Aquaculture HelperTM

Aquaculture is the farming of aquatic organisms - fish, shellfish, algae and aquatic plants - under controlled or semi-controlled conditions. Unlike capture fisheries, growth, feed, water quality and harvest are managed by the farmer.

Aquaculture can be practised in freshwater, brackish water or seawater depending on species requirements and local conditions. Fish and shellfish can be farmed directly in natural waters - for example in cages placed in lakes, fjords, rivers or coastal waters. Farming can also take place in land-based facilities.

Traditionally many farms have been flow-through systems where water is taken from a nearby watercourse through ponds, raceways or culture channels before being returned to the environment. Modern aquaculture also includes recirculating aquaculture systems (RAS), where water is reused and treated in several stages with mechanical filters, biological treatment, degassing, UV or ozone. By controlling water quality, the farmer can create optimal conditions for fish growth while reducing water use and environmental impact.

Aquaculture is today the fastest-growing sector in food production and has become a central part of the world supply of fish and shellfish. According to the Food and Agriculture Organization of the United Nations (FAO), about 141 million tonnes of aquaculture products were produced globally in 2024, including fish, shellfish, crustaceans, algae and seaweeds. Of this, just over 100 million tonnes were farmed aquatic animals such as fish, shrimp and mussels. Aquaculture now accounts for more than half of world production of aquatic animals and is therefore one of the most important protein sources for a growing global population.

The world in brief

Asia dominates global volume. China is the world's largest producer with extensive freshwater and coastal culture (fish, shrimp, mussels, algae). Southeast Asia (e.g. Vietnam, Indonesia, Thailand) is strong in shrimp, pangasius and other warm-water species. India is growing fast in freshwater aquaculture and shrimp. Japan and Korea combine advanced technology with premium species, often in smaller high-value systems. Africa has large potential and expanding pond and tilapia culture, though infrastructure and feed chains vary widely between countries. North America combines traditional farming with investment in land-based salmonids (RAS). South America - especially Chile - is a powerhouse in cage salmon, with growing focus on technology and sustainability. Western Europe and the UK mix cage, raceway and land-based projects; Eastern Europe has ponds, pike-perch/perch and rising interest in modern systems. In the Nordic countries (where DABCE is based), cage farming is strong in Norway, while Sweden, Finland and other inland settings more often work with rainbow trout, Arctic char and land-based or semi-land-based systems - raceways, semi-flow-through and RAS.

See also the sections on production systems and species, and our DABCE FlowFarm model for concept sizing.

Aquaculture offers several major advantages for food systems, the environment and regional development. It can supply protein at industrial scale without increasing catch from already pressured wild stocks, and it can be located where markets, power and skills exist - including close to the consumer.

Local food production near the consumer

Land-based and semi-land-based farms can be located near cities, logistics hubs and power grids. That shortens transport chains, improves traceability and often strengthens the local economy (jobs, suppliers, services). It aligns with the UN Sustainable Development Goals, including SDG 2 (zero hunger) via more access to nutritious food, SDG 8 (decent work and economic growth) via green industry and jobs, and SDG 12 (responsible consumption and production) via more controlled flows and traceability.

Offset overfishing and pressure on wild stocks

When wild stocks are limited or overfished, farming can add volume without increasing fishing pressure in seas and lakes. Aquaculture does not replace responsible fisheries management, but it can close the gap between demand for fish and what sustainable fishing can supply - supporting food security and reduced hunger (SDG 2) as well as life below water (SDG 14).

Advanced culture technology and the water environment

More advanced systems - especially RAS and well-designed semi-flow-through - make it possible to treat and control water quality in ways traditional open-water cage farming cannot. Solids, nutrients and organics can be captured and treated before discharge; oxygen, CO2 and temperature can be held within good limits. The result can be lower load on the receiving water, more predictable water quality in the farm, and better conditions for fish health and welfare than production that relies only on the surrounding water body.

In summary - why it matters

• Stable protein supply when wild stocks are limited.
• Can offset overfishing by adding farmed volume under controlled conditions.
• Local food production near the consumer - traceability, logistics and regional economy (SDG 2, 8, 12).
• Advanced technology can reduce water loading and improve water quality versus traditional cage farming (SDG 6, 14).
• Circular flows: heat, sludge, nutrient recovery and more sustainable production (SDG 12).
• Jobs in rural and coastal regions.
• Reduced hunger, greater sustainability and economic advantages when farms sit near markets and infrastructure.

Aquaculture is not a modern invention. Already more than 4,000-5,000 years ago there is evidence that people actively held and harvested fish in constructed water systems - long before industrial feed, hatcheries and RAS.

Ancient Egypt - paintings and reliefs

In ancient Egypt, tomb paintings and reliefs show fish held and taken in constructed ponds and garden pools along the Nile - early pond culture rather than today's floating net pens. The literature often cites evidence around 2500 BCE (over 4,500 years ago) for built ponds, and more widely accepted scenes of line-fishing for tilapia in irrigated ponds around 2000-1500 BCE. Nile tilapia (Oreochromis niloticus) recurs in art and had its own hieroglyph. (See e.g. the review by Rogers 2024, Journal of Archaeological Research, citing Balarin & Hatton 1979; Brewer & Friedman 1989; Nash 2011; Costa-Pierce 1987.)

China - archaeological finds

In China, archaeology points to even older roots. At the Early Neolithic site of Jiahu (Henan), body-length distributions of carp remains have been interpreted as early managed carp culture/water-level control around 6200-5700 BCE - about 8,000 years ago, contemporary with early rice farming and water management (Nakajima et al. 2019, Nature Ecology & Evolution: "Common carp aquaculture in Neolithic China dates back 8,000 years"). Later written sources include Shang-period oracle bones on fishing in garden ponds and Fan Li's carp-culture treatise (Yang Yu Jing) from about the 5th century BCE.

Pond culture then spread and was refined across Europe and Asia over the centuries. Modern aquaculture accelerated in the 20th century with dry feeds, hatchery methods, cages and later RAS.

Rainbow trout and Atlantic salmon became global industrial species. In the Nordic countries (where DABCE is based), inland farming grew with raceways and ponds while cages dominated in Norway. Land-based post-smolt and food-fish RAS now drive a new investment wave.

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 where DABCE is based.

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 where DABCE is based.

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.

Floating nets in lakes or sea. Dominate Atlantic salmon. Sensitive to lice, escapes and ice/wind in northern inland waters. Need robust mooring and contingency plans.

Farms further offshore with more exposure and heavier structures. Potential for scale and less coastal conflict, but high technical and insurance complexity.

E.g. partial reuse (PRAS), semi-flow-through with strong solids and biofiltration on effluent, or land-based start + cage finish. The FlowFarm concept is a semi-flow-through example with reuse and effluent polish.

Temperature drives metabolism, oxygen demand, immunity and growth. Salmonids prefer cool water; heat increases stress and disease risk.

Rainbow trout often grow well around 10-16 C; risk rises sharply above about 20 C. Arctic char prefers even cooler water.

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 (approximate values)

* Theoretical equilibrium per Henry's law (fresh water, approximate rounded values). Actual production levels are normally lower due to transfer losses, biological consumption, hydraulics and safety margins.

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.

Common sensor principles: electrochemical (Clark-type / galvanic or polarographic membrane probes) and optical (luminescence / fluorescence quenching). Optical probes have become standard on many farms because they often need less maintenance, tolerate longer intervals between calibrations and do not consume oxygen during measurement. Electrochemical probes can be cheaper to buy but often need more frequent membrane, electrolyte and zero checks. Both types usually report mg/L and % saturation when temperature (and sometimes salinity) is compensated correctly - check that the sensor has proper temperature compensation and that % sat is calculated against the right reference (fresh/salt, elevation/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.

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 affects ammonia equilibrium (NH3/NH4+), metal toxicity and fish comfort. Large daily swings stress fish.

Target stable pH within species tolerance; link to alkalinity and CO2 control.

Alkalinity buffers pH and supports nitrifiers. Low alkalinity causes unstable pH in biofilters.

In freshwater RAS, bicarbonate/lime may be needed as make-up.

Calcium and magnesium affect osmoregulation and eggs/hatching. Very soft water may need remineralisation.

Fish excrete nitrogen mainly as ammonia. NH3 (unionised) is toxic; its fraction rises with pH and temperature.

Biofilters convert NH3/NH4+ to nitrite and nitrate. Monitor TAN together with pH.

Nitrite (NO2-) is toxic and impairs blood oxygen transport. Common at biofilter start-up or collapse.

Chloride can temporarily reduce toxicity, but the root fix is always biofilter function.

Nitrate (NO3-) is less toxic but accumulates in RAS. High levels need water exchange or denitrification.

Salinity drives osmoregulation. Salmon smoltify to seawater; trout is farmed in fresh or brackish water. Sudden salt steps stress fish.

Particles stress gills, shade UV and carry bacteria. Drum filters and good hydraulics keep TSS low.

Oxidation-reduction potential is often used with ozone. Excessive ORP damages gills; control and residual ozone handling are required.

A rotating drum with mesh (often 40-90 um) removes faeces and feed fines. Continuous backwash. Main solids step in modern RAS and semi-RAS.

Micron choice: finer mesh clears water but raises backwash and headloss. Balance against biofilter and UV.

Stacked discs as media. Compact alternative to drums for some flows and footprints.

Classic deep-bed filtration. In aquaculture, continuous sand filters (e.g. DynaSand class) are often used on effluent or polishing.

Floating plastic beads provide solids capture and biofilm area. Common in smaller RAS and hatcheries.

Moving Bed Biofilm Reactor: plastic carriers in an agitated tank. Robust nitrification, scalable, tolerates load swings.

Stationary media (plastic, rock, etc.). High area, but clogging risk if solids are not removed first.

Water trickles over packed media in air. Provides nitrification and some gas exchange. Needs pumping to the top.

Mainly seawater: removes surface-active organics via foam. Less common in freshwater salmonids but relevant for marine culture.

Ozone oxidises organics, colour and microbes. Powerful but needs dose control, off-gas handling and often UV/carbon after.

UV inactivates bacteria and viruses without residual chemical. Needs clear water (good UVT). Used on make-up, recycle and/or effluent.

Pressurised water-oxygen contact for high dissolution. Common when pure oxygen is used.

Oxygenates at low head with chambers and gas. Energy-efficient in many RAS designs.

Cascade or packed columns strip CO2 and can maximise air-side oxygen uptake. Basis of air + pure O2 strategies: air first, pure oxygen as additive.

Diffusers, surface aerators and blowers supply air. Cheaper than pure oxygen but lower transfer per flow - needs more water turnover.

Recover heat from effluent or hall air into process water. Critical for energy economics in cold climates.

Hold temperature in warm periods or for cold-preferring species (Arctic char).

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.

Controlled temperature, flow and darkness until eyed stage. Dead eggs are removed to limit fungus.

Stacked trays with upwelling flow - high density in small footprint. Common in salmonid hatcheries.

Shallow tanks with gentle hydraulics after swim-up. First feed, light regime and density are critical.

Iodine baths on arrival of eyed eggs reduce pathogen risk. Follow dose and time; rinse correctly.

Hatcheries are extremely sensitive to power loss. UPS, generator, oxygen reserve and alarms are standard requirements.

Salmo salar. Anadromous: freshwater to smolt, then seawater. Main species in Norwegian cages; land-based post-smolt and food-fish RAS are growing.

Temperature: cool water; avoid summer heat stress. RAS needs strict biosecurity and CO2 control.

Oncorhynchus mykiss. Robust, mature feed and market chain. Farmed in raceways, ponds, cages and RAS. Portion or large fish depending on market.

Typical culture 10-16 C. Pan-size often 12-18 months. Well suited to semi-flow-through and RAS (see FlowFarm).

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.

Several species farmed for meat and caviar. Long generation time, high value, strict traceability. Sometimes linked to premium niche projects (roe/caviar).

Warm-water species, fast growth, large global volume. Less relevant in cold Nordic inland sites without heating.

Various regional species (e.g. Clarias, Ictalurus). Hardy, often pond or tank systems in warmer climates.

Sander lucioperca. High European market value, demanding larval stage. Interest in RAS and pond culture in Europe.

Perca fluviatilis. Local interest in Europe; smaller industrial scale than salmonids.

Marine flatfish, land-based culture in Europe. High value, complex larval rearing.

Litopenaeus etc. Global industry in the tropics; biofloc and RAS for higher latitudes are developing.

Dicentrarchus labrax. Mediterranean cage and land-based culture. Marine species with established feed chain.

• Egg - incubation to eyed stage and hatch.
• Alevin - lives on yolk sac.
• Fry - swim-up and first feed.
• Fingerling - stocking size.
• Juvenile - rapid growth phase.
• Adult - food fish or broodstock.

Salmon also smoltifies before seawater. Trout is often grown entirely in fresh or brackish water.

Gills

Gas exchange (O2/CO2), ion regulation and excretion. Damaged by particles, ammonia, nitrite, ozone and high gas pressure.

Osmoregulation

Freshwater fish take up salt and expel water; reverse in the sea. Stress and wrong salinity cost energy and growth.

Stress, growth, reproduction

Chronic stress (poor water quality, handling, density) lowers immunity and FCR. Broodstock need specific light and temperature regimes.

• Fungus (e.g. Saprolegnia) on eggs and wounds.
• Bacteria: furunculosis, vibriosis, BKD, gill disease, etc.
• Viruses: IPN, IHN, VHS etc. - often notifiable.
• Parasites: Ich, Gyrodactylus, tapeworms, etc.

• Controlled intake of eggs/fish (certification, disinfection).
• Zoning, footbaths, equipment per area.
• Quarantine new animals before mixing with production.
• Vaccination where licensed vaccines and indication exist.

Good welfare means enough oxygen, low stress, gentle handling, suitable density and humane slaughter. Certification and law are tightening in the EU/Nordics.

FCR = kg feed / kg fish gain. Modern salmonid feeds can approach 1.0-1.2 under good conditions. FCR drives both OPEX and environmental footprint.

• Protein - growth and maintenance; fishmeal/alternatives.
• Lipids - energy; high-energy feeds common for salmonids.
• Minerals, vitamins, pigment (astaxanthin) per product goal.

Hand feeding, automatic and demand feeders, and camera-driven AI feeding. Goal: maximise growth without waste that loads water and FCR.

Sludge is faeces, feed fines and biofilm. Rich in organic carbon, nitrogen and phosphorus. Must be collected to protect biofilters, fish and the receiving water.

• Drum filter backwash, settling, cone-bottom tanks.
• Dewatering: geobags, belt filters, decanters, filter presses.

Hydrothermal carbonisation (HTC), anaerobic digestion (biogas), compost/fertiliser and phosphorus recovery support circular economy. Spreading and hygiene rules must be followed.

Oxygen demand depends on biomass, feed, temperature and species. Rules of thumb (kg O2 per kg feed or fish) are used in concept models; site measurement is needed for design.

• Aeration via degasser/diffusers - cheap, limited ΔDO.
• PSA - on-site oxygen generation.
• LOX - liquid oxygen, flexible but logistics-dependent.

Best practice: maximise air-side uptake in degassers, add pure oxygen to raise total ΔDO and cut water flow - do not replace all air oxygen with gas.

Excess total gas pressure (especially N2) causes gas bubble disease. Avoid supersaturated source water without degassing. CO2 control is linked to the oxygen strategy.

DO, pH, temperature, level, flow, ORP, sometimes online TAN/NO2. Calibration and redundancy matter as much as the sensor itself.

• Cameras for appetite and feed waste.
• Image analysis for counting and size distribution.
• Biomass estimators reduce handling stress.

PLC/SCADA controls pumps, valves, oxygen and alarms. Day/night, peak-feed and emergency recipes (oxygen, bypass) must be documented and tested.

CAPEX: tanks, water treatment, oxygen, building, power, project management. OPEX: feed (often largest), energy, oxygen, juveniles, maintenance, labour, sludge.

Use concept tools (e.g. FlowFarm) for early CAPEX/OPEX scenarios - not as bankable design.

• Revenue = sellable biomass x price (whole fish vs fillet).
• Power price and FCR dominate RAS sensitivity analyses.
• Risks: disease, equipment failure, permits, market price.

• Water reuse and clean effluent.
• Carbon footprint: feed, energy, logistics.
• Renewable power and heat recovery.
• Circular sludge and nutrient management.
• Certifications: ASC, GlobalG.A.P., KRAV etc. depending on market.

Alevin - Yolk-sac fry - newly hatched fish living on the yolk sac.

Biomass - Total weight of fish in the system (kg).

CO2 - Carbon dioxide from respiration; must be stripped in RAS.

Degasser - Unit that strips CO2 and can increase air-side oxygen uptake.

DO - Dissolved oxygen in water.

FCR - Feed Conversion Ratio - kg feed per kg fish gain.

Fingerling - Juvenile fish at stocking size.

Hatchery - Facility from egg to fry/fingerling.

LHO - Low Head Oxygenator - oxygenation at low head.

MBBR - Moving Bed Biofilm Reactor.

ORP - Oxidation-reduction potential - often linked to ozone.

PSA - Pressure Swing Adsorption - on-site oxygen generation.

RAS - Recirculating Aquaculture System.

Smolt - Juvenile salmon physiologically ready for seawater.

TAN - Total Ammonia Nitrogen - NH3+NH4+.

TSS - Total Suspended Solids.

UV - Ultraviolet water disinfection.

Welfare - Animal welfare - health, environment and handling.