alkalinity and dissolved oxygen as indicators of biofilter performance. See RAS.

MBBR vs other biofilters

Each technology has advantages, and many modern RAS facilities combine several treatment processes. Orientation, not a ranking.

Limitations

Although MBBRs are highly effective, they are not a complete water-treatment solution. They do not directly remove solids, carbon dioxide, phosphorus, colour or most dissolved organic compounds. These parameters require additional processes such as drum filters, settling systems, protein skimmers, degassing towers and ozone or UV treatment.

Practical rule of thumb

The MBBR is the engine room of biological filtration. As long as oxygen, alkalinity and carrier surface area are sufficient, an MBBR can provide highly reliable nitrification and support large fish biomasses. Most biofilter problems are caused not by the carriers themselves, but by insufficient oxygen, inadequate alkalinity, poor solids management, or attempting to feed the system faster than the bacteria can grow.

Fixed bed biofilters are biological filters that use stationary media to provide a large protected surface area for nitrifying bacteria. The media may consist of plastic modules, structured packing, gravel, rock, lava media, synthetic materials or other high-surface-area substrates that remain fixed in place while water flows through them.

Like all biological filters, their primary purpose is nitrification: ammonia (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). By converting toxic ammonia and nitrite into less toxic nitrate, fixed bed biofilters help maintain water quality and support higher fish stocking densities. See Ammonia / TAN and nitrite.

See also

• MBBR
• Trickling filters
• Dissolved oxygen
• Alkalinity
• pH
• Ammonia (TAN)
• Nitrite

How it works

Water passes through stationary filter media where biofilms containing nitrifying bacteria grow on the media surfaces. As water flows through the biofilter, ammonia is converted to nitrite and nitrite to nitrate. Oxygen and alkalinity are consumed. Because the media remains fixed, biofilms can develop large and stable bacterial populations with excellent treatment capacity. Many fixed bed systems operate under low-energy conditions and require little or no mechanical mixing — flow through the bed is often enough.

Why fixed bed biofilters are used

Fixed bed biofilters have been used in aquaculture for decades and remain popular in hatcheries, small RAS, flow-through farms, recirculation systems and public aquaria. Advantages include:

• Very high protected surface area
• Efficient nitrification
• Low energy requirements
• Simple construction
• Stable bacterial communities
• Good performance under steady operating conditions

Because the biofilm is protected within the media structure, these filters can support large bacterial populations in a relatively compact footprint.

The importance of solids removal

The biggest challenge with fixed bed biofilters is fouling. If suspended solids are not removed before the biofilter, waste particles become trapped within the media and gradually block water flow. Potential consequences include reduced hydraulic performance, channeling and dead zones, oxygen depletion within the media, reduced nitrification rates, increased maintenance and formation of anaerobic zones.

For this reason, fixed bed biofilters are normally installed after effective solids removal such as drum filters, settling systems, microscreens or hydrocyclones. Good solids management is often the difference between a highly efficient fixed bed biofilter and a poorly performing one.

Oxygen and alkalinity demand

Like all nitrifying systems, fixed bed biofilters require dissolved oxygen, alkalinity, stable pH and suitable temperature. Nitrification continuously consumes both oxygen and buffering capacity. If oxygen or alkalinity becomes limiting, ammonia removal declines, nitrite may accumulate and efficiency falls. See dissolved oxygen, alkalinity and pH.

Simple sizing

The same model as for MBBR: peak feed → TAN load → area needed → media volume → bed size. Fixed-bed defaults are a lower specific area (structured plastic, orientation) and a higher fill, with air 0 because the bed is not normally mixed. Conversion per m² is the same variable as in the MBBR calculator — fouling reduces the real area. Not a design calculation.

Simple sizing (orientation)

Scenario

Salmon / trout Sturgeon Tilapia Clarias (African catfish)

Typical culture temperature about 10–16 °C. Conservative conversion per m².

Peak feed (kg/d)

TAN yield (g N/kg feed)

Conversion per m² available area (g TAN-N/m²·d)

Area per m³ media (m²/m³)

Fill fraction (%)

Air (Nm³/m³·h, usually 0)

TAN load–

Biofilm area needed–

Area per m³ media (m²/m³)–

Effective area per m³ bed–

Media volume–

Bed volume–

Rec. bed (incl. 25% margin)–

Oxygen for nitrification–

Alkalinity (as CaCO₃)–

Air (if entered)–

Method / defaults The scenario sets TAN yield and conversion per m² available area. Salmon/trout 35 g N/kg and 0.45 g/m²·d; sturgeon 35 / 0.60; clarias 40 / 0.85; tilapia 38 / 0.80. Fixed bed: media 250 m²/m³ (structured plastic, orientation), 85% fill, air 0 (flow through the bed). O₂ = 4.57 g/g TAN; alkalinity ≈ 7.14 g CaCO₃/g TAN. Not a design calculation — fouling reduces the real area.

© 2026 DABCE AB

Fixed bed vs MBBR

Both perform the same biology. Fixed beds often achieve excellent nitrification; MBBRs are typically more forgiving when feed loads fluctuate or solids removal is imperfect.

Fixed beds in hatcheries

Fixed bed biofilters are particularly common in hatcheries where feed loads are moderate, water quality is closely controlled, stable operation is prioritised and tank biomass is relatively predictable. Many trout, salmon and char hatcheries have successfully used fixed bed biofilters for decades.

Common media types

• Structured plastic blocks and cross-flow plastic media
• Corrugated plastic sheets
• Lava rock and gravel beds
• Synthetic fibre media and modular biofilter elements

The goal is always the same: maximising surface area while maintaining effective water flow and oxygen supply. Specific area differs greatly between gravel and structured plastic — enter the right figure in the calculator.

Limitations

Fixed bed biofilters do not directly remove solids, carbon dioxide, phosphorus, colour or dissolved organic compounds. Additional treatment is typically required, including drum filtration, degassing, oxygenation, UV and sometimes ozone. Poor solids removal can eventually force shutdown and cleaning of the biofilter.

Practical rule of thumb

A fixed bed biofilter can provide extremely efficient nitrification, but only if solids are removed before the water reaches the media. Good solids removal protects the biofilter. Poor solids removal eventually turns the biofilter into a solids filter. The greatest strength is the large protected bacterial surface. The greatest weakness is fouling.

A trickling filter is a biological filter where water is distributed over a stationary bed of media while air circulates freely through the filter. As water trickles downward across the media surface, a biofilm of nitrifying bacteria removes dissolved nitrogen compounds from the water.

Like other biofilters, the primary purpose is nitrification: ammonia (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). Trickling filters are unique because they provide both biological filtration and highly effective gas exchange in the same unit. See Ammonia / TAN, carbon dioxide and dissolved oxygen.

See also

• MBBR
• Fixed bed biofilters
• Carbon dioxide (CO₂)
• Dissolved oxygen
• pH
• Ammonia (TAN)

How it works

Water is sprayed, distributed or dripped across media with a large surface area. As water flows over the biofilm, ammonia is converted into nitrite, nitrite into nitrate, oxygen is absorbed from the air and carbon dioxide (CO₂) is stripped from the water. Unlike submerged biofilters, the media is exposed to both air and water, allowing the bacteria direct access to atmospheric oxygen.

Common media include structured plastic packing, random plastic media, lava rock, synthetic biofilter modules and other high-surface-area materials. The packing must stay open enough for air to pass.

Why trickling filters are popular

Trickling filters are widely used because they combine several treatment processes into one unit. Advantages include:

• Excellent nitrification performance
• Highly efficient oxygen transfer
• Natural CO₂ removal
• Low energy requirements
• Stable biofilm development
• Low risk of carrier loss or movement
• Simple operation

In many aquaculture systems, the CO₂ removal can be almost as valuable as the nitrification itself.

Excellent carbon dioxide removal

One of the greatest strengths of a trickling filter is gas exchange. As water is broken into thin films and droplets, carbon dioxide diffuses into the air, additional oxygen enters the water and excess nitrogen gas may also be reduced. This makes trickling filters particularly attractive in RAS where fish continuously produce large amounts of carbon dioxide.

High CO₂ can reduce growth, worsen feed conversion, increase stress and reduce oxygen-uptake efficiency. A properly designed trickling filter helps address both ammonia and CO₂ at the same time. See carbon dioxide, dissolved oxygen and pH.

Solids management is critical

Like fixed-bed biofilters, trickling filters perform best when solids have already been removed. Excess solids can coat surfaces, restrict airflow, promote unwanted bacterial growth, reduce nitrification and increase maintenance. They are therefore usually located after drum filters, settling systems, microscreens or other primary solids removal.

Oxygen and alkalinity demand

Nitrification still consumes dissolved oxygen and alkalinity. Oxygen transfer is often excellent, but alkalinity consumption remains significant. Monitor TAN, nitrite, pH, alkalinity and dissolved oxygen. See alkalinity and Ammonia / TAN.

Simple sizing

The same model as for MBBR and fixed bed: peak feed → TAN → area → media volume → tower volume. Defaults are a lower specific area (open packing so air can pass) and air 0 (natural draft). The calculator sizes nitrification volume — not CO₂ stripping, hydraulic loading or tower height. Not a design calculation.

Simple sizing (orientation)

Scenario

Salmon / trout Sturgeon Tilapia Clarias (African catfish)

Typical culture temperature about 10–16 °C. Conservative conversion per m².

Peak feed (kg/d)

TAN yield (g N/kg feed)

Conversion per m² available area (g TAN-N/m²·d)

Area per m³ media (m²/m³)

Fill fraction (%)

Fan air (Nm³/m³·h, 0 = natural draft)

TAN load–

Biofilm area needed–

Area per m³ media (m²/m³)–

Effective area per m³ tower–

Media volume–

Tower volume–

Rec. tower (incl. 25% margin)–

Oxygen for nitrification–

Alkalinity (as CaCO₃)–

Fan air (if entered)–

Method / defaults The scenario sets TAN yield and conversion per m² available area. Salmon/trout 35 g N/kg and 0.45 g/m²·d; sturgeon 35 / 0.60; clarias 40 / 0.85; tilapia 38 / 0.80. Trickling: media 150 m²/m³ (open packing for air, orientation), 90% fill, air 0 (natural draft). O₂ = 4.57 g/g TAN; alkalinity ≈ 7.14 g CaCO₃/g TAN. The model sizes nitrification volume, not CO₂ stripping or tower height. Not a design calculation.

© 2026 DABCE AB

Trickling filters in RAS

In recirculating systems they are used as stand-alone biofilters, combined nitrification and degassing units, polishing stages after MBBRs or supplemental CO₂ removal. Typical trains:

Fish tanks → drum filter → trickling filter → oxygenation → fish tanks

Fish tanks → drum filter → MBBR → trickling filter → oxygenation → fish tanks

The exact configuration depends on production goals and system design. See RAS and MBBR.

Trickling filters vs MBBR

Many modern RAS farms combine both: MBBRs for high nitrification capacity and trickling filters for additional gas exchange.

Limitations

Although highly effective, trickling filters do not directly remove suspended solids, phosphorus, colour or most dissolved organic compounds. They also require sufficient height and airflow, which may increase building requirements. In cold climates, heat losses can be greater than in submerged systems because of the large air–water interface.

Practical rule of thumb

An MBBR is primarily a nitrification reactor. A trickling filter is both a nitrification reactor and a degassing system. If ammonia and carbon dioxide are both challenges, a properly designed trickling filter can address both at the same time. For many farms it is not just a biofilter, but part of the facility's gas-management strategy.

Protein skimmers, also known as foam fractionators, remove dissolved and fine particulate organic matter by generating large numbers of small air bubbles. Organic compounds attach to the bubble surfaces and are carried upward as foam, which is collected and removed from the system.

Protein skimming is widely used in marine aquaculture, public aquariums and marine RAS because seawater naturally supports efficient foam formation. In freshwater systems the process is generally much less effective, although some specialised freshwater applications exist.

See also

• Ozone systems
• UV systems
• Dissolved oxygen
• MBBR
• Drum filters
• Salinity

How it works

Many dissolved organic compounds contain both hydrophilic and hydrophobic regions and therefore accumulate at the air–water interface. When fine air bubbles are injected, dissolved organics attach to bubble surfaces, the bubbles rise, foam forms at the surface and the foam is removed as concentrated waste. The process is often called foam fractionation because organics become concentrated within the foam phase.

What protein skimmers remove

• Dissolved organic carbon (DOC)
• Proteins, amino acids and mucus
• Fine suspended solids, feed fines and fish-waste particles
• Surface-active organic compounds

By removing these materials before they break down, skimmers can reduce the load on downstream treatment.

Why protein skimmers matter

Organic waste constantly enters aquaculture systems through feed, fish excretions, mucus and microbial activity. If these organics accumulate they can increase oxygen demand (BOD), promote bacterial growth, reduce water clarity, increase biofouling and contribute to off-flavours. Protein skimmers remove a portion of these compounds before they decompose.

• Improved water clarity and reduced organic loading
• Lower biological oxygen demand
• Improved biofilter stability and reduced fouling
• Better overall water quality

Why they work best in seawater

The effectiveness of foam fractionation increases significantly with salinity. Seawater ions strengthen bubble stability and improve foam formation. As a result, marine RAS, hatcheries, public aquariums and shrimp or marine fish systems often use skimmers as standard equipment. In freshwater salmonid systems they are generally much less efficient and therefore less commonly used. See salinity.

Ozone and protein skimmers

Protein skimmers are frequently combined with ozone. Ozone can break down dissolved organics, improve water clarity, increase skimmer efficiency, reduce colour and turbidity and lower microbial loads. In marine RAS, ozone contact is often followed by a protein skimmer that removes oxidised organic material. See ozone and UV.

Placement in a RAS

Typically after solids removal and before biological filtration. Simplified trains:

Fish tanks → drum filter → protein skimmer → biofilter → oxygenation → fish tanks

Fish tanks → drum filter → ozone contact → protein skimmer → biofilter

The exact arrangement depends on system design. See drum filters and MBBR.

Limitations

Protein skimmers do not replace biological filtration. They do not directly remove ammonia (TAN), nitrite, nitrate, carbon dioxide or most dissolved salts. Those need biofilters, degassing, water exchange or denitrification. View skimmers as organic-load reduction, not a complete water-treatment solution.

The main sizing parameters

Removal is in practice governed by bubble area × contact time × the amount of organic matter. The larger the total bubble area and the longer the contact, the more organic can be removed. Do not size on system volume alone.

• Air flow (m³/h or L/min): more bubbles and more area, but too much air shortens contact, destabilises foam and costs energy.
• Bubble size: smaller bubbles give much more area at the same air volume. Microbubbles via venturi, needle wheel or ejector are common.
• Contact time: organics need time to adsorb. Set by skimmer volume, water flow and the air/water ratio.
• Air/water ratio: too little air means too little area; too much hurts separation. Air and water must be in balance.
• Water flow: more water exposes more DOC but cuts contact time. Always a trade-off between capacity and efficiency.
• DOC load: how much organic is added. Daily feed is usually a better metric than water volume.
• Salinity: far better in sea and brackish water than in freshwater. See salinity.
• Ozone: can make organics more surface-active and improve foam. See ozone.

Skimmer size ∝ feed mass × organic load — not ∝ water volume. Two farms with the same volume but different feed rates can need very different skimmers.

Pre-sizing from feed

Start from the mass load, not the tank volume. A simplified chain: feed → organic waste → solids removal → remaining foamable material → skimmer → foam concentrate.

L_skim = F × f_org × f_diss × f_avail. F is peak feed (kg/d), f_org the share leaving the fish as organic, f_diss the dissolved or very fine share after mechanical filtration, f_avail the share that is surface-active and practically foamable. The last factor is the most uncertain — not all DOC is foamable. Use the highest realistic daily feed, not the annual average, and also check the peak hour.

Hydraulic check: Qs = V × N / 24 is treatment flow from system volume and turnovers per day — a check, not the primary sizing. Contact time tc = V_effective / Qs. Air/water RA/W = Q_air / Qs. Bubble area grows roughly with air flow / bubble diameter. The calculator below takes the feed load and shows load, flow, contact time, air ratio and an indicative contact volume. Not a supplier selection.

Pre-sizing (orientation)

Water

Seawater Brackish Freshwater Ozone before the skimmer

Seawater: better foam. f_avail start 0.30 (0.38 with ozone).

Peak feed (kg/d)

Peak share of the day (–)

f_org (organic waste / feed)

f_diss (dissolved/fine after filter)

f_avail (foamable share of dissolved)

Culture-loop water volume (m³)

Desired turnovers / day

Water flow through skimmer (m³/h)

Air flow (m³/h)

Skimmer contact volume (m³)

Specific load (kg foamable / m³·d)

Safety factor

Loop volume = tanks + recycle this skimmer serves, not the skimmer body. Flow 0 = calculate loop volume × turnovers / 24.

Foamable load L_skim–

Peak load–

Foamable per kg feed–

Water flow Qs–

Turnovers–

Contact time–

Air/water RA/W–

Load per m³ contact–

Rec. contact volume (incl. SF)–

Method / limitL_skim = F × f_org × f_diss × f_avail. The factors are start values you should change — not all DOC is foamable. f_avail rises slightly with salinity and ozone. Rec. volume = L_skim × SF / specific load. Not a supplier selection and not a design calculation.

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In different aquaculture systems

• Marine RAS: very common, often paired with ozone; improves clarity and stability.
• Sea bass and sea bream: frequently used in land-based systems to reduce dissolved organics.
• Marine hatcheries: common during larval production to keep high water quality.
• Shrimp: often used in recirculating and intensive systems.
• Freshwater trout and salmon RAS: less common; benefits may not justify the extra complexity.

Practical rule of thumb

Biofilters remove dissolved nitrogen. Protein skimmers remove dissolved organics. Size first on how much organic is produced (feed load), not on how many cubic metres of water the system holds. The chain is: feed → DOC → bubble area → foam → removal. Final selection against representative water quality, salinity, feed load and supplier or pilot data.

Ozone (O₃) is one of the strongest oxidants used in aquaculture and water treatment. It is widely applied in Recirculating Aquaculture Systems (RAS), hatcheries and marine production systems to improve water quality, reduce dissolved organic matter, improve water clarity and support microbial control. Ozone is also widely used to sterilise supply and effluent water in aquaculture systems.

Unlike chlorine, ozone leaves no long-term disinfectant residual because it naturally decomposes back into oxygen. This makes it attractive for aquaculture, but it also means ozone must be generated on-site and used immediately. Ozone is highly unstable and therefore generated locally rather than stored.

When properly engineered and operated, ozone can significantly improve water quality and production stability. When poorly applied, it can damage fish, destroy biofilter performance, create toxic residual oxidants and present serious safety risks to personnel. Higher ozone concentrations can cause tissue damage, stock mortalities and risks to biofilters.

Why ozone is used

As production intensity increases, organic wastes accumulate within the system. While drum filters and biofilters remove solids and nitrogen compounds, they do not efficiently remove all dissolved organics and fine colloidal particles. Ozone is often used as an additional process step to manage these compounds. Ozone is used to remove organic carbon, turbidity, algae, colour, odour and taste, and to improve system stability.

Potential benefits include:

• Oxidation of dissolved organic matter.
• Reduction of water colour.
• Improved water clarity.
• Improved UV transmittance.
• Reduction of fine colloidal particles.
• Support of foam fractionation and protein skimming.
• Reduced microbial pressure.
• Improved overall water quality stability.

Many operators notice ozone first as “clearer water”, but the underlying benefits often extend throughout the entire treatment train.

How ozone works

Ozone is a highly reactive molecule consisting of three oxygen atoms. The third oxygen atom is weakly bound and readily reacts with other compounds. This makes ozone a powerful oxidising agent. The instability of ozone is what gives it its treatment effectiveness.

When ozone enters water it reacts with:

• Dissolved organic compounds.
• Fine particles.
• Bacteria, viruses and fungi.
• Some protozoan pathogens.
• Colour-causing and odour-causing compounds.

Many of these reactions occur very rapidly. In addition to direct oxidation, ozone may initiate secondary oxidation reactions that further improve water quality.

Typical ozone system layout

Ozone should generally not be applied directly into culture tanks. Instead, it is normally used in a dedicated treatment loop. Water is usually treated in a side-stream and returned only after ozone has reacted or been removed. Side-loop treatment and dedicated contact systems are commonly used to avoid direct fish exposure.

Typical components include:

• Oxygen concentrator or liquid oxygen supply.
• Ozone generator.
• Venturi injector or gas transfer system.
• Contact chamber.
• Protein skimmer or foam fractionator.
• Degassing unit.
• Residual ozone destruction.
• ORP monitoring.
• Safety systems and alarms.

Ozone and protein skimmers

One of the most important practical benefits of ozone is its interaction with foam fractionation systems. Ozone can alter dissolved organic compounds, improve particle aggregation, increase skimmer efficiency, improve water clarity and reduce yellowing compounds. Many marine RAS facilities combine protein skimmers and ozone treatment specifically because the two technologies complement each other.

Ozone and UV systems

Ozone and UV are often treated as competing technologies, but they are frequently complementary. Ozone can improve UV performance by reducing dissolved organics that absorb ultraviolet light. Improved UV transmittance is a recognized outcome of ozonation.

A common approach is: mechanical filtration → ozone treatment → foam fractionation → UV treatment → culture system. This sequence can improve overall treatment efficiency and biosecurity.

Ozone and water clarity

Water colour and turbidity are common challenges in intensive production systems. Over time dissolved organics accumulate and create yellow-brown water colour, lower UV penetration, reduced visibility and increased organic loading. Ozone breaks down many of these compounds and improves visual water quality. The result is often better visibility, improved UV effectiveness and reduced organic load.

Ozone and biofilters

Ozone can support biofilter performance indirectly by reducing organic loading. However, excessive ozone exposure can damage nitrifying bacteria. Potential problems include loss of nitrification capacity, reduced ammonia and nitrite conversion and biofilter instability.

For this reason, ozone is usually applied upstream of residual removal and not directly to biofilters. The goal is improved water quality without exposing biofilter bacteria to harmful residual oxidants.

Ozone in freshwater systems

Freshwater applications are generally more straightforward. Ozone primarily reacts with organic matter and microorganisms. Risks still exist, but chemistry is usually easier to manage than in marine systems. Even in freshwater systems: residual ozone must not reach fish, ORP should be monitored, ozone demand varies with water quality, and dosing should be adjusted gradually.

Ozone in seawater systems

Marine ozone treatment requires significantly greater caution. Seawater contains naturally high levels of bromide. Ozone reacts rapidly with bromide and forms secondary oxidants, often referred to as ozone-produced oxidants. These compounds may persist longer than ozone itself and can create biological risks if not properly controlled. Fish can sometimes be affected even when no measurable ozone remains.

These secondary oxidants may include:

• Hypobromous acid.
• Bromamines.
• Bromate.
• Other brominated oxidants.

Because of this chemistry, seawater ozonation is more complex, marine species may be exposed to secondary oxidants, conservative operation is essential and residual destruction is critical. Safe seawater ozone treatment depends on controlled contact time, off-gas handling, oxidant management and careful monitoring.

ORP control

ORP (Oxidation Reduction Potential) is commonly used for process control. When oxidation increases, ORP generally rises. However, ORP is not a direct ozone measurement. ORP responds to many chemical conditions within the water and should be interpreted as an indicator rather than an ozone concentration measurement.

ORP is best viewed as a process control parameter, an early warning indicator and a trend measurement — not a direct ozone sensor or a replacement for good system design. See ORP.

Safety

Ozone is highly effective but must be treated as an industrial oxidant. Ozone gas can be hazardous to personnel and should never be allowed to accumulate in enclosed spaces. High atmospheric ozone concentrations are hazardous to health.

• Ozone leak detection.
• Ventilation systems.
• Off-gas destruction.
• Emergency shutdown systems.
• Personnel training.
• Oxygen system safety.
• Alarm systems and routine maintenance.

When ozone is most valuable

Ozone often provides the greatest benefit in intensive RAS, hatcheries, smolt facilities, marine RAS, high-density production, systems with low water exchange and facilities requiring high water clarity. The technology becomes increasingly attractive as water reuse rates increase.

Practical rule of thumb

Ozone is not simply a disinfection tool. It is a water quality management technology. In most successful aquaculture applications, ozone improves system performance by reducing dissolved organics, improving water clarity, supporting foam fractionation and lowering microbial pressure. The goal is not maximum oxidation. The goal is controlled oxidation.

Fish should never be exposed to free ozone, marine systems require special attention due to bromide chemistry, and residual destruction should always be part of the treatment train. When properly designed and conservatively operated, ozone can be one of the most powerful tools available for maintaining stable water quality in intensive aquaculture systems.

Orientation — not a dosing protocol or species-specific limits. Verify design against supplier guidance, ORP practice and local water chemistry (especially bromide in seawater).

See also

• ORP
• UV
• Protein skimmers
• Drum filters
• MBBR
• Sensors
• RAS
• Biosecurity
• Turbidity / TSS

Ultraviolet (UV) disinfection is one of the most widely used water treatment technologies in aquaculture. Unlike chemical disinfectants, UV inactivates microorganisms using light rather than oxidation or chemical residuals.

Modern aquaculture facilities use UV systems to reduce pathogen pressure, protect hatcheries, improve biosecurity and support stable production. UV is commonly applied to intake water, make-up water, recirculation loops and effluent streams depending on the farm’s biosecurity objectives.

One of the biggest advantages of UV is that it leaves no chemical residual in the water. There is no chlorine, ozone or disinfectant remaining after treatment. This makes UV particularly attractive ahead of biofilters, fish tanks and sensitive life stages such as eggs, larvae and fry.

How UV works

Most aquaculture UV systems operate using UV-C light at approximately 254 nm. When microorganisms pass through the reactor, UV radiation damages DNA and RNA, preventing replication. The organism may still physically exist, but it can no longer reproduce and infect new hosts.

UV can affect bacteria, viruses, fungal spores, algae, protozoa and some parasite stages. The effectiveness depends on both the organism and the dose delivered. A UV reactor does not sterilise water in the strict sense. Instead, it provides a specific level of inactivation depending on UV dose and reactor performance.

UV dose matters more than lamp power

One of the most common mistakes is focusing on lamp wattage rather than UV dose. The important parameter is UV dose (mJ/cm²). Dose is determined by UV intensity, exposure time, reactor hydraulics, water quality and lamp condition.

A large UV unit can still deliver poor treatment if flow is too high or water quality is poor. Conversely, a properly sized UV unit can achieve excellent performance with relatively modest power consumption. When evaluating a UV system, always ask what UV dose is being delivered at the actual operating flow — not how many watts the lamp consumes.

UV dose, UV flux and biological reality

Many UV suppliers describe performance using UV intensity or UV dose at a specific location within the reactor, typically expressed as mJ/cm² — the flux of UV photons at a certain distance or point. From a disinfection perspective this is entirely reasonable because microorganisms passing through the reactor receive a specific UV exposure based on intensity and residence time.

For barrier-treatment applications such as intake water, hatcheries and effluent disinfection, delivered dose remains the primary design parameter. In these applications the objective is often to achieve a defined level of pathogen inactivation during a single pass through the UV reactor. Both mJ/cm² and related design values matter.

However, recirculating aquaculture systems (RAS) are fundamentally different. In a RAS, the same water continuously circulates through treatment equipment. Water may pass through a UV system dozens or even hundreds of times before a microorganism has sufficient time to reproduce significantly. From a biological perspective, the question is often not only “What UV dose does one pass receive?” but also: how much UV energy is deposited into the recirculating water volume over time?

For this reason, many process engineers find it useful to think in terms of UV energy input per cubic metre of system water, UV power relative to recirculating flow, total UV treatment intensity relative to system volume, or UV energy relative to feed load or biomass. These metrics do not replace traditional UV dose calculations, but they can simplify comparisons between different reactor designs and focus on the biological dose–effect in the system.

The biological reality is that in many RAS facilities, water returns to the UV system much faster than bacteria, viruses or many other microorganisms can reproduce. As a result, total system exposure over repeated passes may become just as important as single-pass dose.

Intake water versus recirculating water

A simple distinction is often useful:

• Intake and biosecurity barriers: UV dose (mJ/cm²), UVT, log-reduction targets and reactor validation are usually most important — both dose and system-energy perspectives matter.
• Recirculation: UV dose still matters, but water turnover, recirculation rate and UV energy per unit water volume become increasingly relevant; biological system response is the ultimate performance indicator.

The most useful engineering approach is often to understand both perspectives. A reactor may have excellent single-pass validation data, yet provide limited overall impact if only a small fraction of system water reaches it. Conversely, a modest reactor treating a very large fraction of recirculating flow may provide substantial biological benefits at the system level.

UV transmittance (UVT)

Water quality has a major influence on UV performance. Before UV light can inactivate microorganisms, it must reach them. UVT describes how much UV light passes through the water. Low UVT may be caused by dissolved organic matter, water colour, fine particles, turbidity, algal cells, iron or manganese. Poor UVT can dramatically reduce delivered dose. Two systems with identical UV equipment may achieve completely different results if one has poor water clarity.

Mechanical filtration comes first

UV works best on clean water. Particles can shield microorganisms from UV radiation. This means bacteria or parasites embedded within particles may survive treatment even when UV dose appears adequate. For this reason, UV systems are typically installed downstream of drum filters, disc filters, microscreens, clarifiers, foam fractionators and ozone systems. The cleaner the water entering the UV reactor, the more effective the UV system becomes.

Common applications

Intake water treatment

Many hatcheries treat all incoming water with UV before it enters production. Benefits include reduced pathogen introduction, improved biosecurity and more predictable production conditions. This is one of the most common UV applications worldwide.

Hatcheries

Eggs, larvae and fry are highly susceptible to disease. UV treatment is therefore widely used in egg incubation systems, fry systems, broodstock facilities and live feed production. Many hatcheries consider UV one of their primary disease-prevention barriers.

Recirculating aquaculture systems (RAS)

In RAS facilities UV may be used to reduce microbial load, improve water hygiene, protect sensitive production stages and support overall biosecurity. Because UV leaves no residual, water can immediately return to the fish after treatment.

Effluent treatment

Some facilities apply UV treatment before water discharge — for pathogen control, biosecurity, environmental protection and regulatory compliance.

UV and ozone

UV and ozone are frequently used together. Rather than competing technologies, they often complement each other. Ozone can reduce colour, improve UVT and remove dissolved organics. UV can then provide microbial control, inactivate pathogens and act as a final barrier before culture water returns to fish. Many high-performance RAS facilities employ both technologies as part of a multi-barrier approach.

UV and advanced oxidation processes (AOP)

UV can also operate as part of Advanced Oxidation Processes (AOP). Examples include UV + ozone, UV + hydrogen peroxide, and UV + ozone + peroxide. These systems create highly reactive hydroxyl radicals. Potential applications include dissolved organic removal, advanced water polishing, reduction of refractory compounds and improved water reuse. AOP systems are considerably more complex than standard UV disinfection and usually require specialist design and operation.

Quartz sleeves and fouling

Most UV systems contain a quartz sleeve that separates the lamp from the water. Light must pass through this sleeve to reach the water. Fouling may occur from biofilms, scale, calcium deposits, iron deposits and organic matter. Even small amounts of fouling can significantly reduce UV performance. Modern units often use automatic wipers, chemical cleaning and UV intensity monitoring. Regular inspection remains essential.

Lamp aging

UV lamps degrade over time. A lamp that still lights up may no longer produce sufficient UV output. This is why commercial UV systems are usually maintained based on operating hours, UV intensity measurements and manufacturer recommendations — not simply on whether the lamp still appears functional. Many facilities lose significant UV performance because aging lamps are not replaced on schedule.

Monitoring and validation

Modern UV systems increasingly include UV intensity sensors, UVT monitoring, flow monitoring, alarm systems, data logging and SCADA integration. Monitoring allows operators to verify that the required dose is actually being delivered. Without monitoring, operators often assume performance rather than measure it.

Strengths of UV

• No chemical residual.
• Immediate treatment.
• Relatively low operating complexity.
• Suitable for fish tanks and biofilters.
• Effective against many pathogens.
• Compact footprint and easy automation.
• Low chemical handling requirements.

For many farms, UV represents one of the simplest and most reliable biosecurity technologies available.

Limitations of UV

• No residual protection downstream.
• Reduced effectiveness in turbid water.
• Particle shielding effects.
• Lamp aging and quartz sleeve fouling.
• Species-specific / organism-specific dose requirements.

Some viruses, protozoa and parasite stages may require significantly higher doses than routine bacterial control. For this reason, UV should be viewed as one barrier within a broader biosecurity strategy rather than a complete disease-prevention solution.

Practical rule of thumb

The effectiveness of a UV system depends far more on delivered dose than installed wattage. Clean water, good UV transmittance, proper hydraulic design, functioning lamps and regular maintenance are what determine performance.

UV suppliers often sell reactors based on delivered dose. Aquaculture operators ultimately care about biological results. For intake-water disinfection, validated UV dose is usually the key design parameter. For recirculating systems, it is often equally important to consider how much UV energy is continuously being applied to the total water volume and microbial population within the system. The reactor treats water once. The system treats the water hundreds of times. Understanding the difference helps explain why two facilities with similar UV reactors may experience very different biological outcomes.

In practice, the most successful aquaculture facilities treat UV as a process system rather than a lamp. Filter first, measure UVT, size for the required dose with a safety margin, monitor performance continuously and maintain lamps, quartz sleeves and sensors on schedule. When properly designed and operated, UV remains one of the most effective and widely used biosecurity barriers in modern aquaculture.

Orientation — not a validated log-reduction protocol or supplier selection. Verify dose, UVT and flow against current reactor data and target organisms.

See also

• Ozone
• Turbidity / TSS
• Drum filters
• Protein skimmers
• Biosecurity
• RAS
• Egg disinfection
• Fry and larval tanks
• Sensors
• SCADA

Oxygen cones (e.g. Speece cone / oxygen cone) are pressurized contactors where water and pure oxygen meet under elevated pressure. That raises oxygen solubility and gives high transfer efficiency — common when pure O₂ is used in RAS, raceways and intensive land-based systems.

When cones are used

• High oxygen demand and limited space for aeration.
• Pure-oxygen strategies (generator, tank or cylinder).
• Need for precise DO boosts in side-stream or main flow.

Design points

• Pressure, residence time and gas/water ratio govern transfer.
• Too much pressure/gas can cause gas supersaturation — see TGP/supersaturation.
• Degassing and pressure reduction after the cone must be controlled.
• Often combined with LHO, diffusers or tank dosing depending on hydraulics.

Operation

Monitor DO before/after, oxygen pressure and leaks. Calibrate DO sensors at critical points. The cone does not replace good hydraulics or a sound overall oxygen strategy (air first, pure O₂ as additive where it pays).

See also

• LHO
• Aeration
• Oxygen supply
• Dissolved oxygen
• Gas supersaturation
• Degassers

An LHO (Low Head Oxygenator) oxygenates water at low hydraulic head. Water is distributed over chambers or packing where oxygen (or oxygen-enriched gas) contacts the water without needing the high pump pressures of a deep oxygen cone. That makes LHOs energy-efficient in many RAS designs.

Advantages

• Low head → lower pumping energy than a high-pressure cone.
• Good oxygen transfer with pure O₂ when designed correctly.
• Can be built into process-floor hydraulics.

Limitations and risks

• Needs even water distribution across chambers.
• Wrong gas balance can cause N₂ supersaturation or poor O₂ utilisation.
• Packing/chambers must be kept clean of biofilm and solids.
• Not always right for very high ΔDO — sometimes a cone or complementary dosing is needed.

In the oxygen strategy

Many facilities first maximise air-side uptake in degassers/trickling, then use an LHO or cone for the final O₂ boost. See degassers, oxygen cones and oxygen demand.

See also

• Oxygen cones
• Degassers
• Aeration
• Oxygen demand
• Dissolved oxygen
• Gas supersaturation

Degassers (CO₂ strippers) remove carbon dioxide and can simultaneously increase air-side oxygen uptake. Common designs are cascades, packed columns and other air/water contactors where water is broken into films and droplets while air flows counter- or co-currently.

In RAS, fish continuously produce CO₂. High levels reduce growth and welfare long before DO becomes critical. Degassing is therefore often as important as pure oxygen dosing.

What a good degasser provides

• CO₂ stripping to air.
• Increased DO from air (cheap “free” oxygen uptake).
• Lower nitrogen-gas risk when design is sound.
• Basis for the strategy: air first, pure O₂ as additive.

Design notes

• Air flow, packing height and water loading (m³/m²·h) govern performance.
• Too little air → residual CO₂; too much water per area → short contact time.
• Trickling filters can combine nitrification and stripping — see trickling.
• Place stripping logically in the loop (after fish / before return) depending on hydraulics.

See also

• Carbon dioxide
• Trickling filters
• Aeration
• LHO
• Dissolved oxygen
• RAS

Aeration supplies air (~21% O₂) to water via diffusers, surface aerators, blowers, venturis or cascades. It is the most common and usually cheapest way to raise DO — but gas transfer per flow is lower than with pure oxygen, so intensive systems need more turnover or complementary O₂.

Common techniques

• Bottom/pipe diffusers with blowers.
• Surface aerators and paddlewheels (common in ponds).
• Venturis and injectors in side-stream.
• Cascades and drops that combine aeration and some stripping.

When aeration is enough — and when it is not

• Ponds, raceways and low–medium intensity: often aeration + good hydraulics.
• High density / RAS: aeration + degasser first; pure O₂ for peak demand.
• Night-time algal respiration in ponds: aeration is life insurance.

Operation and energy economy

Blowers and surface aerators can dominate the power bill. Maintain diffusers (fouling cuts transfer), avoid unnecessary over-aeration, and control against DO sensors with sensible deadbands. Always compare lifecycle cost with pure oxygen — see CAPEX/OPEX and oxygen supply.

See also

• Dissolved oxygen
• Degassers
• Oxygen cones
• LHO
• Oxygen supply
• Ponds
• CAPEX & OPEX

Heat pumps move heat from a source (effluent, raw water, hall air, ground or waste heat) into culture water or the building. In cold climates, heating can be one of the largest OPEX items in land-based farming — so heat recovery is often critical for energy economics.

Typical aquaculture applications

• Heat recovery from effluent into intake/make-up.
• Tempering of RAS halls and process water.
• Coupling to industrial waste heat or district heating where available.

Why it pays to think early

• Power price and heat demand drive profitability more than “installed kW”.
• Good insulation and hydraulics reduce the heat duty required.
• Size for both winter peak and summer conditions.
• Maintain filters, heat exchangers and refrigerant per supplier.

Heat pumps raise CAPEX but can cut OPEX sharply over the facility lifetime — a classic CAPEX/OPEX trade-off. See temperature, CAPEX & OPEX and sustainability.

See also

• Chillers
• Temperature
• CAPEX & OPEX
• RAS
• Sustainability

Chillers remove heat from culture water or hall air when temperature would otherwise rise above the species comfort zone. Demand is high in warm climates, in tightly insulated RAS halls with high internal heat load, and for cold-preferring species such as Arctic char.

When cooling is needed

• Summer peaks in land-based systems.
• High pump power, lighting and biofilter aeration that heat the water.
• Species with a narrow temperature window (e.g. Arctic char, some salmon production).
• Hatcheries and fry where stable cool temperature is required.

Design and operating notes

• Calculate heat load (fish, pumps, air, building) — do not just guess “one chiller”.
• Consider free cooling, groundwater or seasonal storage before full mechanical cooling.
• Redundancy: chiller failure on a hot day can become an acute welfare risk.
• Maintain condensers and heat exchangers — fouling cuts efficiency quickly.

Cooling is energy-intensive. Minimise internal heat (pump efficiency, insulation, ventilation) before sizing chillers. See heat pumps, temperature and energy under CAPEX/OPEX.

See also

• Heat pumps
• Temperature
• Arctic char
• CAPEX & OPEX
• RAS
• Backup power

Monitoring is the basis for safe farming around the clock - whether the site is an earthen pond, raceway, net pen or full RAS. Without knowing what is happening in the water, you only react when the fish already show stress. With the right measurements, problems can be caught early: falling oxygen, a sudden temperature swing, a stopped pump, a full settler or a failed intake.

What always has to be measured

Whatever the production system, two parameters are practically indispensable: dissolved oxygen (DO) and temperature. Oxygen drives respiration, growth and survival - and changes quickly with feed, density, algal blooms, ice, pump failure or heat. Temperature drives metabolism, oxygen demand, immunity and the species comfort zone (e.g. Arctic char vs tilapia). Without some form of measurement of these, you are flying blind.

How you measure differs a lot. On a simple pond farm, a handheld DO/temperature probe a few times a day, plus eyes and experience, may be enough. On a commercial RAS you expect continuous sensors, PLC/SCADA, trends, SMS/email alarms and often redundant measuring points. In between - raceways, semi-flow-through, net pens - sites often combine spot checks with fixed sensors at critical points.

Differences between production systems

Ponds (mud / earthen ponds)

Large volume, natural primary production and strong day-night swings (algal photosynthesis raises DO by day; night risk of low DO). Measurement often focuses on DO and temperature at surface and depth, Secchi depth, and sometimes pH/alkalinity. Continuous sensors become more common at higher density, but many farms still rely on handheld checks, night watches and aeration started manually or via simple DO switches.

Flow-through and raceways

Water quality is driven strongly by intake flow and source condition. It is important to track temperature and DO on inlet and outlet water, plus flow (too little flow means oxygen risk and ammonia risk). Handheld checks can suffice on small sites; larger raceways and hatcheries often have fixed DO/temp sensors, flow switches and alarms on pump or intake failure. Solids and turbidity from the source may need turbidity or simple optical checks.

Net pens / cages

Fish live in the surrounding water body - measurement is about understanding the environment around the pen, not a closed loop. Typically: temperature profiles, DO (especially in summer stratification, algal blooms or ice), sometimes current, salinity and weather. On modern marine sites, buoys, underwater sensors and feed/camera monitoring are common; simpler lake cages may still use daily handheld checks and observation. Alarms for low oxygen or storms are more "environmental warning" than "pump trip".

RAS and semi-flow-through

Here monitoring is a process-safety issue. The system is tightly coupled: pump failure, biofilter trouble, rising CO2 or low oxygen can escalate in minutes. Standard is continuous sensors for DO, temperature, pH, often ORP/redox, level, flow and pressure; plus 24/7 alarms to staff. SCADA/PLC controls oxygen, valves and emergency modes. Spot checks (TAN, nitrite, nitrate, alkalinity, CO2) complement - some online, many still lab or handheld kits. The higher the reuse, the more critical the measurement becomes.

From handheld checks to SCADA

• Handheld probe / kit - cheap and flexible, but depends on routine and night staffing.
• Simple loggers and alarms (DO/temp) - continuous trend without full automation.
• Fixed sensors + PLC - automatic oxygen control, pump trips, level protection.
• SCADA/HMI with SMS/email/app - overview, history, remote alarms, multi-user.
• Integration with feed, cameras and AI - predictive alarms and optimisation (see AI section).

Typical measuring points on more advanced farms: DO and temperature in tanks and critical return lines; pH in the biofilter loop; flow and level in sumps; alarms on power, emergency power and oxygen reserve. Calibration, sensor placement (dead zones vs representative mixing) and spare sensors matter as much as "having a sensor".

The potential ahead

Better monitoring is not only fewer emergency trips - it enables higher density with maintained welfare, lower feed waste, faster troubleshooting and documentation for permits and customers. Data from DO, temp, flow and feed can link to growth models, energy control and early disease warning. Remote monitoring cuts the need for constant presence on distant pens or inland sites, while responsibility still requires someone who can act when the alarm sounds.

The potential is greatest where risk is fast and value high: RAS, hatcheries, high density and species with a narrow temperature window. But pond and cage farms also gain by moving from handheld-only checks to at least continuous DO/temp with alarms. The next steps - digital twins, AI feed control and automatic oxygen/flow optimisation - always rest on the same base: reliable measurements, especially oxygen and temperature.

TyrFlow (DABCE™)

DABCE TyrFlow™ is a triple-redundant sensing system with edge computing - developed among other uses for aquaculture, where reliable measurement and fast local processing are critical. Three independent measuring paths and computation near the sensors give more robust data, fewer false alarms and continued function even when cloud connectivity is limited. Contact DABCE for more on TyrFlow in farming, RAS and other demanding water applications.

Aquaculture facilities are highly vulnerable to power outages — whether hatchery, RAS, raceway, pond with aeration, net pen with feed/cameras or shrimp farm. Even short interruptions can lead to oxygen depletion, loss of circulation, reduced filtration performance, temperature instability, and significant stock losses. Reliable backup systems are therefore considered essential infrastructure in all intensive aquaculture — not merely an operational convenience.

A typical backup strategy uses multiple layers of protection. Design is based on the acceptable time-to-failure of critical life-support functions: aeration/oxygen, circulation pumps, biofilters, monitoring and communication are usually prioritised. Required backup duration may range from a few minutes in grid-stable areas to several days in remote locations.

UPS — uninterruptible power

Uninterruptible Power Supplies (UPS) provide immediate short-term power for PLCs, sensors, SCADA systems, communication equipment, alarms, and critical controls while larger backup systems start. Without a UPS the control system dies the instant the grid fails — even if the generator starts within a minute.

Generators

Diesel or natural gas generators are the most common solution for extended outages. Generators are typically sized to operate life-support systems such as pumps, blowers, oxygen generators, feeding equipment, and control systems. Automatic Transfer Switches (ATS) detect outages and automatically transfer loads from the utility grid to backup generators or batteries.

Battery energy storage (BESS)

Battery Energy Storage Systems (BESS) are becoming increasingly common. Lithium-ion battery banks can bridge generator startup, support critical loads for several hours, and integrate with renewable energy systems. Some facilities use batteries as the primary backup with generators as secondary support. Solar PV combined with batteries is increasingly used for remote and off-grid facilities — usually still supplemented by generators for long-duration reliability.

Emergency oxygen — often more important than power

Emergency oxygen reserves are often considered more important than backup power itself. Liquid oxygen (LOX) tanks, compressed oxygen cylinders, or oxygen cones supplied from reserve storage can maintain fish or shrimp survival during prolonged power failures even if full water circulation cannot be maintained. Size oxygen against peak feed/biomass and worst-case DO demand — see dissolved oxygen and oxygen demand.

Redundancy, alarms and prioritisation

• Redundant pumps and blowers (N+1 or N+2) reduce single points of failure.
• ATS + automatic start of generator/battery.
• Remote alarms (SMS, app, cloud) on power loss, low DO, pump failure, etc.
• Prioritise: oxygen/aeration → circulation → controls/alarms → other loads.
• Test regularly: generator, ATS, UPS, alarms and oxygen reserve — untested backup is a risk.

For intensive aquaculture, backup power and emergency oxygen are risk management: they protect animal welfare, production assets and business continuity. See RAS, hatchery, monitoring systems, SCADA and oxygen.

Egg incubators are used to provide stable and controlled conditions during the sensitive early life stages of fish. Their purpose is to maximize hatching success, reduce disease risk, and ensure uniform development from fertilized eggs to first-feeding larvae or fry.

Successful incubation depends on maintaining the correct water temperature, dissolved oxygen level, flow rate, and water quality for the species being cultured. Most incubators are kept in low light or darkness, as fish eggs naturally develop in shaded environments and excessive light can increase stress and reduce hatch quality in some species.

Flow and water quality

Water flow serves several functions. It delivers oxygen, removes carbon dioxide and metabolic waste products, prevents stagnant zones, and helps keep eggs clean. Flow must be strong enough to maintain good water quality but gentle enough to avoid damaging delicate eggs.

Dead or unfertilized eggs should be removed regularly. These eggs are often quickly colonized by fungi such as Saprolegnia, which can spread to healthy eggs and cause significant losses. Modern hatcheries may use manual inspection, siphoning, automated sorting systems, or image-based monitoring to identify and remove non-viable eggs. See also egg disinfection and biosecurity.

Common incubator designs

Different incubator designs are used depending on the species:

• Vertical tray incubators - common for salmonids (Atlantic salmon, rainbow trout, Arctic char, brown trout). See Heath trays.
• Upwelling jars (McDonald jars) - widely used for species with small pelagic eggs or eggs that benefit from gentle suspension in the water column.
• Heath tray incubators - simple, proven and common in many hatcheries; own section under hatchery.
• Specialized systems - marine fish, sturgeon, carp, tilapia, shrimp and others with their own flow, salinity and density needs.

Temperature and development

Temperature has a major influence on development speed. Warmer water generally accelerates embryonic development, while colder water slows it down. However, temperatures outside the species-specific optimum range may increase deformities, mortality, or poor larval quality. For this reason, many hatcheries use temperature-controlled water systems to maintain precise conditions throughout incubation. See temperature and dissolved oxygen.

Monitoring in modern hatcheries

Modern commercial hatcheries increasingly monitor egg survival, hatch rate, temperature, dissolved oxygen, pH, flow rate, and developmental progress using digital sensors and automated alarm systems. Early detection of problems can significantly improve productivity and biosecurity. Hatcheries are also extremely sensitive to power loss - see backup power systems.

Good egg incubation practices often determine the performance of the entire production cycle. High-quality eggs, stable environmental conditions, careful handling, and effective disease prevention are among the most important factors for achieving strong fry and fingerlings.

Key parameters to monitor

• Water temperature
• Dissolved oxygen (DO)
• Water flow rate
• pH and alkalinity
• Ammonia and nitrite
• Egg mortality rate and hatch percentage
• Biosecurity and fungal growth

Typical species using hatchery incubators

• Atlantic salmon (Salmo salar)
• Rainbow trout (Oncorhynchus mykiss)
• Arctic char (Salvelinus alpinus)
• Brown trout (Salmo trutta)
• Sturgeon (Acipenser spp.)
• Common carp (Cyprinus carpio)
• Tilapia (Oreochromis spp.)
• Sea bass (Dicentrarchus labrax)
• Sea bream (Sparus aurata)
• Various shrimp and other marine fish species

Sizing (orientation)

Incubator sizing is usually based on annual fry production, egg-to-fry survival, and how long eggs stay in the incubator. A simple chain:

• Eggs per year: N_eggs = N_fry / S (S = survival as a decimal).
• Incubation time: days = degree-days (°D) / temperature (°C).
• Simultaneous inventory (steady year-round production): N_inv ≈ N_eggs × (days / 365).
• Volume: V = N_inv / (eggs per litre); design volume often 1.2–1.5 × V.
• Flow (salmonid rule of thumb): Q ≈ 0.02–0.05 L/min per 1000 eggs × N_inv.

Typical egg incubation characteristics by species

The table gives approximate orders of magnitude for egg size, packing density, degree-days to hatch and typical incubation temperature. Strain, broodstock status, salinity and hatchery practice cause large differences — use as orientation, not a protocol.

* Shrimp are often incubated/larval-reared at much higher density and in different units than salmonid eggs — do not compare directly with salmonid eggs/L. Teaching ranges; strain and hatchery practice govern.

Typical commercial scale

Orders of magnitude for hatchery batches. A “batch” can mean a hatch window or a production round — not a global standard.

Scale varies enormously with company, market and species. Use as order-of-magnitude in early sizing.

The calculator below uses mid-range values from the table for common presets (salmon, trout, Arctic char, tilapia, carp). Adjust eggs/L, °D and temperature to your protocol.

Calculator: egg incubator sizing (orientation)

Species / preset Arctic char Atlantic salmon Rainbow trout Tilapia Carp Custom

Target fry / year

Survival egg → fry (%)

Eggs per litre

Degree-days to hatch (°D)

Water temperature (°C)

Flow (L/min per 1000 eggs)

Spare factor (1.2–1.5 typical)

Incubator type

Incubator type Auto (recommended) Stacked trays (Heath / vertical tray) Upwelling jar (McDonald jar) Trough / bulk tray

Trays per stack

Egg volume per tray (L)

Volume per unit (L)

Height per unit (m)

Eggs to incubate / year—

Incubation duration—

Eggs present at once—

Incubation volume—

Design volume (with spare)—

Water flow (total)—

Recommended type—

Number of units (stacks / jars / troughs)—

Total trays—

Typical height per unit—

Layout (orientation)—

Method / defaults N_eggs = N_fry / S. Simultaneous inventory ≈ N_eggs × (days/365). V = inventory / (eggs per L); design = V × spare. Flow Q = q × N_inv / 1000. Units: stacks = design volume / (trays × L/tray); jars/troughs = design volume / L per unit (rounded up). Type data (orientation): Heath stack 8 trays × ~2 L ≈ 16 L/stack, height ~1.6–2.0 m; McDonald jar ~6–16 L, height ~0.5–0.8 m; trough ~20–100 L. Auto: salmonids → stack; tilapia/carp or high eggs/L → jar/trough. Orientation — not a hatchery project.

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Example (worked illustration, Arctic char preset): 1,000,000 fry/year, 80% survival → 1,250,000 eggs/year. At 6 °C and 550 °D → about 92 days; simultaneous inventory ≈ 314,000 eggs; at 10,000 eggs/L ≈ 31 L; with 1.5× spare ≈ 47 L active incubation volume across trays or units. Try the same numbers in the calculator. Oxygen-based flow sizing (Q = M_O₂ / ΔDO) can become limiting near hatch — see dissolved oxygen.

Levelling biology over the year

The calculator above effectively assumes steady year-round production: eggs are incubated more or less continuously so inventory and tank capacity can be sized on “eggs at once”. In reality salmonids in general are driven by seasonal breeding cycles — rainbow trout, Atlantic salmon, Arctic char, brown trout and others — as long as the broodstock environment is not controlled with light and temperature. Wild stocks and farmed broodfish under natural light/temp spawn in a limited window. Without deliberate planning the hatchery gets a large egg pulse over a few weeks and then long periods with empty trays. That loads incubators, staff, fry tanks and later grow-out unevenly — and means the “tonnes per year” in a spreadsheet do not match the actual biology.

If you want to level the flow of fry and food fish over the year, there are essentially two strategic paths (often combined):

1. Freeze / bank eggs

Milt (sperm) from salmonids can be cryopreserved (frozen) with established technique and thawed when needed. Unfertilised eggs are far more sensitive: egg banking by deep-freezing ova is not standard to the same extent as for milt, although research and specialised methods continue. In practice, “freezing the biology” therefore often means: store frozen milt, and/or hold fertilised eggs in controlled, slow development (cold water, planned °D budget) so hatch is spread — more “tempered delay” than a freezer. Some hatcheries also buy eyed eggs from several suppliers or seasons to fill gaps. The same logic applies to rainbow trout, salmon and Arctic char: without light-/temperature-controlled broodstock (or bought eggs from several windows), egg supply clumps — planning egg supply and incubation temperature is then critical if you want to avoid a single annual peak.

• Advantages: partly decouples the broodstock week from when fry are needed; frozen milt gives breeding flexibility and backup.
• Limits: ova are harder to store long-term than milt; quality, logistics and rules around brood material; not a “freezer = level production” shortcut.

2. Manipulate broodstock cycles

The second path is to control when broodstock spawn. Salmonids — rainbow trout, Atlantic salmon, Arctic char and others — respond strongly to light (photoperiod) and temperature. Under natural light and seasonal temperature, spawning is therefore seasonal. With controlled light regimes — and often temperature — one can shift, compress or spread spawning: e.g. several brood groups out of phase, out-of-season eggs, or more than one spawn season per year in closed systems. That is a standard tool in many commercial salmonid hatcheries, not only for one species. In land-based farming (RAS/semi-flow) the reason is especially clear: you want to fill tanks evenly year-round to use CAPEX, labour and market capacity.

• Photoperiod: longer/shorter day length drives maturation and spawning in many salmonids — needs dark halls, timers and discipline.
• Temperature: stable cold for Arctic char; wrong temperature stresses broodfish and egg quality.
• Several brood groups: different light cycles → staggered spawns → more continuous egg supply.
• Cost: more broodstock space, power for light/cooling, skill and strict biosecurity between groups.

In practice many commercial salmonid hatcheries combine both: controlled broodstock to get eggs when needed, frozen milt as backup, and incubation at a chosen temperature to fine-tune hatch date within the °D window. The sizing calculation (steady inventory) then becomes a target to approach — not a law of nature. Without egg banking or cycle control you must instead size incubators, fry tanks and staff for the peak season and accept lower utilisation the rest of the year.

Heath trays, also known as vertical incubators, are among the most widely used egg incubation systems in salmonid hatcheries. Multiple trays are stacked vertically, allowing large numbers of eggs to be incubated within a relatively small floor area. Water typically enters at the top of the stack and flows downward through each tray, providing oxygen and carrying away waste products.

The design offers excellent space efficiency and is particularly suited for species with relatively large demersal eggs, such as Atlantic salmon, rainbow trout, Arctic char, brown trout, and Pacific salmon species. Individual trays can be removed for inspection, grading, disinfection, or egg picking without disturbing the entire incubation system.

Typical loading densities range from several thousand to tens of thousands of eggs per tray, depending on egg size and hatchery practices. A single Heath stack may hold hundreds of thousands of eggs, making the system highly scalable for both small hatcheries and commercial facilities producing millions of fry annually.

Advantages and requirements

Advantages include high egg density, simple construction, easy inspection, good water distribution, and proven performance over decades of commercial use. However, careful management of flow distribution and routine removal of dead eggs are important to prevent fungal outbreaks and maintain high survival rates. See also egg incubators, egg disinfection and biosecurity.

Typical species

• Atlantic salmon (Salmo salar)
• Rainbow trout (Oncorhynchus mykiss)
• Arctic char (Salvelinus alpinus)
• Brown trout (Salmo trutta)
• Chinook salmon (Oncorhynchus tshawytscha)
• Coho salmon (Oncorhynchus kisutch)

Typical design parameters

Teaching ranges. Flow, load and temperature depend on species, egg size and hatchery standard — not fixed rules.

Simple tray-count sizing

Number of trays ≈ (number of eggs / eggs per tray) × safety factor, where the safety factor is often about 1.1–1.5 (spare, uneven loading, picking). Number of stacks ≈ trays / trays per stack (e.g. 8).

Example: 1,000,000 Arctic char eggs, 25,000 eggs/tray, safety factor 1.25 → trays = 1,000,000 / 25,000 × 1.25 = 50 trays. With 8 trays/stack ≈ 7 stacks (rounded up). The same logic is used in the calculator under Egg incubators (choose stacked trays).

• Check flow per tray so DO is held and dead zones are avoided.
• Pick dead eggs routinely; fungus spreads quickly in dense trays.
• Darkness/low light, stable temperature and biosecurity between stacks.

Fry and larval tanks are used after hatching and yolk sac absorption, when young fish begin exogenous feeding and become fully dependent on environmental conditions and nutrition. This stage is often one of the most sensitive in aquaculture, as small changes in water quality, feeding, light, or stocking density can strongly affect survival, growth, and later production performance.

Larval tanks are typically shallow and designed with gentle hydraulics to minimize stress and ensure that weak-swimming larvae can easily access feed. Water flow must provide sufficient oxygen and waste removal without creating excessive currents that force larvae to expend energy or become trapped against screens.

First feeding

The transition to first feeding is a critical developmental milestone. Larvae must locate and consume feed before yolk reserves are exhausted. In many marine species, live feeds such as rotifers and Artemia are used during the earliest stages before gradual introduction of formulated feeds. Freshwater species such as salmonids typically transition directly to commercial starter diets after yolk sac absorption.

Light, density and grading

Light intensity and photoperiod are important design and operational considerations. Many species require specific light regimes to locate feed, regulate behaviour, and support growth. Excessive light can increase stress, while insufficient illumination may reduce feeding efficiency. Hatcheries often use programmable lighting systems. Stocking density must be carefully controlled. High densities can improve space utilization but may increase competition, stress, oxygen consumption, disease transmission, and size variation. Frequent grading is often required to reduce cannibalism in species such as perch, pike-perch, barramundi, and many marine fish.

Modern fry-rearing systems increasingly incorporate automated feeding, oxygen control, image-based monitoring, and environmental sensors for more consistent results. See monitoring systems, dissolved oxygen and feed.

Typical species

• Atlantic salmon, rainbow trout, Arctic char
• Tilapia, common carp
• Pike-perch, European perch
• Sea bass, sea bream, Atlantic cod, halibut
• Shrimp (e.g. Litopenaeus vannamei) — see differences vs fish below

Typical design parameters (fish)

Teaching ranges. Fry and larvae are sensitive — aim toward the safer side of the ranges. Not a protocol.

Simple sizing (fish)

Volume ≈ number of fry / target density (fish/m³), or volume ≈ biomass / target biomass density (kg/m³). Example: 100,000 salmon fry at 10,000 fry/m³ → 10 m³. Alternatively 200 kg fry at 20 kg/m³ → 10 m³.

Important operational factors

• First-feeding success often determines overall hatchery performance.
• Oxygen demand increases rapidly as feeding begins.
• Uneaten feed must be removed to prevent water quality deterioration.
• Frequent grading reduces size variation and cannibalism.
• Stable temperature and photoperiod improve growth uniformity.
• Backup oxygen systems are strongly recommended — young fish are sensitive.

How shrimp tanks differ from fish tanks

Fish use the water column and swim. Shrimp are largely bottom-dwelling: they walk, graze, hide and moult. Design therefore differs clearly even though both are reared in “tanks”.

Area vs volume

Fish tanks are usually sized as density in kg/m³ (or fish/m³). For shrimp — especially larger juveniles and grow-out — the limit is often animals/m² or kg/m² because animals occupy the bottom. A shrimp tank may therefore be shallower and wider than a fish tank holding similar biomass. Many systems add extra surface: meshes, sheets, net curtains, artificial seaweed or biofloc structures — several times the floor area. For fish, extra surface is rarely relevant except as biofilter media.

Hydraulics, bottom and moulting

Fish tanks may use circular flow for swimming stimulus and self-cleaning. Shrimp tanks need gentler hydraulics: avoid strong currents, protect moulting animals, avoid feed in corners. Flow is mainly for oxygen and solids. Faeces and feed sink quickly; bottom deposits favour Vibrio. Hence: central drains, sloped floors, bottom flushing, frequent sludge removal. During moulting shrimp are vulnerable to cannibalism, mechanical damage and water-quality swings — habitat, stable salinity, alkalinity and calcium/magnesium become critical (often monitored as carefully as DO).

Density, feed and biofloc

Intensive shrimp culture can reach animal densities impossible for most fish. Orientation: extensive ponds a few shrimp/m²; semi-intensive often tens to ~150/m²; intensive lined ponds and indoor RAS much higher (hundreds–thousands/m² depending on stage). Fish are more often limited earlier by oxygen and swimming behaviour. Shrimp feed mainly on the bottom, more slowly and continuously — multiple daily feeds, feed trays, automatics, acoustic sensors and cameras are common. Biofloc (BFT) is widespread in shrimp: microbial aggregates consume ammonia and are grazed by shrimp; FCR and water exchange can improve. Fish RAS often aims to remove suspended solids, whereas biofloc intentionally maintains them.

Simple shrimp nursery sizing: area (m²) ≈ number of shrimp / target density (shrimp/m²). Example: 100,000 nursery shrimp at 500/m² → 200 m². Fish systems are normally sized primarily from tank volume and biomass density instead.

In short: fish tanks are generally designed around water volume and swimming behaviour; shrimp tanks often around bottom area, surface area, feeding behaviour and moulting biology. That is why modern indoor shrimp farms frequently use shallow raceways, lined ponds, stacked nursery tanks and large amounts of artificial substrate, while salmon, trout, char, sea bass and tilapia are more commonly reared in circular tanks, raceways or cages designed around volume and hydraulic performance.

Egg disinfection is a standard biosecurity measure in many hatcheries and is commonly performed when fertilized or eyed eggs arrive from external suppliers. The goal is to reduce the risk of introducing bacterial, fungal, viral, or parasitic pathogens into the hatchery. Treatment is typically performed shortly after egg arrival, during transfer between facilities, or before eggs enter the hatchery water system.

Iodophor (iodine) — standard method

Iodophor-based disinfectants are the most widely used treatment for routine disinfection of eyed salmonid eggs and remain the industry standard in many hatcheries worldwide. They are effective against many surface-associated pathogens while being relatively safe for fish eggs when applied according to the manufacturer's instructions. Disinfection does not sterilize the egg interior but helps reduce contamination on the egg surface and shell.

Correct concentration, exposure time, water quality, and thorough rinsing are essential. Excessive dosage or prolonged exposure can damage developing embryos and reduce hatch rates. Follow species-specific recommendations, veterinary requirements and local rules — this page does not give doses.

• Most established method for eyed salmonid eggs (salmon, trout, char, brown trout, etc.).
• Bath with measured concentration, controlled contact time and clean rinse water.
• Combined with dead-egg picking and other hatchery hygiene — not a standalone protection.

Alternative: hydrogen peroxide (H₂O₂)

Hydrogen peroxide is used in some hatcheries and applications, among other things to control fungal growth (Saprolegnia) on eggs. It breaks down into water and oxygen and leaves no long-term residue. It is not the global standard for routine salmonid egg baths in the same way as iodophor, but can be a complement or alternative where protocols and rules allow.

• Advantages: effective against fungus in many cases; no long residue; adds oxygen as it decomposes.
• Limitations: overdosing damages embryos; protocols differ by species and stage; requires strict control.

Alternative: ozone (O₃)

Ozone is an extremely powerful oxidant and disinfectant. It is common for hatchery and RAS water treatment (make-up, bio-barrier, clarity) — see ORP and ozone systems — but it is rarely the first choice for a direct egg bath. Residual ozone has a narrow safety margin and can quickly damage egg membranes and embryos.

• Advantages: very powerful disinfection of water paths; no long residue after decomposition.
• Limitations: narrow margin for eggs; mainly applied to water in system design, not free on eggs without residual control.

Commonly targeted pathogens

• Saprolegnia spp. (water moulds on eggs)
• Bacterial contaminants on egg surfaces
• Certain viral agents transmitted externally on eggs
• Opportunistic pathogens introduced during handling and transport

Design and zoning

• Dedicated disinfection station
• Measured disinfectant concentration and controlled contact time
• Gentle mixing for uniform exposure
• Clean rinse water after treatment
• Separate clean and dirty handling zones

Operation

• Follow species-specific recommendations and manufacturer limits — never exceed concentration or contact time.
• Follow local veterinary and regulatory requirements.
• Handle eggs carefully — mechanical damage raises mortality and fungus risk.
• Remove dead eggs during incubation; keep treatment records for traceability.

Good egg disinfection should be combined with supplier health certification, quarantine, equipment sanitation, water treatment, and regular hatchery health monitoring. Disinfection reduces risk but cannot compensate for poor hygiene or infected broodstock. Particular attention when eggs move between regions or countries — an important pathway for disease transmission.

Key principle: the best programme combines healthy broodstock, clean water, biosecurity procedures and careful egg handling. No disinfectant can compensate for poor hatchery hygiene. Especially common in hatcheries for Atlantic salmon, rainbow trout, Arctic char, brown trout, sturgeon and many marine finfish traded internationally. See Heath trays, egg incubators, biosecurity, ORP and ozone.

Aquaculture Egg Disinfection – Simple SOP guide

Pick method and egg stage. The tool sets orientation C and t, computes CT = C × t and builds a simple checklist. Always adjust against an approved product, label and veterinary advice — defaults are teaching start values for salmonids, not legal requirements.

Aquaculture Egg Disinfection – Simple SOP guide

Orientation for salmonids in hatcheries — not a product label or veterinary protocol. Adjust C and t against an approved product. Never exceed manufacturer limits.

Method Iodophor (standard) Hydrogen peroxide (H₂O₂) Ozone (O₃) — water path

Egg stage Eyed (most common) Green / newly fertilised (more sensitive)

Concentration C

Contact time t (min)

CT (C × t)—

CT = concentration × time (orientation)mg/L · min

Suggested SOP (checklist)

About defaults Iodophor: often-cited orientation ~50–100 mg/L active iodine × about 10 min for eyed salmonid eggs (many protocols about 100 mg/L × 10 min). H₂O₂: highly species-specific; start value orientation only. Ozone: mainly residual in water path (µg–low mg/L), not a free egg bath — residual CT must stay low. Always rinse, zone and document.

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Atlantic salmon (Salmo salar) is an anadromous species that hatches and grows in freshwater before migrating to the ocean, where it spends most of its grow-out phase. Atlantic salmon is the dominant aquaculture species in Norway, Scotland, the Faroe Islands, Iceland, Canada and Chile, accounting for the majority of global farmed salmon production. Traditionally produced in sea cages, the industry is increasingly adopting land-based systems for smolt production, post-smolt rearing and, in some cases, full grow-out to harvest size.

Atlantic salmon performs best in cool, well-oxygenated water. Growth rates are strongly influenced by temperature, with optimal growth typically occurring between approximately 10 and 16 °C. Elevated temperatures increase metabolism, oxygen demand and disease risks, while prolonged exposure to warm water can lead to reduced feed intake, stress and mortality. Temperature control is therefore a critical design consideration in both freshwater and seawater RAS.

Water quality and RAS

Modern RAS salmon production requires strict management of water quality. Key parameters include dissolved oxygen (DO), carbon dioxide (CO₂), ammonia (TAN), nitrite, alkalinity, pH, salinity and suspended solids. Atlantic salmon are particularly sensitive to elevated CO₂ concentrations, making degassing systems an essential component of recirculating facilities. Efficient oxygenation and robust biofiltration are required to support high biomass densities while maintaining fish welfare and growth performance.

Biosecurity

Biosecurity is a major driver behind land-based salmon farming. Closed systems can reduce exposure to sea lice, pathogens, harmful algal blooms and environmental fluctuations that affect open-net pen production. However, this requires rigorous disinfection protocols, quarantine procedures, water treatment systems and continuous monitoring to prevent disease outbreaks within the facility.

Typical production stages

• Broodstock
• Egg incubation
• Freshwater fry and parr
• Smolt production
• Post-smolt grow-out
• Harvest-size production

Atlantic salmon remains the benchmark species for large-scale recirculating aquaculture technology and has been a major driver of innovation in biofiltration, oxygenation, water treatment, automation, fish health monitoring and backup life-support systems. Today, salmon RAS facilities range from small freshwater hatcheries producing a few million smolts annually to industrial-scale land-based farms targeting tens of thousands of tonnes of harvest-size fish per year.

Production timeline and growth (orientation)

Atlantic salmon has one of the longest production cycles in aquaculture. Exact duration depends on strain, temperature, feeding, photoperiod and whether production is in traditional sea cages or RAS. The tables below are typical ranges under favourable conditions — not guaranteed protocols.

Ranges under favourable conditions. Strain, temperature, density and health change the timeline.

Example growth progression

Approximate progression under favourable conditions (orientation):

Modern post-smolt RAS often delivers 300–1,000 g before sea transfer; classic hatcheries often deliver 80–150 g smolt. Fully land-based food fish omits the marine phase.

• Rule of thumb: about 8–18 months egg → smolt
• About 16–24 months smolt → harvest (3–6 kg)
• About 24–36 months total egg → 4–6 kg harvest

This long production cycle is one reason salmon farmers place strong emphasis on biosecurity, backup power, oxygen supply and water quality management, as losses at any stage can represent years of biological investment.

Calculator: time-to-weight & production chain

The tool estimates time from start phase/weight to target weight (based on the orientation growth curve above) and proposes a simplified tank/stage chain toward an annual production target. Density, survival and number of tank sizes are adjustable. Orientation — not bankable facility design.

Calculator: Atlantic salmon — time-to-weight & production chain

1. Time from start to target weight

Start phase Fertilised egg Hatch / alevin First feeding Fingerling 5 g Parr 10 g Parr 50 g Smolt 100 g Smolt 150 g Post-smolt 500 g Post-smolt 1 kg Custom start weight

Start weight (g)

Target weight (g)

Growth tempo Slow (+15% time) Typical (orientation) Fast (−15% time)

Estimated time—

Time from egg to target—

Milestones on the way—

Approximate growth (orientation)

Curve uses the same orientation data as the time calculator; tempo scales the x-axis. Logarithmic weight axis. Orientation — not a feed-control growth model.

2. Production chain toward annual target

Target production (tonnes/year)

Harvest weight (kg/fish)

End product Smolt (~150 g) Post-smolt (~0.8 kg) Food fish (harvest weight above)

Facility starts with Eggs / hatchery Fry (~0.5 g) Purchased smolt (~100 g)

Survival start → end (%)

Stockings / year (continuous ≈ 12)

Number of stages / tank sizes (1–12)

Tank shape Round (circular) Octagonal

Height/width (H/W)

  Set optimal (0.30)

H/W = water depth ÷ diameter (round) or flat–flat span (octagon). Optimal default 0.30 ≈ Ø:depth 3.3:1 (typical circular RAS tanks). Lower = flatter/larger footprint; higher = deeper/narrower.

Density (kg/m³) by fish size — adjust to strategy. More stages ⇒ shorter time per tank and sizing to exit weight ⇒ lower total volume.

Fry / small parr

Parr

Smolt

Post-smolt

Grow-out

Fish / year (at end)—

Required at start / year—

Cycle start → end—

Total tank volume—

Stages in the chain

Stage	Weight (g)	Months	Fish in stage (avg)	Biomass (t)	kg/m³	Volume (m³)

Suggested tank plan

Tank class	Stage	Days in tank	Unit volume	Depth	Area / tank	Diameter / span	Units	Class volume

Tank classes—

Total tanks—

Production stages—

Required volume (with 15% spare)—

Installed tank volume—

Total tank footprint—

3. Layout generator (to scale, area = CAPEX) (Experimental)

Packing biased to the golden ratio (φ≈1.618): D+1.0 m cells, grid/shelf and a segment meta-grid so the hall is not one long thin strip.

Segments / hall modules

Return line (from tanks) Buried Not buried (floor/ground)

Outgoing / supply water line Mounted under roof On floor / ground level

Water treatment area (% of tank footprint)

Hall layout to scale SCALE

Tank area—

Service area (mandatory)—

Hall area—

Packing efficiency—

Unused area in hall—

Water treatment—

Total plan area (CAPEX)—

m² / tonne production—

Packing runs as JS in the browser (grid + shelf). Shared service D+1.0 m. Dashed circle = 0.5 m service zone. Orientation — not construction documentation.

Time from log-interpolated orientation curve (~10–14 °C). Stage count N splits growth logarithmically from start to end weight (1–12 tank classes). Volume is sized to exit weight in each stage (avg fish × w_exit / density) — total volume falls as N rises. Geometry: chosen H/W (default 0.30) sets depth and diameter/span from unit volume (V = A·H, H = (H/W)·W). Layout plan is to scale (metres + scale bar). Orientation — not bankable design.

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See also

• Temperature
• Dissolved oxygen
• CO₂
• RAS
• Egg incubators
• Backup power systems
• Monitoring systems

Rainbow trout (Oncorhynchus mykiss) is one of the world’s most widely farmed salmonids. It is robust, has a mature feed and market chain, and is grown in raceways, ponds, cages, semi-flow-through and RAS. Unlike Atlantic salmon, trout is usually farmed for the whole cycle in fresh or brackish water — portion (pan-size) or larger food fish depending on market.

Optimal growth typically sits around 10–16 °C. Trout tolerates somewhat higher temperatures than Arctic char but still responds to heat with higher oxygen demand and stress. In RAS, stable temperature, high DO, controlled CO₂ and good solids handling are the keys to high density with maintained welfare.

Systems and use

• Raceways and flow-through — classic production where water rights and receiving water allow.
• Ponds — lower intensity, often in warmer climates or combined with other species.
• Cages in lakes/sea — used regionally; freshwater cages common inland.
• Semi-flow / FlowFarm and RAS — cut water use and discharge; well suited to rainbow trout.

Typical production stages

• Broodstock
• Egg incubation
• Fry / first feeding
• Fingerling
• Juvenile
• Food fish (portion or large)

Compared with Atlantic salmon the cycle to portion size is shorter, and seawater smoltification is not standard in Nordic inland farming. Large fish (2–4+ kg) appear in premium and fillet markets. See also FlowFarm for semi-flow concepts.

Production timeline and growth (orientation)

Exact duration depends on strain, temperature (~12–16 °C often favourable), feed and density. Tables are typical ranges under favourable conditions — not protocols.

Ranges under favourable conditions. Strain, temperature and health govern real time.

Example growth progression

Rule of thumb: portion fish often about 10–16 months from egg; large fish another 6–12+ months. Orientation.

• Rule of thumb: about 10–16 months egg → portion (~0.3–0.4 kg)
• About 16–24 months egg → 2–3 kg
• Level year-round production needs controlled broodstock / frozen milt — see egg incubators

Calculator: time-to-weight & production chain

Same type of tool as for Atlantic salmon, but with trout’s faster growth curve and end products (portion / large fish). Density, survival, tank classes and layout generator included. Orientation — not bankable design.

Calculator: rainbow trout — time-to-weight & production chain

1. Time from start to target weight

Start phase Fertilised egg Hatch / alevin First feeding Fingerling 5 g Parr 10 g Parr 50 g 100 g 150 g 500 g 1 kg Custom start weight

Start weight (g)

Target weight (g)

Growth tempo Slow (+15% time) Typical (orientation) Fast (−15% time)

Estimated time—

Time from egg to target—

Milestones on the way—

Approximate growth (orientation)

Curve uses the same orientation data as the time calculator; tempo scales the x-axis. Logarithmic weight axis. Orientation — not a feed-control growth model.

2. Production chain toward annual target

Target production (tonnes/year)

Harvest weight (kg/fish)

End product Portion fish (~350 g) Large trout (~2.5 kg) Food fish (harvest weight above)

Facility starts with Eggs / hatchery Fry (~0.5 g) Fingerling (~50 g)

Survival start → end (%)

Stockings / year (continuous ≈ 12)

Number of stages / tank sizes (1–12)

Tank shape Round (circular) Octagonal

Height/width (H/W)

  Set optimal (0.30)

H/W = water depth ÷ diameter (round) or flat–flat span (octagon). Optimal default 0.30 ≈ Ø:depth 3.3:1 (typical circular RAS tanks). Lower = flatter/larger footprint; higher = deeper/narrower.

Density (kg/m³) by fish size — adjust to strategy. More stages ⇒ shorter time per tank and sizing to exit weight ⇒ lower total volume.

Fry / small parr

Fingerling

Juvenile

Portion / large

Grow-out

Fish / year (at end)—

Required at start / year—

Cycle start → end—

Total tank volume—

Stages in the chain

Stage	Weight (g)	Months	Fish in stage (avg)	Biomass (t)	kg/m³	Volume (m³)

Suggested tank plan

Tank class	Stage	Days in tank	Unit volume	Depth	Area / tank	Diameter / span	Units	Class volume

Tank classes—

Total tanks—

Production stages—

Required volume (with 15% spare)—

Installed tank volume—

Total tank footprint—

3. Layout generator (to scale, area = CAPEX) (Experimental)

Packing biased to the golden ratio (φ≈1.618): D+1.0 m cells, grid/shelf and a segment meta-grid so the hall is not one long thin strip.

Segments / hall modules

Return line (from tanks) Buried Not buried (floor/ground)

Outgoing / supply water line Mounted under roof On floor / ground level

Water treatment area (% of tank footprint)

Hall layout to scale SCALE

Tank area—

Service area (mandatory)—

Hall area—

Packing efficiency—

Unused area in hall—

Water treatment—

Total plan area (CAPEX)—

m² / tonne production—

Packing runs as JS in the browser (grid + shelf). Shared service D+1.0 m. Dashed circle = 0.5 m service zone. Orientation — not construction documentation.

Time from rainbow trout orientation curve (~12–16 °C). Faster to portion size than Atlantic salmon; often entire cycle in freshwater. Orientation — not bankable design.

© 2026 DABCE AB

See also

• Temperature
• Dissolved oxygen
• RAS
• Flow-through
• Raceways
• Egg incubators
• Backup power systems
• Atlantic salmon

Arctic char (Salvelinus alpinus) is a cold-preferring salmonid with a circumpolar range in Arctic and sub-Arctic fresh waters - and in some stocks also in brackish water. In the Nordics it carries both cultural and commercial weight: wild and farmed char sell as premium fish, often at a higher price per kilo than rainbow trout, thanks to flavour, texture and a "Nordic" market profile.

Arctic char in aquaculture

Farmed Arctic char is a niche next to global salmonids, but it is strategically important in cold inland sites in Sweden, Finland, Iceland, Canada and Alaska. Production uses ponds and raceways with cold spring or lake water, cages in cold lakes, and increasingly land-based systems (semi-flow-through and RAS) where temperature can be held stable year-round.

Compared with rainbow trout, char grows more slowly at the same temperature and tolerates heat less well. Optimal growth often sits in the cooler range (roughly 8-14 °C depending on strain and life stage); above about 16-18 °C stress, reduced feed intake and health risks rise quickly. That makes the species more demanding in warm summers and more dependent on cold intake, shading, deep basins or active cooling in land-based farms.

Systems and technology

• Flow-through and raceways with cold spring or mountain water - classic inland solution where flow and temperature are enough.
• Cage culture in cold lakes - lower CAPEX but exposed to ice, under-ice oxygen risk, escapes and local environmental rules.
• Semi-flow-through / partial reuse - cuts freshwater use and discharge while keeping temperature control from the source.
• RAS with cooling - high control and biosecurity; energy for cooling and pumping becomes central to CAPEX/OPEX.

Water-quality demands are high: stable dissolved oxygen, low ammonia/nitrite, good solids control and gentle hydraulics. Char is sensitive to poor water chemistry and sudden swings - the same filtration, gas and monitoring principles as for other salmonids, but with even less temperature margin.

Market and economics

The premium position means better price but needs consistent quality, traceability and often limited volume. Longer production time and cooling needs raise cost per kg versus trout; profitability depends on differentiating the product (whole fish, fillet, smoking, local/Nordic branding) rather than only chasing lowest production cost. For project developers, char is therefore interesting where cold water is a natural advantage - and where the market pays for the niche.

Level production over the year

As with rainbow trout and Atlantic salmon, Arctic char spawning is seasonal if the broodstock environment is not controlled with light and temperature. A farm that wants to sell food fish every month — and fill RAS/semi-flow evenly — cannot rely on a single natural egg pulse. To level the biology you need either frozen milt / planned egg supply and delayed incubation, or control of the broodstock cycle (photoperiod and temperature, several brood groups out of phase), often combined. Otherwise hatchery and grow-out are sized for the peak season and sit partly empty the rest of the year. See Egg incubators (levelling biology over the year).

Typical production stages

• Broodstock
• Egg incubation (often cool, higher °D than trout)
• Fry / first feeding
• Fingerling
• Juvenile
• Food fish (often 0.8–2.5 kg depending on market)

Production timeline and growth (orientation)

Char grows more slowly than rainbow trout at the same temperature, but under good cool conditions (~10–12 °C) grow-out is much faster than an overly pessimistic cold-water curve. A useful orientation: about 9 months from ~20 g to ~900 g. Exact time depends on strain, temperature, feed and density — tables are typical ranges, not protocols.

Ranges under favourable cool conditions. Heat, density and health lengthen the cycle. Orientation: ~9 months from 20 g to 900 g.

Example growth progression

Key interval: ~20 g → ~900 g in about 9 months under good cool conditions. Orientation.

• Rule of thumb: about 9 months from ~20 g to ~900 g
• About 15–18 months egg → ~0.5–0.9 kg
• About 18–22 months egg → ~1–1.5 kg (common premium)
• About 24–32 months egg → ~2–2.5 kg
• Keep temperature in the cool window — heat costs growth and welfare

Calculator: time-to-weight & production chain

Same tool family as for Atlantic salmon and rainbow trout, but with char’s slower growth curve (~8–14 °C) and typical harvest weights. Density, survival, tank classes, growth chart and layout generator included. Orientation — not bankable design.

Calculator: Arctic char — time-to-weight & production chain

1. Time from start to target weight

Start phase Fertilised egg Hatch / alevin First feeding Fingerling 5 g Parr 10 g Parr 50 g 100 g 150 g 500 g 1 kg Custom start weight

Start weight (g)

Target weight (g)

Growth tempo Slow (+15% time) Typical (orientation) Fast (−15% time)

Estimated time—

Time from egg to target—

Milestones on the way—

Approximate growth (orientation)

Curve follows char’s slower orientation data; tempo scales the x-axis. Logarithmic weight axis. Orientation — not a feed-control growth model.

2. Production chain toward annual target

Target production (tonnes/year)

Harvest weight (kg/fish)

End product Smaller food fish (~500 g) Large char (~2–2.5 kg) Food fish (harvest weight above)

Facility starts with Eggs / hatchery Fry (~0.5 g) Fingerling (~40 g)

Survival start → end (%)

Stockings / year (continuous ≈ 12)

Number of stages / tank sizes (1–12)

Tank shape Round (circular) Octagonal

Height/width (H/W)

  Set optimal (0.30)

H/W = water depth ÷ diameter (round) or flat–flat span (octagon). Optimal default 0.30 ≈ Ø:depth 3.3:1 (typical circular RAS tanks). Lower = flatter/larger footprint; higher = deeper/narrower.

Density (kg/m³) by fish size — adjust to strategy. More stages ⇒ shorter time per tank and sizing to exit weight ⇒ lower total volume.

Fry / small parr

Fingerling

Juvenile

Grow-out / food fish

Food fish / premium

Fish / year (at end)—

Required at start / year—

Cycle start → end—

Total tank volume—

Stages in the chain

Stage	Weight (g)	Months	Fish in stage (avg)	Biomass (t)	kg/m³	Volume (m³)

Suggested tank plan

Tank class	Stage	Days in tank	Unit volume	Depth	Area / tank	Diameter / span	Units	Class volume

Tank classes—

Total tanks—

Production stages—

Required volume (with 15% spare)—

Installed tank volume—

Total tank footprint—

3. Layout generator (to scale, area = CAPEX) (Experimental)

Packing biased to the golden ratio (φ≈1.618): D+1.0 m cells, grid/shelf and a segment meta-grid so the hall is not one long thin strip.

Segments / hall modules

Return line (from tanks) Buried Not buried (floor/ground)

Outgoing / supply water line Mounted under roof On floor / ground level

Water treatment area (% of tank footprint)

Hall layout to scale SCALE

Tank area—

Service area (mandatory)—

Hall area—

Packing efficiency—

Unused area in hall—

Water treatment—

Total plan area (CAPEX)—

m² / tonne production—

Packing runs as JS in the browser (grid + shelf). Shared service D+1.0 m. Dashed circle = 0.5 m service zone. Orientation — not construction documentation.

Time from Arctic char orientation curve (~8–14 °C). Slower than rainbow trout; heat-sensitive. Orientation — not bankable design.

© 2026 DABCE AB

See also

• Temperature
• Dissolved oxygen
• RAS
• Flow-through
• Egg incubators
• Backup power systems
• Rainbow trout
• Atlantic salmon

Sturgeon (Acipenseriformes: Acipenser, Huso and others) are farmed for meat and especially caviar (roe). It is a high-value niche with long generation times, strict traceability and often CITES/species rules. Several species and hybrids are used in aquaculture; Siberian sturgeon (Acipenser baerii) is among the most common in European land-based production.

Unlike salmonids, females take many years to mature — often 5–15+ years depending on species, temperature and feeding. That makes CAPEX, broodstock planning and cash flow critical: caviar revenue comes late, while meat from males and juveniles can bring earlier turnover. Premium niche projects (roe/caviar) therefore need long-term financing and a clear product strategy.

Common culture species (orientation)

Age to first caviar is site-, temperature- and strain-dependent. Orientation — not a breeding protocol.

Systems and water

• RAS and semi-flow — most common for controlled caviar production in Europe.
• Raceways / long tanks — used where flow and temperature allow.
• Ponds — more extensive in some regions; lower control.
• Temperature often about 16–22 °C for many culture species (species-dependent); stability matters more than “max growth”.

Water quality: good DO, controlled ammonia/nitrite, low solids load and gentle handling. Sturgeon are benthic and sensitive to poor tank hygiene and grading stress. Sexing (ultrasound) and individual tagging are standard in serious caviar production.

Caviar, roe and traceability

Caviar yield is often stated as a share of female body weight (orientation typically about 8–15% depending on species, condition and processing). Not all egg mass becomes saleable caviar — process yield, grading and market requirements differ. CITES, species labelling and chain documentation are central; farmed caviar must be distinguishable from wild-caught.

After first spawning many females do not spawn every year — intervals of 1–3 years are common. Size the broodstock against the share that is “ready” in a given year, not against 100% of mature females yielding caviar annually.

Calculator: caviar / roe from mature females

Pick sturgeon species (preset), number of mature females and average weight. The tool estimates caviar (kg/year), egg numbers for hatchery use and approximate male needs. Orientation — not a caviar plant or CITES application.

Calculator: sturgeon — caviar / roe from mature females (orientation)

Sturgeon species / preset Siberian sturgeon (A. baerii) Russian sturgeon (A. gueldenstaedtii) Sterlet (A. ruthenus) White sturgeon (A. transmontanus) Beluga (Huso huso) Adriatic sturgeon (A. naccarii) Hybrid (e.g. bester) Custom

Mature females (n)

Average female weight (kg)

Caviar yield (% of body weight)

Share spawning this year (%)

Eggs per kg body weight

Females per male (breeding)

Caviar (kg/year)—

Eggs / year (orientation)—

Males (orientation)—

Total broodstock (♂+♀)—

Years to ♀ maturity (orientation)—

Spawn interval (years)—

Caviar_kg ≈ N_females × BW × (caviar_%/100) × (spawn_%/100). Eggs ≈ N × BW × eggs/kg × (spawn_%/100). Spawn interval > 1 year means share < 100% per calendar year. Presets are teaching mid-values — species, age, condition, ultrasound selection and process yield govern real caviar. Orientation — not CITES, breeding or factory design.

© 2026 DABCE AB

See also

• RAS
• Raceways
• Egg incubators
• Egg disinfection
• Biosecurity
• Backup power systems

Tilapia is the common name for several cichlids in the family Cichlidae; Nile tilapia (Oreochromis niloticus) and related Oreochromis species (and hybrids) dominate world production. It is one of the most farmed fish groups globally — robust, fast-growing in warm water, omnivorous and backed by a mature feed chain. Volume is largest in Asia, Africa and Latin America; in cold Nordic inland sites heating or indoor RAS is required for the species to be relevant.

Historically tilapia ranks among the world’s oldest farmed fishes: in ancient Egypt Nile tilapia appears in art and hieroglyphs, and pond culture along the Nile goes back thousands of years. In modern times selection, monosex (often male) production and intensification in ponds, cages and RAS have made tilapia a staple protein species in many countries — closer to chicken as “volume protein” than to a premium niche.

Biology and temperature

Tilapia is a warm-water species. Optimal growth typically sits around 28–32 °C; below about 20–22 °C growth slows sharply and below about 12–15 °C mortality risk rises. That is why Nordic outdoor projects without heat are rarely realistic, while heated RAS, greenhouse ponds or tropical sites work. Some strains tolerate a wide salinity range (fresh to brackish), opening coastal areas where freshwater is limited.

Reproduction starts early and often — without control it leads to overcrowding and small fish in ponds. Male monosex (genetic YY, hormone-treated juveniles where allowed, or hybrid systems) is therefore standard in intensive production. Mouthbrooding (the female broods eggs in the mouth) gives high fry survival but complicates breeding comparisons with salmonids.

Production systems

• Ponds — globally dominant volume; semi-intensive to intensive with fertilisation, feed and aeration.
• Cages in lakes and reservoirs — common in Africa, Asia and Latin America.
• Raceways and tanks — where flow and heat allow.
• RAS and biofloc — higher control and density; energy for heat/pumping becomes central in temperate zones.
• Aquaponics — tilapia is a common “engine” species with plants.

In ponds the system often acts as an ecosystem: phytoplankton and natural production complement pelleted feed. In RAS/tanks feed and water quality are more “closed” — DO, TAN, nitrite, CO₂ and solids are managed technically. Tilapia tolerate higher density and more challenging water quality than salmonids, but that is no excuse for poor husbandry: gill disease, streptococci and poor FCR still hit hard under stress.

Feed, growth and harvest

As an omnivore tilapia can use more plant-based feeds than salmon, which lowers feed cost and environmental footprint per kg protein in many LCA comparisons. Grower feed protein is often lower than for salmonids (orientation about 28–36% depending on stage). Typical portion fish is about 400–600 g; cycle time from stocking to harvest is often about 4–8 months at good temperature (sometimes faster in the tropics). Several cycles per year are common in warm regions — a key difference from the long single salmonid cycle.

FCR in well-run intensive culture often sits around 1.3–1.8 (lower in tropical ponds with natural production, higher under stress, low temperature or poor feed quality). Fillet yield is lower than for salmonids — typically about 30–35% for skin-on fillet depending on size and trim — which matters when comparing “kg fish” with “kg saleable fillet”.

Production timeline and growth (orientation)

Exact duration depends on strain (often improved Oreochromis lines), temperature (~28–30 °C favourable), feed, density and whether monosex is used. Tables are typical ranges under favourable conditions — not protocols.

Ranges under favourable temperature. Strain, monosex, feed and health govern real time.

Rule of thumb: ~0.5 kg often about 5–7 months from ~20 g at 28–30 °C. Orientation.

• Rule of thumb (warm water): juveniles ~10–50 g → ~0.5 kg in about 5–7 months
• Pond density: from a few t/ha (extensive) to tens of t/ha (intensive with aeration)
• Tank/RAS density: often tens of kg/m³ depending on oxygen and treatment
• Several cohorts/year in the tropics → steadier delivery and higher use of pond/tank capacity

Water quality, health and risks

Tilapia tolerate more than salmonids — but DO below about 3–4 mg/L, high nitrite, sudden temperature swings and poor bottom hygiene quickly cause growth stop, gill damage and secondary infections. In ponds, night-time oxygen crashes (after algal blooms) are a classic risk; aeration and contingency are therefore central in intensive pond culture. In RAS the same logic as for other species applies: biofilter, solids removal, gas exchange and alarms.

Important culture health risks include among others streptococci (Streptococcus spp.), Francisella, columnaris and parasites — regionally variable. Biosecurity at stocking, water intake and visitors matters even for “hardy” species. Off-flavour (MIB/geosmin) from cyanobacteria or biofilm can make fish unsaleable until depurated in clean water — a practical cost and logistics issue before harvest.

Market and Nordic perspective

Globally tilapia is a volume product with price competition. In Europe much frozen fillet is imported; fresh local culture in heated RAS can reach niche prices but must carry the energy cost. In a Nordic climate tilapia is therefore mainly relevant as a comparison species, heated indoor/RAS concept or technology example (water treatment, oxygen, monitoring) — not as a cold-water outdoor species without heat.

In tropical and subtropical countries tilapia is often staple protein with a short cycle, local feed chains and a low technical threshold in ponds — while intensification (biofloc, RAS, cages) is rising. Technology export and advice on water quality, oxygen and monitoring are therefore relevant even when the species itself is not farmed in the Nordics.

Production sizer (tank / pond)

The tool estimates fish numbers, standing biomass and volume or pond area from target production (tonnes/year), harvest weight, cycle length, survival and density. Switch between tank/RAS (kg/m³) and pond (kg/ha). Pick a climate preset for typical cycle times and densities. Orientation — not bankable facility design.

Production sizer: tilapia (orientation)

Climate / tempo Tropical (fast, ~5 mo) Warm (typical, ~6.5 mo) Temperate/RAS (slower, ~8 mo)

System Tank / RAS (kg/m³) Pond (kg/ha)

Target production (tonnes/year)

Harvest weight (g/fish)

Stocking weight (g/fish)

Cycle time stocking → harvest (months)

Survival stocking → harvest (%)

Density at harvest (kg/m³)

Density at harvest (kg/ha)

FCR (kg feed / kg gain)

Fish harvested / year—

Fish stocked / year—

Cycles / year—

Fish per cohort (at harvest)—

Standing biomass (orientation)—

Tank volume (m³)—

Pond area (ha)—

Feed (tonnes/year)—

Cycles/year ≈ 12 / cycle months. Fish/year = tonnes×1000 / (harvest_kg). Stocked = harvested / survival. Standing biomass ≈ (harvest biomass per cohort) × 0.55 (mid-cycle orientation). Volume = standing biomass / density (tank). Pond ha = standing biomass / (kg/ha). Feed ≈ gain × FCR. Orientation — not bankable design; heat, oxygen and water quality govern real density.

© 2026 DABCE AB

See also

• Temperature
• Ponds
• RAS
• Dissolved oxygen
• Feed types
• Biosecurity
• Monitoring systems
• Egg incubators

“Catfish” in farming is not one species but several families with similar use: hardy warm-water fish in ponds, tanks and RAS. The two industrially most important lines are African catfish (Clarias, especially C. gariepinus and hybrids) and American channel catfish (Ictalurus punctatus). Pangasius (Pangasianodon) is also farmed widely in Asia, and European wels (Silurus glanis) in more niche European projects. Shared traits are often high hardiness, relatively fast growth in warm water and a role as volume protein rather than a premium niche.

Clarias is especially relevant for intensive land-based systems: the species has air-breathing organs and can sustain very high densities when water quality, feed and handling are maintained. Ictalurus dominates the classic US pond industry with large volume and mature logistics. In a Nordic and European context Clarias is the usual “catfish” reference in heated RAS — while channel catfish is mainly a comparison and pond reference.

Common culture species (orientation)

Temperature and system choice are species- and site-dependent. Orientation — not a culture protocol.

Biology and temperature

Catfish are warm-water species. For Clarias, fast growth typically sits around 26–30 °C; below about 20–22 °C growth slows and below about 15–17 °C stress and disease risk rise. Channel catfish has a similar warm preference but is often farmed in seasonal ponds where summer drives growth. Air-breathing in Clarias means the species can cope with lower water DO than salmonids — but that is no excuse to run hypoxic: growth, feed conversion and gill health still suffer, and air-breathing raises the fish’s energy cost.

Clarias is omnivorous/carnivorous with aggressive feed intake and cannibalism risk in fry and juvenile stages — grading, density and correct feed size are critical. Channel catfish is a more “classic” pond fish with an established feed chain and less cannibalism in grow-out, but sensitive to off-flavour and summer/oxygen crashes in ponds.

Production systems

• Ponds — dominate channel catfish (USA) and much Clarias/pangasius volume in Africa and Asia.
• Tanks and raceways — common intensive Clarias production with high density and easy harvest.
• RAS — European and export-relevant Clarias culture; heat, biofilter and solids handling drive CAPEX/OPEX.
• Flow-through / semi-flow — where water and receiving water allow; lower technical complexity than full RAS.

In intensive Clarias tanks, densities often sit far above salmonids (tens to hundreds of kg/m³ depending on oxygen, surface air access, treatment and fish size). Design to real hydraulics and gas exchange — not to “record figures”. In ponds density is more often tonnes per hectare, with aeration as the bottleneck in intensive operation.

Feed, growth and harvest

Clarias often takes relatively high-protein grower feeds (orientation about 30–40%+ depending on stage) but can use more animal and plant raw materials than salmon. FCR in well-run intensive culture often sits around 1.0–1.5. Channel catfish has a mature US feed industry with typically lower protein than carnivorous salmonids and FCR often around 1.5–2.0 in ponds depending on natural production and season.

Typical harvest weight for Clarias often sits around 0.8–1.5 kg (sometimes larger); channel catfish fillet industry often runs about 0.5–1+ kg depending on market. Cycle time from stocking to harvest is often about 4–8 months in warm water — several cycles per year are realistic in the tropics and in heated RAS.

Production timeline and growth (orientation)

Exact duration depends on species (Clarias vs Ictalurus), strain/hybrid, temperature, feed and density. Tables below are mainly Clarias-oriented under good warm conditions — not protocols.

Ranges under favourable temperature. Cannibalism and grading govern real survival more than in tilapia.

Rule of thumb: ~1 kg often about 5–7 months from ~20 g at 26–30 °C. Orientation.

• Rule of thumb (Clarias, warm): ~15–50 g → ~1 kg in about 5–7 months
• Tank/RAS density: often high tens–low hundreds kg/m³ (oxygen + air space above surface)
• Pond density: from a few t/ha to tens of t/ha with aeration (species- and region-dependent)
• Several cohorts/year in the tropics and in heated RAS

Water quality, health and risks

Clarias tolerate more solids and lower DO than salmonids, but nitrite, high TAN at high pH, poor bottom hygiene and abrupt cooling still cause growth stop and disease. In tank/RAS, solids removal, biofilter and gas exchange are bottlenecks at high density — even when the fish “cope” with murkier water. Ensure free surface access for air-breathing in Clarias systems.

Health risks include among others bacterial infections (e.g. Aeromonas, Edwardsiella), parasites and viruses — regionally variable. Cannibalism and grading injuries are practical loss sources. In ponds (especially channel catfish), off-flavour (MIB/geosmin) and summer oxygen crashes are classic market and operational risks. Biosecurity at stocking and water intake matters even for “hardy” species.

Market and Nordic perspective

Globally catfish is volume protein: channel catfish in the USA, pangasius fillet from Asia, Clarias in Africa and growing intensive systems in Europe/Asia. In Europe Clarias is often sold as fresh or processed product in regional chains; price competition with imported fillet is real. Heated RAS in the Nordics must carry the energy cost — the species is therefore mainly a comparison, technology export (water treatment, oxygen, monitoring) or heated concepts, not a cold outdoor species without heat.

Production sizer (tank / pond)

The tool estimates fish numbers, standing biomass and volume or pond area from target production (tonnes/year), harvest weight, cycle length, survival and density. Switch between tank/RAS (kg/m³) and pond (kg/ha). Climate/intensity presets are Clarias-oriented; adjust density for channel catfish ponds. Orientation — not bankable facility design.

Production sizer: catfish / Clarias (orientation)

Climate / intensity Tropical intensive (~5 mo, high density) Warm typical (~6.5 mo) Temperate/RAS (~8 mo)

System Tank / RAS (kg/m³) Pond (kg/ha)

Target production (tonnes/year)

Harvest weight (g/fish)

Stocking weight (g/fish)

Cycle time stocking → harvest (months)

Survival stocking → harvest (%)

Density at harvest (kg/m³)

Density at harvest (kg/ha)

FCR (kg feed / kg gain)

Fish harvested / year—

Fish stocked / year—

Cycles / year—

Fish per cohort (at harvest)—

Standing biomass (orientation)—

Tank volume (m³)—

Pond area (ha)—

Feed (tonnes/year)—

Cycles/year ≈ 12 / cycle months. Fish/year = tonnes×1000 / (harvest_kg). Stocked = harvested / survival. Standing biomass ≈ (harvest biomass per cohort) × 0.55. Volume = standing / density (tank). Pond ha = standing / (kg/ha). Feed ≈ gain × FCR. Presets are Clarias-oriented (higher density than tilapia due to air-breathing). Adjust for Ictalurus ponds. Orientation — not bankable design; oxygen, surface air access and treatment govern real density.

© 2026 DABCE AB

See also

• Temperature
• Ponds
• RAS
• Raceways
• Dissolved oxygen
• Turbidity / TSS
• Feed types
• Biosecurity
• Tilapia

Pikeperch (Sander lucioperca; Eng. pikeperch or zander) is one of Europe’s most valuable freshwater fishes and is regarded as one of the most promising species for diversifying European aquaculture. It is valued for firm, relatively bone-light and mild flesh, which gives a high market value compared with many other freshwater species. Interest in pikeperch farming has grown strongly in Europe, especially in recirculating aquaculture systems (RAS), where production can be controlled year-round.

Unlike established volume species such as salmon, rainbow trout and tilapia, pikeperch is often seen as a premium product with strong willingness to pay in the restaurant and fresh-fish market. It has a shorter industrial farming history than many other commercial fish species, but market demand continues to rise while wild catches in Europe have declined.

Fry and larval phase – the critical bottleneck

The greatest challenge in pikeperch culture is the fry and larval phase. Early feeding is critical and the species is sensitive to stress, unsuitable light regimes and uneven growth. Cannibalism can occur among fry, which requires regular size grading. Low survival and deformities during the larval stage have long been among the main bottlenecks for commercial production.

Many farms therefore choose to buy fingerlings from specialised fry producers or to run separate nursery units before fish are moved to grow-out facilities. Producing the full chain from eggs to market size requires substantially more biological competence and carries higher risk than focusing on grow-out alone.

Grow-out and production systems

Once pikeperch has passed the early fry phase it is well suited to intensive production. RAS often uses relatively warm temperatures to maximise growth, while high water quality, good oxygenation and gentle handling remain important because the species is stress-sensitive. Pikeperch can be farmed at relatively high densities and is therefore suited to commercial land-based production.

Common production systems

• RAS (Recirculating Aquaculture Systems) – year-round production with high control of temperature, light and water quality.
• Pond culture – traditional production form, especially common in Central Europe.
• Combined systems – pond or nursery for fry production and RAS for grow-out to market size.

Besides pure RAS production, pikeperch is farmed extensively in traditional pond systems, especially in Central Europe. Many producers combine pond culture for fry production with RAS for intensive grow-out. The combination can reduce production costs while giving better control of quality, growth and delivery reliability in the later production phase.

Typical culture conditions

The values above are general guidelines. Actual performance depends on strain, feed, health status, temperature, water quality and production strategy.

Production timeline and growth

Exact growth rate varies with temperature, feeding, genetic background, light management and culture conditions. The table below shows typical development stages in commercial production. The production sizer below counts from stocking (fingerling) to harvest.

A full production cycle from hatch to about 1 kg market size is often stated as roughly 15–18 months under good production conditions.

Why farm pikeperch?

• High market value compared with many other freshwater species.
• Sought-after premium fish in the restaurant and fresh-fish segment.
• Relatively bone-light fillets with good yield.
• Well suited to intensive RAS production.
• Growing European market and rising interest in species beyond the salmonid segment.

Challenges

• Complex larval and fry production.
• Stress-sensitive species that needs careful handling.
• Cannibalism in early stages.
• Needs high water quality and careful husbandry.
• Longer production cycle than many tropical warm-water species.

Market

Pikeperch is today an established premium species on the European market with strong demand from restaurants, retail and the fresh-fish segment. It is produced commercially in several European countries and is widely seen as an important candidate for further development of sustainable land-based aquaculture. The combination of high market value, good fillet quality and suitability for intensive RAS culture continues to drive investment in the species.

Production sizer (RAS / pond)

The pikeperch production sizer estimates fish numbers, standing biomass, tank volume or pond area from target production, harvest weight, cycle time, survival and density. The tool is intended as an orientation calculation for concept development and early project work. Results do not replace detailed biological, technical or economic facility design.

Production sizer: pikeperch (orientation)

Tempo / intensity (grow-out) Fast RAS (~14 mo → ~1 kg) Typical RAS (~16 mo) Conservative (~18 mo)

System Tank / RAS (kg/m³) Pond (kg/ha)

Target production (tonnes/year)

Harvest weight (g/fish)

Stocking weight (g/fish)

Cycle time stocking → harvest (months)

Survival stocking → harvest (%)

Density at harvest (kg/m³)

Density at harvest (kg/ha)

FCR (kg feed / kg gain)

Fish harvested / year—

Fish stocked / year—

Cycles / year—

Fish per cohort (at harvest)—

Standing biomass (orientation)—

Tank volume (m³)—

Pond area (ha)—

Feed (tonnes/year)—

Cycles/year ≈ 12 / cycle months (long cycle → fewer cycles). Fish/year = tonnes×1000 / harvest_kg. Stocked = harvested / survival. Standing biomass ≈ cohort harvest × 0.55. Volume = standing / density (tank). Pond ha = standing / (kg/ha). Presets are fingerling→market, not larval survival. Orientation — not bankable design; stress, light and water quality govern real density.

© 2026 DABCE AB

See also

• Temperature
• RAS
• Ponds
• Dissolved oxygen
• Fry and larval tanks
• Biosecurity
• Monitoring systems
• Perch
• Tilapia
• Rainbow trout

European perch (Perca fluviatilis) is one of Europe’s best-known freshwater fishes and attracts interest for sport fishing, food and aquaculture. It is valued for firm, mild flesh and is a recurring premium product in regional fresh-fish and restaurant markets. Compared with salmonids (salmon, rainbow trout) and tropical volume species, the industrial farming scale is still smaller, but interest in intensification — especially in RAS — has grown in Central Europe and the Nordics.

Perch belongs to the same family as pikeperch (Percidae). Many challenges are therefore similar: a demanding larval phase, cannibalism risk in fry, stress sensitivity and the need for stable water quality. Market size varies regionally — from smaller portions to fillet fish around 200–1000+ g. That affects cycle time, density and economics.

Fry and larval phase – the critical bottleneck

For European perch (Perca fluviatilis), the larval and fry phase is usually the greatest biological and technical challenge in the whole production chain. Although the species grows well in modern RAS and has an attractive market value, commercial expansion is often limited by the difficulty of producing large volumes of high-quality fingerlings.

Perch fry are very small at hatch and have limited energy reserves. Early and correct feeding is therefore critical. Many culture systems use live feed, for example rotifers and Artemia, before fry are gradually weaned onto dry feed. Failed weaning can lead to high mortality and uneven growth.

Fry are also sensitive to light conditions, water quality, handling and sudden environmental changes. Stress in the first weeks can affect both survival and later growth performance. Stable temperature, good water hygiene and steady operating conditions are therefore especially important in this period.

One of the largest problems is the strong size variation that appears early in rearing. Fast-growing individuals can quickly become much larger than their siblings, leading to aggressive behaviour and cannibalism. Regular size grading and close monitoring of the population are required to limit losses. Without effective grading, substantial parts of a year-class can be lost.

Historically, relatively low survival in the larval and fry phase has made it hard to scale production from eggs to harvest fish. Although techniques have improved over recent decades, fingerling production is still a specialised activity that needs high biological competence, established routines and experienced staff.

For this reason many commercial producers choose to buy fry or fingerlings from specialised hatcheries instead of owning the full production chain themselves. A common strategy is to run a separate nursery phase up to a robust fingerling size before fish are moved to the grow-out system. This reduces biological risk and simplifies production.

For new projects and investors it is therefore often more realistic to focus on the grow-out phase and secure fry supply through external suppliers. Establishing full production from eggs to market-ready fish can create more value in the long run, but also means substantially greater technical complexity, higher capital need and greater biological risk.

Typical challenges in the perch fry phase

• Very small larvae with high demands on early feeding.
• Often a need for live feed before transition to dry feed.
• Sensitivity to stress, light and water quality.
• Large size variation already early in production.
• Cannibalism when growth is uneven.
• Need for regular grading.
• Lower survival than in several established salmonid species.
• Requires high biological competence for large-scale production.

Typical production strategies

• Hatchery: eggs to larva.
• Nursery: larva to fingerling (usually 1–20 g).
• Grow-out: fingerling to market size (200–1000+ g depending on market).
• Alternative model: purchase fingerlings from specialised producers and focus on grow-out.

For most new RAS projects, access to high-quality fingerlings is a more important success factor than the grow-out technology itself. A secure and predictable fry supply is often the real bottleneck for commercial expansion of perch production.

Grow-out and production systems

Once perch has passed the fry phase it is suited to intensive grow-out in controlled systems. Optimal growth is often stated around 22–24 °C — somewhat cooler than many pikeperch RAS targets. High DO, good solids handling and gentle handling are important. In comparative trials RAS has often given better survival, FCR and weight uniformity than cages, partly through better control of temperature and light-managed reproduction.

Common production systems

• RAS – year-round production with control of temperature, light and water quality; the most common path for intensification.
• Pond culture – traditional and regional production, often lower intensity and seasonal.
• Cages in lakes/ponds – have been tested; less control of growth and maturation than RAS.
• Combined systems – pond/nursery for fry and RAS for grow-out.

Industrial scale is still smaller than for salmonids. That means fewer “turnkey” references, more site-specific solutions and often a greater need for local market knowledge. At the same time the smaller scale and premium profile make the species interesting for regional land-based projects.

Typical culture conditions

The values above are general guidelines. Actual performance depends on strain, feed, health status, temperature, light, water quality and production strategy.

Production timeline and growth

Exact growth rate varies with temperature, feeding, genetic background, light management and culture conditions. The table follows the usual chain hatchery → nursery → grow-out. The production sizer below counts from stocking (fingerling) to harvest — not larval survival.

Many new projects buy fingerlings and focus on grow-out. A full chain from eggs increases value creation but also risk and complexity.

Why farm perch?

• Regional premium demand in Europe and the Nordics.
• Familiar species for consumers — “perch” needs less market introduction than many exotic species.
• Suitable for intensive RAS once fry/fingerlings are secured.
• Diversification beyond salmonids alone in land-based projects.
• Fingerling supply often decides whether a project can scale — grow-out is more proven.

Challenges

• Complex larval and fry production — often the real bottleneck.
• Cannibalism and size variation in early stages.
• Stress and disease sensitivity (incl. bacteria and parasites) requires good biosecurity.
• Smaller industrial scale than salmonids — fewer large reference farms.
• Early maturation can disrupt growth if light/temperature are not managed.

Market

Perch is sold as fresh fish, fillet and in regional specialty segments. Price per kilo can be high compared with volume species, but the smaller industrial scale also means limited and uneven supply. Farmed perch competes with wild catch and with other percids (e.g. pikeperch). Strong local demand and traceable land-based production are therefore important arguments in concepts and sales.

Production sizer (RAS / pond)

The perch production sizer estimates fish numbers, standing biomass, tank volume or pond area from target production, harvest weight, cycle time, survival and density. The tool is intended as an orientation calculation for concept development and early project work. Results do not replace detailed biological, technical or economic facility design.

Production sizer: perch (orientation)

Tempo / intensity (grow-out) Fast RAS (~10 mo → ~250 g) Typical RAS (~12 mo → ~400 g) Conservative (~16 mo → ~800 g)

System Tank / RAS (kg/m³) Pond (kg/ha)

Target production (tonnes/year)

Harvest weight (g/fish)

Stocking weight (g/fish)

Cycle time stocking → harvest (months)

Survival stocking → harvest (%)

Density at harvest (kg/m³)

Density at harvest (kg/ha)

FCR (kg feed / kg gain)

Fish harvested / year—

Fish stocked / year—

Cycles / year—

Fish per cohort (at harvest)—

Standing biomass (orientation)—

Tank volume (m³)—

Pond area (ha)—

Feed (tonnes/year)—

Cycles/year ≈ 12 / cycle months. Fish/year = tonnes×1000 / harvest_kg. Stocked = harvested / survival. Standing biomass ≈ cohort harvest × 0.55. Volume = standing / density (tank). Pond ha = standing / (kg/ha). Presets are fingerling→market, not larval survival. Orientation — not bankable design; light, stress and water quality govern real density. Fingerling supply is often the real bottleneck.

© 2026 DABCE AB

See also

• Pike perch
• Temperature
• RAS
• Ponds
• Dissolved oxygen
• Fry and larval tanks
• Biosecurity
• Rainbow trout

Turbot (Scophthalmus maximus) is a high-value marine flatfish naturally found in the Northeast Atlantic, the Baltic Sea and the North Sea. It is well known in gastronomy and valued for firm white flesh and a high market price. Commercial farming takes place mainly in Europe, using both coastal flow-through systems and modern land-based recirculating aquaculture systems (RAS).

Turbot suits land-based culture because it lives on the bottom and can be held at relatively high biomass densities in shallow tanks. Its flat body shape means tank bottom area is often more important than tank volume when sizing the system. Good water quality, stable temperature and high dissolved oxygen are decisive for optimal growth and fish health.

Fry and larval phase – the main bottleneck

The greatest challenge in turbot farming is larval production. Larvae are very small and need advanced rearing with live feeds such as rotifers and Artemia before weaning onto dry feed. Metamorphosis from a symmetrical larva to a flatfish, with one eye migrating to the opposite side of the head, is a biologically complex process sensitive to nutritional deficiencies and culture conditions. The larval and fry phase therefore needs high competence and specialised equipment.

As with many marine fish species, larval production is the most technically demanding part of the chain. Commercial success often depends on the ability to produce large volumes of robust fingerlings of even quality. Many commercial producers therefore buy juveniles from specialised hatcheries instead of running full-scale reproduction and larval rearing themselves. For new projects, access to high-quality juveniles is often a more important success factor than the grow-out technology itself.

Typical production strategies

• Hatchery: eggs → larva (live feed, weaning).
• Nursery: larva/fry → juveniles after metamorphosis.
• Grow-out: juveniles → market size in shallow tanks (flow-through or RAS).
• Alternative model: buy juveniles and focus on grow-out.

Typical culture conditions

Values are general guidelines. Actual performance depends on strain, feed, salinity, temperature, water quality and production strategy.

Growth and market size

Turbot is usually sold at weights between 1 and 3 kg depending on market. Growth time varies with temperature, feeding and production strategy, but production to market size normally takes several years and needs long-term planning of facility capacity. The production sizer below counts from stocking (juveniles) to harvest and sizes mainly bottom area (kg/m²), not volume as for roundfish.

Long cycle → fewer cycles per year and more standing biomass per tonne of annual production than in fast-growing warm-water species.

Advantages

• Very high market value.
• Premium product in restaurant and export markets.
• Suitable for land-based culture.
• Relatively good feed conversion under optimal conditions.
• Benthic species that uses shallow tank area efficiently.

Challenges

• Complex and costly larval production.
• High competence required in hatchery and nursery.
• Sensitive metamorphosis during early development.
• Marine culture requires management of salinity and corrosion.
• Access to juveniles can limit expansion.
• Long grow-out cycle needs long-term capital and capacity planning.

Market

Turbot is an established premium species in European gastronomy and export. Farmed turbot complements limited wild catch and can be delivered more evenly through the year from land-based facilities. A high price per kilo must be weighed against the long cycle, marine infrastructure and juvenile supply.

Production sizer (land-based, bottom area)

The turbot production sizer estimates fish numbers, standing biomass and required tank bottom area (m²) from target production, harvest weight, cycle time, survival and density in kg/m². Optional water depth gives an orientation tank volume. The tool is an orientation calculation for concept development — not detailed facility design.

Production sizer: turbot (orientation, bottom area)

Tempo / target size (grow-out) Faster (~18 mo → ~1.2 kg) Typical (~24 mo → ~2 kg) Conservative (~30 mo → ~2.5 kg)

Target production (tonnes/year)

Harvest weight (g/fish)

Stocking weight (g/fish)

Cycle time stocking → harvest (months)

Survival stocking → harvest (%)

Density at harvest (kg/m² bottom)

Mean water depth (m, optional)

FCR (kg feed / kg gain)

Fish harvested / year—

Fish stocked / year—

Cycles / year—

Fish per cohort (at harvest)—

Standing biomass (orientation)—

Tank bottom area (m²)—

Tank volume (orientation)—

Feed (tonnes/year)—

Cycles/year ≈ 12 / cycle months (long cycle → fewer cycles). Fish/year = tonnes×1000 / harvest_kg. Standing biomass ≈ cohort harvest × 0.55. Bottom area = standing biomass / (kg/m²). Volume ≈ area × depth (orientation). Presets are juvenile→market, not larval survival. Orientation — not bankable design; salinity, oxygen and juvenile quality govern real density.

© 2026 DABCE AB

See also

• RAS
• Flow-through
• Salinity
• Dissolved oxygen
• Fry and larval tanks
• Temperature
• Biosecurity
• Perch

Shrimp (Litopenaeus spp., Penaeus spp. and other farmed shrimp) is the world’s largest aquaculture product by trade value and one of the most important protein sources from aquaculture globally. The dominant farmed species is whiteleg shrimp (Litopenaeus vannamei), which accounts for most of international production. Traditionally shrimp are farmed in earthen ponds and coastal systems in tropical regions, but in recent years intensive biofloc culture and land-based recirculating systems (RAS) have gained commercial interest.

Land-based shrimp farms offer high biosecurity, reduced pathogen spread and the chance to produce fresh premium shrimp near consumer markets in Europe and North America. Technology has made it possible to farm tropical shrimp even in cold climates, but energy costs for heating and water treatment remain an important challenge.

Main farmed species

The production sizer below is mainly oriented toward L. vannamei grow-out.

Temperature

Shrimp are poikilothermic and growth is strongly driven by water temperature. Temperature control is one of the main cost factors in land-based production in northern Europe.

Orientation — strain, stage and oxygen affect real tolerance.

Salinity and mineral balance

A major advantage of L. vannamei is its euryhaline nature: the species can be farmed across a wide salinity range. Optimal salinity often sits around 10–30 ppt; culture can go toward nearly freshwater and also full seawater (~35 ppt). At low salinities ion balance becomes critical — sodium, potassium, calcium and magnesium for osmoregulation, shell formation and growth. Many low-salt systems therefore need mineral supplementation.

Oxygen demand

• Desired DO: >5 mg/L.
• Critical level: <3 mg/L.
• Biofloc systems often need very strong aeration.
• Oxygen shortage quickly causes stress, reduced growth and higher disease sensitivity.

Biofloc technology (BFT)

Biofloc technology is one of the main innovations in modern shrimp farming. Adding a carbon source (e.g. molasses or starch) stimulates bacterial growth that converts ammonium into microbial biomass. Small flocs (biofloc) are kept suspended by intensive aeration and can be grazed by the shrimp.

• Advantages: lower water exchange, lower ammonia, natural protein supplement, high productivity per volume.
• Disadvantages: high aeration energy, complex operation, risk of oxygen swings, high particle load.

Biofloc is used across much of the tropical shrimp industry and is often seen as an alternative or complement to classic RAS technology.

RAS for shrimp culture

Recirculating systems enable production with very low water intake and high biosecurity. Typical components: culture tanks, solids filtration, biological treatment, CO₂ stripping, UV or ozone, oxygenation and temperature control. Advantages are especially large where land, water or biosecurity limit traditional pond culture. See also fry tanks for differences between fish and shrimp tanks (bottom area, substrate, moulting).

Biomass density

Maximum density is often limited by oxygen, biofilter and organic-matter handling. Orientation.

Feed and FCR

• High-protein pellets, typically about 30–40% protein.
• FCR often about 1.1–1.8.
• Feed cost often exceeds 50% of production cost.
• Automated feeding and camera systems are increasingly used to cut waste.

Growth and production cycle

Under optimal conditions L. vannamei can reach market size in a relatively short time. Growth is strongly affected by temperature, feed quality, genetics and water quality.

Orientation under good temperature. The production sizer counts from stocking (PL/juvenile) to harvest.

Disease and biosecurity

Historically disease has been the largest economic risk in shrimp production. Important examples: White Spot Syndrome Virus (WSSV), EMS/AHPND (Early Mortality Syndrome), IMNV (Infectious Myonecrosis Virus) and EHP (Enterocytozoon hepatopenaei). Many modern RAS projects exist specifically to reduce that risk through controlled water supply and strict biosecurity.

Economics and market

Demand is global — from low-cost production in tropical ponds to premium segments with locally produced fresh shrimp. Drivers for European RAS production include shorter supply chains, higher freshness, reduced antibiotic use, better traceability and lower environmental impact. Despite higher production costs, locally produced premium shrimp can often sell at substantially higher prices than imported frozen products.

Key sizing parameters

• Production (t/year), harvest weight (g), biomass density (kg/m³).
• Feeding (kg/day), FCR, temperature, salinity.
• DO, TAN, nitrite, nitrate, alkalinity and pH.
• Aeration and oxygen capacity; heat demand and energy use.

For land-based shrimp farms, temperature control, oxygenation and biofilter capacity are normally the three most important sizing factors.

Production sizer (pond / biofloc / RAS)

The production sizer estimates numbers, standing biomass and culture volume from target production, harvest weight, cycle time, survival and density (kg/m³). Pick a system preset for typical densities and cycle times (L. vannamei-oriented). Orientation for concept development — not bankable facility design.

Production sizer: shrimp / L. vannamei (orientation)

System Pond (lower density) Biofloc RAS (high density)

Tempo / target size Fast (~3.5 mo → ~20 g) Typical (~4.5 mo → ~25 g) Larger (~5.5 mo → ~35 g)

Target production (tonnes/year)

Harvest weight (g/shrimp)

Stocking / PL weight (g/shrimp)

Cycle time stocking → harvest (months)

Survival stocking → harvest (%)

Density at harvest (kg/m³)

FCR (kg feed / kg gain)

Shrimp harvested / year—

Stocked / year—

Cycles / year—

Shrimp per cohort (at harvest)—

Standing biomass (orientation)—

Culture volume (m³)—

Feed (tonnes/year)—

Cycles/year ≈ 12 / cycle months (short cycle → several cycles). Count/year = tonnes×1000 / harvest_kg. Standing biomass ≈ cohort harvest × 0.55. Volume = standing / density (kg/m³). Presets are PL/juvenile→market. Orientation — not bankable design; temperature, oxygen, biosecurity and mineral balance govern real density. In cold climates heat cost often dominates.

© 2026 DABCE AB

See also

• Temperature
• Salinity
• Dissolved oxygen
• RAS
• Ponds
• Fry and larval tanks
• Biosecurity
• Feed types
• Mineral and ion management

European sea bass (Dicentrarchus labrax) is one of the Mediterranean’s most important farmed fish species and has for decades been a cornerstone of European marine aquaculture. It is farmed mainly in Greece, Turkey, Spain, Italy and Croatia, traditionally in sea cages but increasingly also in land-based recirculating systems (RAS). High market demand, established feed programmes and good adaptability make sea bass an attractive species for modern mariculture.

Biology and distribution

Sea bass occurs naturally along Europe’s Atlantic coast and in the Mediterranean and Black Sea. It is a predator that feeds on smaller fish, crustaceans and other marine organisms. The fish is euryhaline and can tolerate large salinity variation, so it can be farmed in both marine and brackish waters.

Temperature

In northern Europe heating is often needed for maximum growth in land-based farms. Orientation.

Salinity

• Optimal salinity often about 25–40 ppt.
• Can be farmed in brackish water.
• Tolerates short-term variation better than many marine species.
• High tolerance gives flexibility when choosing a water source for RAS.

Oxygen demand

• Desired DO: >6 mg/L.
• Critical levels: <4 mg/L.
• High biomass needs effective oxygenation.
• Too-low oxygen → reduced appetite, slower growth and higher disease sensitivity.

Culture systems

Sea cages

The most common production form globally. Advantages: low capital cost, natural water exchange and proven technology. Disadvantages: exposure to disease and parasites, risk of storm damage and escapes, and limited control of environmental conditions.

Land-based RAS

Land-based sea bass production is growing. Advantages: full control of temperature and water quality, high biosecurity, less environmental impact on coastal waters and production closer to consumer markets. Disadvantages: higher capital cost, higher energy use and more advanced operation.

Growth

Sea bass has a moderate growth rate compared with many salmonids. Growth is strongly affected by temperature and feeding strategy.

Orientation under favourable conditions. The production sizer counts from stocking (juvenile) to harvest.

Feed and FCR

• Carnivorous species; high-energy pellets.
• Protein typically about 40–50%; fat about 15–25%.
• FCR often about 1.2–1.8.
• Modern feeds increase the share of plant ingredients, algae and alternative marine raw materials.

Larval rearing and fry production

The most technically advanced part is often the hatchery and fry phase: egg incubation, larval rearing with live feeds (rotifers, Artemia) and gradual weaning onto dry feed. Fry production is today well developed and commercially established across much of the Mediterranean — many grow-out projects therefore buy juveniles.

Health and biosecurity

• Bacterial infections.
• Parasites.
• Stress-related problems.
• Water-quality-related disease.

RAS is increasingly used to reduce infection risk and improve control of the production environment.

Market and economics

Sea bass is an established premium fish on the European market and appears in restaurants, fish counters and retail. The product is sold mainly as whole fresh fish, boneless fillets and chilled premium products. The market benefits from high consumer acceptance, established logistics, stable demand in Europe and good willingness to pay for fresh products.

Key sizing parameters

• Annual production (t/year), market weight (g), biomass density (kg/m³).
• Daily feed, temperature, salinity, DO.
• TAN, nitrite, nitrate, CO₂, pH and alkalinity.
• UV or ozone dosing; heat and energy demand.

Sea bass is today regarded as one of the most mature marine species for land-based culture. Strong market demand, well-established fry production and relatively well-known culture parameters make it one of the leading candidates for marine RAS production in Europe.

Production sizer (RAS / grow-out)

The production sizer estimates fish numbers, standing biomass and tank volume from target production, harvest weight, cycle time from stocking, survival and density (kg/m³). Presets are grow-out-oriented (juvenile → market). Orientation for concept development — not bankable facility design.