The Free Online Aquaculture Dictionary



Biochemical oxygen demand. A measure of the organic substances/matter (and also chemicals which may use oxygen) present in the water. Measured in terms of the oxygen requirements of the organic matter in the water in units of mg/l. The oxygen concentration of a sample of water is taken before sealing it in a dark container, with no entrapped air, where it is held for five days at 20oC. The oxygen concentration is then taken at the end of the five day period, and the B.O.D. is the value of the oxygen at the start minus the oxygen at the end. For samples where the B.O.D. is thought to be higher than 7mg/l, the sample should be diluted with distilled water to avoid a false result as all the oxygen will be used up before all the organic matter is used up. Often several samples are taken at various dilutions to ensure a result. BOD is sometimes written BOD5 which indicates that the BOD was measured over a 5 day period. Some users may use different time periods, in which case the number of days is usually denoted (e.g. BOD3 = 3 days) but the 5 day period is the usual one used. The initials BOD may also be suffixed by "ATU" i.e. BOD5ATU. This indicates that chemicals have been added to the water to inhibit the oxidation of ammonia (nitrification). The amount of B.O.D. produced by fish is dependant on the quality of feed and the efficiency with which it is fed and converted. Typical values for high quality feed, which is fed well and converted well are in the region of 330 - 480 grams of B.O.D. for every kilogram of feed given.


Also known as diatoms. Free floating, microscopic unicellular algae surrounded by a cell wall which is highly impregnated with silica (SiO2 :nH2O). Around 10,000 different species, which comprise approximately 20% of all primary production. Typically brown in colour (as a result of their containing the carotenoid - fucoxanthin). Impact in two ways in aquaculture 

1. Some species (such as Skeletonema costatum (6 microns diameter), Chaetoceros calcitrans (2.5 microns diameter), and Thalassiosira pseudonana (5.5 microns diameter)) are cultured for feeding to rotifers and other zooplankton, which are then fed in the larval rearing stages of some species. 

2. Some of the larger species of diatoms in supply waters can also be responsible for damage to small fish, especially in the gill lamellae. The silicon outer shell of some species is very sharp, and when water is drawn across the gill for respiration, the diatoms can scratch and cut the gills. In extreme cases this can lead to mortalities. This can sometimes be the cause of apparent respiratory problems in spring and summer, when the fish are producing a lot of gill mucus in response, without any apparent pathogen or water chemistry problem. This problem is often mistreated as bacterial gill disease, with limited success. Mechanical filtration of water can remove many of the diatoms and prevent the problem.

Back flushing

The general term used to describe the process where filters are cleaned. Also sometimes called back washing. The washing can be either from flows in filters a being reversed, filter beds being agitated by air or some mechanical means, or the jetting of water/air onto screens to clean them. Although many of these operations are not technically correct under this heading, they all tend to be grouped into the back washing/flushing term

Back Washing

see back flushing


Usually single celled microscopic organism about 1 micron in diameter. See biological filtration, bacterial diseases.

Bacterial diseases

Diseases caused by bacteria. Bacterial diseases fall into two categories, those that affect specific areas of the body (localised) and those that affect the entire body (systemic). External bacterial diseases usually respond well to chemicals such as Chloramin T and the quaternary ammonium compounds. Systemic infections (such as Enteric Red Mouth Yersinia ruckeri) usually require antibiotic treatment. Many pathogenic bacteria are present in the body tissues and environment all the time, a trigger such as stress is often responsible for lowering the fishes defence mechanism (see Corticosteroid) allowing the bacteria to multiply.

Bacterial Gill Disease

A condition where the gill lamellae of the fish are infested by colonies of filamentous bacteria which cause the gills to produce excessive amounts of mucous and stick together. Both of these have the result of limiting the transfer of gasses across the gill surface and lead to respiratory stress. The disease, which in Salmonids and pond fish is usually caused by the bacteria Flavobacterium branchiophila although in warm water fish, the bacteria Cytophaga (formerly Flexibacter) columnaris is often the culprit.  Small fish tend to be more susceptible to bacterial gill disease (BGD) and infections often follow periods of stress. Fish can often be seen with their opercula sticking out due to the excess mucous build up of the gills, and breathing seems rapid. Cause of most BGD outbreaks in salmonids. Infections are often reported following exposure to high ammonia, but there can also be many other non-specific causes such as high suspended solids.

Treatment is usually by the use of quaternary ammonium compounds such as Benzalkonium Chloride at  1-2ppm (active ingredient) and are often followed by a treatment of Chloramine T or a similar chemical. The rational behind this is that the latter is better at killing the bacteria, but cannot effectively penetrate the thick mucous layers that have built up. In addition to the antibacterial effects, the quaternary ammonium compounds have the added benefit of "washing" most of the excess mucous off the gills). Care should be taken using quaternary ammonium compounds in soft water.

When BGD, caused by C. columnaris, occurs in pondfish, external treatments for the control of columnaris disease are used. Diquat at 8.5ppm, Copper sulphate at 0.5 ppm, or Potassium permanganate at 2 to 4 ppm can be added to ponds and allowed to dissipate over time. If copper sulfate or potassium permanganate are used, treatment levels may have to be adjusted, depending on water chemistry. In soft water, 0.5 ppm copper sulfate may be toxic; and if pondwater is high in organic material, potassium permanganate concentrations must be increased.

Bacterial kidney disease

Bacterial kidney disease (BKD) is a systemic infection, caused by the bacteria Renibacterium salmiarum that commonly causes high mortality in populations of both wild and farmed salmonids. First recognised in wild salmon from the river Dee in Scotland in1933. As a result of this it is sometimes called Dee disease. Brook trout (Salvelinus fontinalis) would appear to to be the most susceptible species followed by Brown trout (Salmo trutta) and Atlantic salmon (Salmo salar). Causes most problems in fish which have been infected and are transferred to sea, due to the damage caused to the kidney, which makes the fish unable to cope with the changes in the osmoregulatory system.  This can lead to rapid mortality. In freshwater the disease is typically chronic, but acute outbreaks sometimes occur, especially at temperatures between 13 - 18C. Due to the grey/white lesions that occur in the kidneys, spleen, and liver; BKD has also been called white boil disease. External blisters in the skin, exophthalmia and small ulcers are typical. Often associated with infections of diphyllobothrium. Gram positive bacteria, resistant to most antibiotics. Some success has been found with Erythromycin


A vaccine that is prepared from bacteria which have been inactivated by a heat or chemical treatment which does not affect the cell antigens.


A chemical that kills bacteria


A chemical which inhibits the multiplication of bacteria and so enables the animal to cope more easily with the infection.


A device, such as a plate or screen, installed to alter / reduce the velocity of water.

Bag Filters

Enclosed vessels designed for the removal of fine solids. A mesh, or felt bag is inserted in the vessel and water passes from the inside to the outside. Filtration to less than 1 micron is possible with such filters although the cost of replacement of the bags, and the cost of pumping (due to the headloss) is high. Useful for applications such as marine hatcheries where very finely filtered water, in relatively small volumes is required. Bags may be dried out and used several times, but there is always some residue left in the bag, which limits it's performance the next time round. Depending on solids loadings and how much the bag gets clogged before it is cleaned, one can typically get three to six uses from a bag , before it needs replacing.


See Net Bagging


A fleshy protrusion from the body, usually the mouth. Used by some fish to feel for food.

Barometric pressure

The same as atmospheric pressure. The force exerted by the atmosphere at any point. Most commonly expressed in mmHg (millimetres of mercury) from the original equipment used to measure this which consisted of a tube of mercury which rose or fell with changes in the pressure.

Barr marks

Vertical strips of colour on fish


A chemical that has the ability to react with acids to form salts

Base metabolism

The minimum amount of energy required for the body to perform it's vital life functions i.e. excludes the energy required for growth, activity etc.

Bath Treatments

Chemical treatments of the fish, where the flow to the tank/pond is closed off or a tarpaulin is pulled around a cage. The chemical is added to the water, and the tank remains without any inflow or outflow for a pre-determined period, (often an hour or so). Oxygenation or aeration is usually applied to a tank, whilst the water flow is turned off. The main advantage of bath treatments is that they are easy to administer, they require no special equipment (such as dosing pumps) and they generally use less chemicals than flush treatments.


Order of fish (part of the superorder Paracanthopterygii) includes fish such as Toadfish (Opsanus)

Beer's Law

The light passing through a coloured liquid decreases as the concentration of the substance dissolved in the liquid increases. See also spectrophotometer.


Gas bubbles formed in the blood stream and tissues. See gas bubble disease


Not a risk to health.


Collective term for organisms living in or on the sediments.

Benzalkonium Chloride

Sometimes referred to as Roccal, or B.A.C.. A quaternary ammonium compound used as per Hyamine 3500. Typically supplied as a 10,20 or 30% solution, so care should be taken when using the chemical to ensure that the right amount of active ingredient is used. Dose rates are typically 1ppm in poorly buffered water to 4ppm in well buffered water for a period of one hour.


A commonly used and widely available anaesthetic (ethyl-4-amino-benzoate). Administered at a dose rate of 50mg/l, with the benzocaine first being dissolved in a small amount of alcohol or acetone before adding to the water.


Order of fish (part of the superorder Acanthopterygii) includes fish such as squirrelfish (Beryx)


A salt of carbonic acid (i.e.HCO3). Having an HCO3 group, such as sodium bicarbonate (NaHCO3)


Alkaline liquid produced by the liver and stored in the gall bladder. Assists in the digestion and absorption of fats by the action of bile salts, which chemically reduce fatty substances an decrease the surface tension of fat droplets so that they are broken down.


A substance used in the production of feeds to help the pellet stay together. Such substances include alginates and molasses.

Biochemical Oxygen Demand




A chemical which will indiscriminately kill any life in the water. In addition to chemicals whish are simply toxic to all life for (example derris), this also includes chemicals (such as cyanide) which take all the oxygen out of the water.


see Biological filtration

Biological Filtration


The growing of bacteria colonies on a media surface over which the water passes to remove nutrients form the water. Used as an essential part of most water recirculation systems and also sometimes for treatment of outlet water from a farm to reduce waste loadings entering a river or stream to comply with regulations. Although the process of biological filtration handles many different types of waste, the main ones that we are concerned with in aquaculture are BOD, ammonia and nitrite. The BOD is oxidised by a group of bacteria called heterotrophic bacteria. These are fast growing, dominant bacteria which often comprise a high percentage of the "sewage fungus" found in tanks, pipes, sumps, channels etc. The ammonia is converted to nitrite by a group of bacteria called nitrosomonas bacteria, the nitrite is converted to relatively harmless nitrate by a group of bacteria called nitrobacter. The process of conversion of ammonia to nitrite and then nitrate is commonly known ad nitrification. Because the heterotrophic bacteria are more dominant than the nitrification bacteria, the fist 25% of biofilters often comprises a high percentage of heterotrophic bacteria and a small percentage of nitrification bacteria, the rest of the filter being mainly nitrification bacteria. Because of the ratios of BOD to ammonia in fish waste, and the way that a biological filter functions, it is generally held that if ammonia levels are kept in check by biological filtration, then the BOD will also be kept in check. The ammonia is therefore used as the main indicator that full biological filtration is taking place. There are three main types of biological filter ; trickle, submerged and fluidised. The sizing of biological filters is calculated by knowing the surface area of the media which is being used, for example pre-formed, plastic media typically has a surface area between 100 and 800 m2m3 (depending on the design). Once the amount of ammonia that the fish are producing has been calculated, the media surface area required can be calculated by using the rule of thumb 0.4 - 1.0 g ammonia removed per day per m2 of media surface area. This figure equates to temperature operation of between 10oC - 30oC respectively. Although the process of biological filtration involves changing the water quality, sudden changes in the incoming water quality to the biological filter can "stress" the bacteria lead to poor performance. Such changes may include sudden changes in pH, flow rates, temperature etc. The processes of oxidation of BOD and nitrification are aerobic processes. Another type of biological filtration which is sometimes used in aquaculture, especially in high rate recirculation systems is denitrification. Denitrification filters convert nitrate to nitrogen gas, the bacteria in such filters are anaerobic

Biological Oxygen Demand



The total weight of all living organisms. Note that for a recirculation system, the biomass should include bacteria in the biofilters. May sometimes however be used in texts to refer to the farmed stock, rather than the total of all living organisms.


A chemical which prevents organisms from multiplying without necessarily killing them, which makes it easier for the body to rally it's natural defences and deal with them.  The used of biostatic chemicals rather than those that kill the organisms can result in increased immunity to subsequent outbreaks.


Term used to describe the use of any living organisms in industrial and production processes.

Biotic index

A figure derived from tables of species  with scores against them, relating to the quality of water. Used by many authorities to determine the pollution effect that a farm is having on the watercourse. Points are awarded for the type of species (those that prefer clean environments getting a higher score), the number of each species found in a sample (higher scores for high numbers) and the number of different species found (higher scores for greater number of species). The difference in scores between the inlet and outlet determines the level of impact on the watercourse. This method is often preferred by farmers as it is more generally representative of the impact rather than water quality samples, which may reflect a particular practice (such as tank cleaning) at the time of sampling. The results are open to speculation as the method of sampling may differ between operators . For example if  a kick sample from a stream is taken (where the bed is disturbed and a net held downstream) the results will be dependant on the force used and duration of agitation of the bed.


One of the water soluble B-complex vitamins.


Fish eating birds can cause great damage to a fish farm. The damage is only partially in the animals eaten by the birds. For each fish successfully caught and eaten, many others are damaged (see picture). In the case of shellfish (e.g. mussels on ropes), species such as the Eider duck can strip the ropes, partly though consumption but mainly through knocking several animals off the ropes for each animal eaten. Birds also present a threat through transmission of disease to the farm from other farms and watercourses in the area. They can also play a role as an intermediary host in some diseases such as eye fluke. Wherever possible, birds should be eliminated from entering the farm by use of netting or other scaring devices. Netting is the only complete solution as birds will very quickly become used to most scaring devices and choose to ignore them.

Bird Netting

Many farms are plagued by fish eating birds. These are best excluded by the complete covering of the farm with netting, which although more expensive to install than wires or strings is more effective.


(see Nipping)


Common term for animals in the class Bivalvia Also sometimes referred to as lamellibranchs which is actually a sub-class of Bivalvia, and contains most of the families and almost all the edible forms.


Class of animals. Part of the phylum Mollusca Freshwater, brackish and marine forms. Shell is comprised of two hinged valves under which lie two large gills. Class includes fixed forms such as mussels and clams, and also burrowers such as shipworm and razorshell.


see Bacterial Kidney Disease.

Black Spot

Often used to refer to the black cysts caused by the intermediate stages of trematodes in fish. 


A ball of cells seen in the early stages of development following fertilisation. See also Cell Division


(Sodium Hypochlorite) Chlorine based chemical which is sometimes used in aquaculture for footbaths etc. Their efficiency and period of effectiveness is not as good however as iodine based products as it is inactivated by organic matter quicker. They are also used for disinfection of tank walls, floors etc., however the chlorine (which is the "active ingredient") will often evaporate out of the solution as it is swilled around the tanks or across floors. The rapidly reduces the effectiveness of the product, and can lead to incomplete disinfection. Very corrosive to metals, avoid contact.


Suborder of fish (part of the order Perciformes) including fish such as Blenny (Blennius)


See algal bloom


Devices which provide pressurised air, typically at pressures between 50mbar and 500 mbar. Most blowers use a turbine principle to provide the air pressure. Blowers have the advantage of being able to be all but closed off on the discharge side of the blower without damaging the blower or the discharge pipe. This is not the case with compressors. Most types of blower can also operate as low vacuum air suction pumps. Typical applications for blowers include; vacuum degassing, aeration using coarse or medium bubble diffusers, aeration using some makes of fine membrane diffusers in shallow water, provision of air for fractionation and bubble degassing of carbon dioxide. See also friction loss-air.

Blue sac disease

A condition of sac fry (alevins) in which the yolk sac takes on a blueish colour. Brought on by a lack of oxygen and / or high Carbon dioxide concentrations which limit the uptake of oxygen into the bloodstream.

Blue tinge

Irritation of the skin on fish can cause an excessive production of mucus which can give the fish a pale blue colour, especially when viewed from above in the water. Particularly associated with the parasite costia Can also be brought on by malnutrition, especially a lack of biotin in the diet.. Should not be confused with the blue colour that some naturally sterile and triploid fish (especially salmonids) can exhibit.

Blue-green algae

see cyanobacteria

Bohr Effect

Discovered by C. Bohr (1855-1911). High concentrations of carbon dioxide in the blood, causes the blood to become more acidic. This has the effect of inducing the haemoglobin to release any bound oxygen in to the tissues, but has the opposite effect at the lungs/gills, where the haemoglobin is less inclined to take up oxygen. Can lead to respiratory stress and exhaustion as the fish has to breath harder to maintain it's oxygen supply. Can be caused by high carbon dioxide levels in the water, which inhibit the release of carbon dioxide from the haemoglobin into the water.

Bony Fishes

A common term used for Teleosts


A shaft drilled into a water bearing rock or fault line in an impervious rock formation. Water is then either pumped out of the rock, or exits as a result of natural geological pressures (artesian well). Boreholes are usually sunk for freshwater supplies, but some farms sink boreholes close to the sea where they can pump seawater as it is drawn inland through porous rocks or sand / gravel. The pumping of seawater from boreholes can often bring about conflict with agriculture as, the increasing salinity of the ground water around the borehole area (salinisation), can lead to crop failure. Borehole water is sometimes wrongly assumed to be 100% disease free. Many borehole supply sources are from underground streams and channels, which may carry viruses and other pathogens. This is especially so if they are located near to a farm with earth ponds, where water from the ponds, seeps back into the ground. Borehole supplies are often high in dissolved nitrogen and carbon dioxide and low in oxygen. As a result of this they should always be degassed before use. The cost of pumping from borehole usually prohibits their use in ongrowing systems, but they are used widely in hatcheries. Water quality from boreholes tends to stay constant with temperatures higher than ambient in the winter and lower than ambient in the summer (for example almost all boreholes in temperate areas will have a temperature between 9 and 11oC all year round). see also water table and aquifer



Group comprising the crabs. Includes important aquaculture species such as Scylla serrata, the mud crab. The culture of crabs is at present restricted to either holding facilities, or facilities where the animals are held until they moult and are then sold as soft shell crabs.


Slightly salty. Used to describe waters which are nether freshwater or seawater, but have a salinity which falls between the two (usually between 0 - 25 ppt.). It may also be used to describe areas (e.g. estuaries) where, due to tidal exchanges, the environment fluctuates between seawater and freshwater.


The gills (singular - branchia)


The part of the skeleton that the gill arches are attached to


see gill rot


Salty water. Generally regarded as water with a salinity of greater than 45ppt.

Brine Shrimp

see Artemia


One of the most common chemicals found in seawater (full seawater contains 65mg/l bromine). Often left out of artificial seawater mixes where ozone is used in a system, as the production of hypobromous acid (a reaction of the bromine with the ozone) can cause toxicity problems to the fish

Brood Chamber

An area formed on the underside of some crustaceans to carry fertilised eggs prior to hatching. Also occurring in flat oysters, where the eggs are incubated on the gills.


A fish which is being kept with the intention of using it for reproduction. Broodstock are usually kept at very low stocking densities in an effort to reduce stress, promote growth rates and reduce the risk of diseases.


Animals which eat continuously. Bites are not very well defined. Includes mostly herbivorous species. See also Grazers

Bubble Curtain

A constant stream if bubbles provided by a submerged diffuser (usually a tube type), which surrounds a specified area. the action of the bubbles forms a "wall" through which most fish will not swim. It is used for maintaining fish in an area, when screens or nets cannot be used. Fish may swim through the "curtain" in a fright response.

Buccal Cavity

The mouth cavity

Buccal force plate

The plate feature at the bottom of the mouth which is controlled by muscles to pump water through the gills and also to give a coughing action to prevent choking or clogging of the gills

Buccal incubation

The incubation of eggs held in the mouth by one or both of the parent fish. The movement of water through the mouth and across the gills keeps the eggs supplied with oxygen and also keeps the eggs in motion, preventing the build up of fungus or other infections. If stressed (especially if the fish feels threatened) the eggs may be swallowed. Common term for this is mouthbrooding or oral incubation . Practiced in particular by some cichlids such including tilapia (e.g. Oreochromis mossambicus)

Budget Quotation

An approximate quotation. Often used by companies in the initial dealings with a customer to give a guideline of the price. The budget quotation saves companies the effort of detailed quoting, whilst giving the customer an idea of price. If the budget quote is in the region of the price that the customer expects to pay, more information is usually sought by the supplying company before a full quotation is offered. Used more often in the supply of systems, rather than individual items of equipment where the price tends to be fixed


The process of adding carbonate and/or bicarbonate ions to water to reduce acidity and increase pH. Buffering systems are often used in recirculation systems , to counter the acidity produced during the nitrification process, and also the acidity caused by the carbon dioxide in the water. Systems usually involve using a type of lime which is either dosed into the water once it as been dissolved, used in a pelleted form with the water flowing through it (often in a fluidised bed), or added directly to sumps etc. in granular or powdered form. The effect of a buffering system used on the farm is that the operator will se a rise in the pH of the water. Systems may be manually controlled or automatically controlled by feedback from a pH controller


Almost any building type can be used to house an aquaculture facility, however some aspects which can improve working and operating conditions are as follows; 1. Ventilation - poorly ventilated aquaculture buildings become very humid, and provide a perfect breeding ground for fungal spores and bacteria on spilled food etc. They are very unpleasant places for staff to work which can often result in poor staff performance. 2. Insulation - Note should be taken on the temperature of operation of the farm, especially if water is being recirculated, which can lead to a build up of heat in the system. The heat gain in to the building should be taken account of as well as the heat loss. 3. Wind loadings and snow loadings should be taken account of. One of the most common cost effective means of housing aquaculture facilities is in polytunnels. These can be built to varying degrees of insulation and lightproofness


An area of containment. Used to describe features such as dams, pond walls and flood banks. Also used to describe areas for containment of chemical spillage.

Butt welding

A process of joining two pipe sections together using a heat source to melt the pipes, and allowing the melted plastic form each pipe to amalgamate, and then letting the joint cool to form a hard plastic again. Special equipment is usually required to provide the heat for an effective butt weld. The most commonly butt welded material are polypropylene and polyethylene

Butterfly Fillet

A double fillet, which leaves both sides of the fish attached above the backbone


Synthetic rubber compound often used in sheet form for lining ponds etc. Ability to vulcanise makes it more suitable for large ponds where multiple sheets are required. Unlike some other compounds it degrades over time in sunlight, especially where areas are exposed for long periods without covering of plants, soil or water


The threads that some molluscs (such as blue mussels - Mytilus edulis) use for attachment to a substrate or cultch