A short glossary for aquaculture, water quality, RAS and related technology. Terms are meant for quick lookup — deeper treatment is in the respective topic sections. Swedish and English variants are given where helpful.
A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · Q · R · S · T · U · V · W · Y
A
Aeration — Supply of air (~21% O₂) via diffusers, surface aerators or blowers. Cheaper than pure oxygen; also used for mixing and CO₂ stripping. Swedish: luftning.
Alevin — Newly hatched fish still living on the yolk sac before external feeding. Swedish: gulesäcksyngel.
Alkalinity — Buffering capacity of the water (usually mg/L CaCO₃). Critical in RAS because nitrification consumes alkalinity. Low alkalinity leads to unstable pH.
Ammonia (NH₃) — The toxic, un-ionized form of inorganic nitrogen. The proportion of NH₃ increases with higher pH and temperature. See also TAN.
Aquaponics — Combined culture of aquatic animals and plants in which nutrients from fish waste feed the plants and the plants help clean the water.
ASC — Aquaculture Stewardship Council. Voluntary sustainability certification for farmed seafood.
B
BAP — Best Aquaculture Practices. Certification programme for farms, feed mills, hatcheries and processing plants.
Biofilm — Microbial community growing on a surface. In a biofilter the biofilm is the active nitrifying or denitrifying surface.
Biofilter / biological filter — Unit that converts toxic ammonia to less harmful nitrate via nitrifying bacteria. Common types: MBBR, fixed-bed, trickling filter.
Biofloc (BFT) — Culture method using suspended microbial aggregates that take up nitrogen and can be grazed by the animals. Common in intensive shrimp farming.
Biomass — Total weight of cultured organisms in the system (kg or tonnes).
Biosecurity — Practices and measures that prevent the introduction and spread of pathogens. Especially important in intensive land-based systems.
Brackish water — Water with salinity between freshwater and seawater (roughly 0.5–30 ppt). Swedish: bräckvatten.
Broodstock — Sexually mature fish used for egg and milt production. Swedish: avelsfisk.
Bypass — Pipework or operating mode that routes water past a unit such as a filter, UV or ozone. Planned, or used as an emergency path.
C
Cage / net pen — Floating or moored net volume in a lake or the sea, where fish are cultured in the surrounding water. Swedish: kasse or nätkasse.
CAPEX — Capital expenditure. Investment cost to build the facility (tanks, treatment systems, buildings, etc.).
CO₂ / carbon dioxide — Gas produced by fish respiration and bacterial activity. High levels stress fish. Removed by degassers / CO₂ strippers.
CT value — Concentration × contact time (for example mg·min/L). Used when sizing ozone or other disinfection.
D
Dead zone — Poorly mixed area where water quality, for example oxygen, can be much worse than at the sensor.
Degasser / CO₂ stripper — Equipment that removes carbon dioxide, and sometimes nitrogen, from water, usually by exposing water to air. Swedish: CO₂-avdrivare.
Degree-days — Sum of temperature × time. Used to estimate development time, for example to hatch.
Denitrification — Microbial conversion of nitrate to nitrogen gas under low-oxygen conditions. Used to control nitrate in high-reuse RAS.
Density / stocking density — Biomass per volume or area, for example kg/m³. Affects oxygen demand, water quality and welfare.
Diffuser — Device that breaks air or oxygen into bubbles for gas transfer.
Dissolved oxygen (DO) — Oxygen dissolved in water (mg/L or % saturation). One of the most critical parameters for fish health and growth.
Drum filter — Rotating screen filter that removes suspended solids. The primary solids-removal step in most RAS. Swedish: trumfilter.
E
EC / conductivity — Electrical conductivity. A fast picture of total dissolved ions, not which ions are present.
Effluent — Water that leaves the facility to the receiving water or to further treatment.
F
FCR (Feed Conversion Ratio) — Amount of feed given divided by biomass produced. Key economic parameter.
Fingerling — Juvenile fish at stocking size, after the fry stage. Swedish: sättfisk.
Flow-through system — System in which water is used once (or with limited reuse) and then discharged. Swedish: genomströmning.
Fouling — Biofilm, algae or animals on nets, sensors, pipes and heat exchangers. Impairs flow, measurement and UV.
Fry — Early life stage after the alevin, when the fish has started external feeding. Swedish: yngel.
G
Gas supersaturation / TGP — Total gas pressure above equilibrium. Can cause gas bubble disease. Watched especially with pressurised aeration and groundwater.
H
Hardness — Concentration of calcium and magnesium (usually mg/L CaCO₃). Distinct from alkalinity. Swedish: hårdhet.
Hatchery — Facility for egg incubation and the early stages through to fry or fingerling. Swedish: kläckeri.
Head loss — Pressure or level loss across pipes, filters or packing. Often rises as a filter clogs.
HRT — Hydraulic Retention Time. Average time water spends in a tank or reactor.
Hypoxia — Too little dissolved oxygen for the species. Causes stress, poorer growth and, at worst, mortality.
I
IMTA — Integrated multi-trophic aquaculture. Several trophic levels are farmed together, for example fish, shellfish and algae, so nutrients are taken up again.
Intake / make-up water — New water added to replace losses, backwash or to dilute salt and nutrients. Swedish: spädvatten.
Ion — Charged particle in the water, for example Ca²⁺, Na⁺, Cl⁻ or HCO₃⁻. Fish exchange ions across gills, skin and gut.
Ion-selective electrode (ISE) — Sensor for one ion, for example ammonium or nitrate. Often needs frequent calibration.
J
Juvenile — Young animal between larva or fry and a sexually mature adult. Often the stage moved into grow-out.
K
Kelp / macroalgae — Large marine algae farmed for food, feed, materials or to take up nutrients, for example in IMTA.
L
Larva — Early free-living stage of many fish, shrimp and shellfish. Often highly sensitive to water quality and feed.
LHO (Low Head Oxygenator) — Device that adds pure oxygen with a relatively small elevation difference.
Live feed — Live prey, for example rotifers or Artemia, used especially in larval culture.
M
MBBR (Moving Bed Biofilm Reactor) — Biofilter with freely moving plastic carriers providing large surface area for nitrifying bacteria.
Mortality — Death rate, usually as a percent of stock or a count per unit of time.
N
Nitrate (NO₃⁻) — End product of nitrification. Much less toxic than ammonia or nitrite; accumulates in RAS.
Nitrification — Two-step bacterial oxidation of TAN to nitrite and then to nitrate. Consumes oxygen and alkalinity.
Nitrite (NO₂⁻) — Intermediate product in nitrification. Toxic to fish, especially salmonids. A common problem in a young biofilter.
O
Off-flavour — Unwanted taste in the fish, often geosmin or MIB from microbes. Often managed with treatment and a depuration period before harvest.
On-growing — Grow-out phase from fingerling or juvenile to harvest size.
OPEX — Operating expenditure. Ongoing costs of running the farm (feed, energy, labour, oxygen, etc.).
ORP — Oxidation-reduction potential. Often used to monitor ozone residual. It is not a direct ozone measurement.
Osmoregulation — The fish’s work to keep its internal salt and water balance against the surrounding water.
Oxygen saturation — DO as a percent of the equilibrium value at a given temperature, salinity and pressure.
Ozone (O₃) — Strong oxidant used for disinfection, removal of dissolved organics and improved water clarity. Residual must be removed before water returns to fish.
P
pH — Measure of acidity or alkalinity. Affects ammonia toxicity, biofilter performance and fish physiology.
Photoperiod — The daily light and dark cycle. It influences smoltification, growth and spawning in many species.
PLC — Programmable logic controller. Industrial controller for pumps, valves, oxygen and alarms.
ppt / PSU — Salinity units: parts per thousand and practical salinity unit. In farming they are often roughly interchangeable.
Protein skimmer / foam fractionator — Removes dissolved organics and fine particles by generating foam. Most effective in seawater or brackish systems. Swedish: proteinskummare.
PSA — Pressure swing adsorption. Technology that generates oxygen on site from air.
Pure oxygen — Gas with a high oxygen content, often above 90%, from a cylinder, tank or generator. Higher transfer than air.
Q
Quarantine — Isolation of new stock or new water streams so disease is not carried in.
R
Raceway — Long, narrow channel with directed flow. Common in trout and salmonid farming, usually as flow-through.
RAS (Recirculating Aquaculture System) — System that treats and reuses the majority of the water. Typical exchange is only a few percent per day.
Rotifer — Small zooplankton, for example Brachionus, used as live feed in marine larval culture.
S
Salinity — Concentration of dissolved salts. Affects oxygen solubility, ammonia toxicity and osmoregulation.
SCADA — Supervisory control and data acquisition. Overarching monitoring and control of sensors, processes and alarms.
Secchi depth — A simple clarity reading with a black-and-white disc. In ponds, a rough measure of turbidity and algae.
Sedimentation / settling — Particles that settle in a basin, cone or swirl separator.
Semi-flow-through — A hybrid of flow-through and RAS: some water is reused, but exchange is higher than in full RAS.
SGR — Specific growth rate. Usually percent body weight per day.
Sludge — Solid waste stream from aquaculture (mainly faeces and uneaten feed). Contains a large share of the nutrients that entered with the feed. Swedish: slam.
Smolt — Juvenile salmon physiologically ready for transfer to seawater.
Soft water — Water with low hardness, little calcium and magnesium. Can increase sensitivity to some metals and may need minerals added.
Speece cone / oxygen cone — Pressurised contactor that dissolves pure oxygen into water at high efficiency.
Stocking — Introduction of fish, shrimp or other organisms into a culture unit. Swedish: utsättning.
Sump — Collection and pump basin in a RAS or an aquarium system.
Swim-up — Stage when fry leave the bottom and the yolk-sac posture and swim up to feed.
T
TAN (Total Ammonia Nitrogen) — Sum of un-ionized ammonia (NH₃) and ionized ammonium (NH₄⁺). The toxic fraction is NH₃.
Temperature — Water temperature. Governs metabolism, growth, how much oxygen can dissolve and how toxic ammonia is.
Trickling filter — Biofilter where water trickles over packing with an airflow. Provides nitrification and some CO₂ stripping.
TSS (Total Suspended Solids) — Particles suspended in the water. High TSS increases oxygen demand and fouls biofilters.
Turbidity — Cloudiness, an optical measure of particles and colloids. Often a fast companion reading to TSS.
U
UPS — Uninterruptible power supply. Battery backup that keeps the PLC, sensors and alarms alive until a generator starts.
UV disinfection — Use of UV-C light (~254 nm) to inactivate microorganisms. No residual chemical.
UVT — UV transmittance: how much of the UV light passes through the water. Low UVT needs a higher dose or better pre-filtration.
V
Vaccination — Immunisation against specific diseases. An important part of preventive health on many commercial farms.
W
Water exchange rate — How quickly water is exchanged or turned over, for example percent per day or tank volumes per hour.
Weaning — Transition from live feed to formulated feed in larvae and fry. Swedish: avvänjning.
Welfare — Animal welfare: health, environment, behaviour and handling. Good water quality and low stress are the base.
Y
Yolk sac — Nutrient reserve of a newly hatched fish before it takes external feed. Swedish: gulesäck.
Definitions are orientation, not legal text or species protocols. Limits, toxicities and sizing depend on species, salinity, temperature and system design.