The Free Online Aquaculture Dictionary



Small crustaceans <1.5mm that comprise an important group in freshwater zooplankton. Commonly called "Water Fleas". Of importance in live feed for freshwater species, especially aquarium fish, where the daphnia are bred for sale in aquarium shops. Rarely used in intensive aquaculture due to difficulties in breeding when compared to artemia.


The recording of a series of readings/measurements. May be carried out either by stand alone instruments which are set to record data into a memory, or directly into a computer where data is stored in the computer's memory. Almost any type of data can be recorded, including water quality parameters, the times when equipment starts and stops and fails, the times when alarms are responded to etc. One of the most frequent problems associated with datalogging is that far too much data, than can be effectively used, is recorded. This often ends up in none of the data being used as it is too big a task to separate the meaningful data from that which is unmeaningful. Before starting to log data, time should be taken to think carefully about the reasons why you are datalogging, and how that data is going to be used at a future data, to avoid the pitfalls of ending up with an overwhelming amount of logged data.


Important to aquaculture for a number of reasons ; 

1. Many species will only feed during daylight hours, and therefore by increasing the length of the daylight hours by using lights, can often result in faster growth rates or better food conversions rates as their feed is spread out over a longer period. 

2. Spawning control - many species such as the salmonids use the daylength as the trigger for starting the maturation and spawning process. Manipulation of daylength (photoperiod control) can help the farmer obtain eggs all year round, by artificially adjusting the seasons that the fish are experiencing. 

3. Daylength also acts as a trigger for other physiological processes, for example the smolting process of Atlantic salmon is governed largely by daylength. 

See also light intensity


The largest group of the crustaceans and the most important in terms of crustaceans and aquaculture. Includes most of the cultured species including shrimps, prawns, crabs and lobsters. Characterised by five pairs of walking legs, the first of which are adapted with pincers. Often divided into the Natantia and Reptania. The former being swimmers and including the Penaeids (shrimps) and the latter being crawlers and including the Brachyura (crabs). Distinguished from other Malacostracans in that the first three pairs of legs are adapted as feeding appendages (or maxillipeds)


The removal of chlorine from water. Chlorine is often added to the water by domestic water suppliers and also by industries who use river or seawater for cooling, where the chlorine is added to reduce fouling in the pipes. Chlorine removal is commonly achieved through three methods, air stripping, addition of sodium thiosulphate and by filtering through activated carbon beds. Air stripping involves using vigorous bubble diffusion or aerated packed columns where the chlorine dissipates into the air. The process is slow, especially when the chlorine concentrations are low at the start and efforts are being made to reduce them further.. Sodium thiosulphate can be added to the water and will require 7mg/l of sodium thiosulphate for every 1 mg/l of chlorine. Sodium thiosulphate will also however, remove oxygen from the water and so should be used with care and a reliable control system. Activated carbon is also used, but it's high costs generally limit it to small hatcheries and research systems. Activated carbon has the benefit that it will also remove other chemicals added to municipal water supplies such as fluorine. Note that the chemical  chloramine, used for treatment of fish behaves differently to chlorine, and cannot be removed by carbon.

Dee Disease

See Bacterial Kidney Disease


The lack of a substance that is required by the animal.


A process which is used to remove undesirable gasses, that because of processes which have occurred, are present in greater concentrations in the water than would otherwise naturally be found. When they are in this state, such gasses are termed supersaturated gasses. A supersaturated gas has a natural tendency, when exposed to an interface between air and water to escape out of the water and into the surrounding air. (This is like in fizzy drinks, where whilst enclosed in a can and under pressure the gas stays in solution, when the can is opened the pressure of the liquid reduces suddenly. The ability of the drink to hold such an amount of gas is reduced and the gas has a route to escape through the surface of the drink. The gas forms bubbles in the glass as it tries to escape. The main function of a degasser is to create a large interface between the water and the air, this is achieved in a variety of ways, by heavy aeration, where the surface area of the bubbles creates a large interface as they rise through the water. Aeration also creates a lot of turbulence, bringing water to the surface where the gas can escape directly to the atmosphere. Letting the water fall over weirs and cascades, where it is broken up into droplets, thereby increasing the interface also degasses the water. Packed columns, which are vessels filled with a type of media over which the water runs are also commonly used. Air is drawn through the column against the film of water which coats the media. To achieve full degassing, as is desirable with supersaturated nitrogen, it is necessary to create a vacuum at the air water interface, this has the effect of "drawing" the supersaturated gas out of the water at a faster rate than would otherwise be achieved. These devices are known as vacuum degassers. See also carbon dioxide, nitrogen, gas bubble disease.

Degree days

A value used, especially during egg incubation, to estimate and predict the various stages of development. Calculated by multiplying the average temperature by the number of days. For example 300 degree days may be 30 days at 10oC, 100 days at 3oC or any other multiple that results in 300

Demand Feeder

A feeder which is operated by the fish, usually by a rod extending down into the water. The fish hit the rod and food falls into the water. Often associated with wasted feed (as more feed than is required can be released, especially if the feeder is knocked accidentally) and a large difference in the growth rate of fish in the tank (as some become more dominant and so restrict the access of other, smaller fish to the feeder area. Useful if fast growth is required, where food conversion is not the main concern.


Living and feeding in the water column (i.e. rather than at the surface or on the sea bed)

D-ended tanks

Type of holding unit. Very economical in terms of space, can be constructed from most materials, including fibreglass and concrete. Enables a lower tank turnover time, without compromising velocity rates and self cleaning abilities. Inlet pipes and aeration / oxygenation devices are positioned to create the desired water velocity rate. Useful in situations where space and make up water are limited.


The process where bacteria are used to convert nitrate into nitrogen gas. There main use is in high rate recirculation systems , where the amount of water brought into the system, is insufficient to dilute the nitrate levels (produced as a result of nitrification by the biological filters) to concentration which are safe for fish. The denitrification process is an anaerobic process and requires a carbon source for the bacteria to perform their task properly. The most common carbon source used is methanol, which is dosed into the systems. 1.90 mg methanol required to provide sufficient carbon to remove 1 mg nitrate. As it is an anaerobic process, the oxygen concentrations in the denitrification filter must be kept below 2 mg/l. In addition to providing a carbon source, methanol can also be used for lowering he oxygen concentration as the water enters the filter. 0.67mg methanol is required to remove 1mg oxygen from the water. It is not necessary to put the full flow of a recirculation system through a denitrification filter, only a small percentage (in the region of 4-8% of the system flow) is required to keep nitrate levels at a minimum. A general rule of thumb is that a properly functioning denitrification filter will remove 0.232 grams of nitrate per m2 of media surface area.


Term used to describe a fish with a vertically flattened body such as types of flatfish and rays.


Referring to the skin tissues


Plant of which the extract is used to produce Rotenone. Can be used as a pesticide in plant form, though not as certain as using prepared rotenone. Used at a rate of 75g/hectare/15cm water depth


The removal of salts and minerals form water (usually seawater) to make freshwater. Desalination plants use reverse osmosis technology to produce water with a salinity of typically <0.1parts per thousand. They are expensive to operate, primarily due to the cost of pumping through several stages of filtration and the reverse osmosis systems itself. They are used mainly in marine hatcheries for species where salinity is critical (for development or broodstock conditioning) and on remote farm locations for domestic water supplies. Operating costs are dependant on the quality of water and level of filtration required. The cleaner the water, the lower the operational costs and the simpler the system design.

Desoxycorticosterone acetate

Chemical sometimes used in the artificial spawning (hypophysation) of fish. Not used often due to harmful side effects, such as infections under the skin following injection.


The addition of flow devices to a large body of water to eliminate the stratified layers and mix the water. This decreases the risk of low oxygen areas and provides a healthy environment in all areas of the pond. usually carried out by using aeration devices. The best systems will mix all the water (including bottom, middle and surface waters) together.


The dead and/or decaying remains of plants and animals.


22:6n-3 highly unsaturated fatty acid (HUFA). Essential for some species such as carnivorous marine larvae. Rotifers can accumulate DHA but artemia cannot. See EFA.


See bacillariophyta

Diatomaceous Earth Filters

Diatomaceous Earth is a granular media, made up of the skeletal remains of diatoms (bacillariophyta). They are sometimes used as an alternative to cartridge filters, where very fine filtration is required. Available in different grades, which will filter to different levels, diatomaceous earth is available to filter as fine as 1 micron. The filters are comprised of three main items; 1. The centre core, which is a porous structure (often plastic) 2. The filter sleeve, which is a fine mesh that fits over the core. These are then housed in a vessel. When starting up, clean water mixed with Diatomaceous Earth is pumped into the filter and as the water passes through the filter sleeve, the Diatomaceous Earth coats the sleeve, the water to be filtered is then pumped into the filter (with Diatomaceous Earth) and as it passes through the filter sleeve, the layer (or cake) of Diatomaceous Earth gradually increases. There comes a point where the cake of Diatomaceous Earth becomes so thick that the water cannot pass through any more and the filter must then be backwashed. This requires very little pressure and the dirty Diatomaceous Earth cake is washed to waste. There are also some Diatomaceous Earth filters which are available as cartridge filters, where the cartridges are already coated with Diatomaceous Earth. In general the cost of purchasing and operating Diatomaceous Earth filters is high and only feasible for hatcheries where high value species are being reared.


Herbicide used for the control of submerged weeds. Applied at; Depth <0.6m - 5.5 kg / ha, Depth >0.6m - 10 kg / ha. Note: Dose rates are for the active ingredient. Formulations will contain diluted amounts. Adjust dose accordingly. 


Organophosphate pesticide added to water to treat pathogens such as sea lice (lepeophtheirius), typically used at a concentration of 1 mg/l for 1 hour. Most common trade name is Nuvan. Very toxic to humans and also shellfish beneath, and in the vicinity of cages. Now banned in many countries.


A 24 hour day that includes a day and night period

Diesel Fuel (1 of 2)

Used as an insecticide on small ponds. Sprayed on to surface to prevent oxygen transfer across the water / air interface. Also inhibits animals from penetrating water surface to breath. Used for the control of mosquito larvae in some areas.

Diesel fuel (2 of 2)

Used a s a herbicide - to control floating weeds. Applied at 470 litres per ha, sprayed directly on to floating weeds 



Device for producing bubbles. Diffusers can generally be divided up into four categories : coarse bubble (>5mm diameter), medium bubble (2-5mm diameter), fine bubble ( 1-2mm diameter) and ultra fine bubble (<1mm diameter). The bubble size produced is dependant on may factors; 

1, The aperture or pore size of the material - the smaller the diameter of the hole, the finer the bubble. Some membrane diffusers have slots which increase in diameter as more pressure and more air flow is applied. 

2. The air flow rate - In general the higher the air flow rate and air pressure, the larger the bubble size.

3.The depth of water that the diffuser is to be used  Bubble size in some diffusers, especially the membrane type, will decrease as the water deepens

4.The water quality - in general the more organic matter in the water the coarser the bubbles. the more saline that water becomes, the smaller the bubbles and the more efficient the diffuser.


A function with only two states. Typically ON or OFF. It cannot give a proportional output, such as instruction a valve to be 30% open….40% open etc. It can only instruct the valve to be either open or closed. Digital outputs use relays to change their state. See relays for more details of their functions. See also Analogue


A substance which is used to dissolve and dilute another substance. E.g. in a solution where a small amount of chemical is mixed with a bucket of water so that it can be spread over a large area, the water in this case is the diluent.


Diverse group of unicellular algae. True dinoflagellates possess flagellae, however some non-motile algae are included (such as filamentous).

Di-n-butyl tin oxide

Chemical used for treatment of tapeworm infestations. Usually dosed in feed at a rate of 250mg/kg body weight per day for a period of five days.


A short treatment (usually less than 2 minutes), where fish are immersed in a chemical solution for a short period, before being returned back to their holding unit. Used for the treatment of some external parasite diseases and for some vaccination methods. Useful for treating individual fish, without affecting the entire tank, or in situations where the chemical in use is expensive and so only a small quantity can be used.

Diphyllobothrium sp.

Tapeworm. Parasite of the gut. Able to pass from prey to predator and re-encyst in the predator. Therefore large predatory fish are usually the most heavily infected. Some links with bacterial kidney disease have been made but remain largely unproven. May infect man if fish is eaten without cooking or freezing. Species include D.latum (esp. North America and Great Lakes system), D.dendriticum (primarily in European Salmonids), D.dallie and D.pacificum (both marine species).


A cell that contains two sets (pairs) of chromosomes (e.g. xx or xy). The normal number carried in all body cells. A diploid is a "normally" sexed fish i.e. one that has not had it's sex altered or manipulated see also haploid, triploid, polyploid


See Eye fluke


Herbicide used for the control of floating and submerged macrophytes. Applied at <0.3m depth 0.5 litres / 100m2 at >0.3m depth 1.0 litres / 100m2. Completely de-activated by mud. Ineffective in muddy waters or areas where sediments have recently been disturbed. 


Self cleaning mechanical filters using a mesh to screen out solids particles from the water. Disc filters typically operate in the 10 micron to 100micron range. There are two types of disc filter 

1. Flat discs of mesh material, aligned vertically across the water flow in a channel. The discs rotate, lifting particles which have become stuck to the screen out of the water, where they are washed off by jets of water fired through from the back of the filter mesh. The wash water and the solids are collected in a trough and discharged from the filer. Such filters have limitations in that the particles which are removed, must be able to stick to the filter mesh, so that they do not fall off, as the disc lifts them out of the water. 

2. The second type of disc filter described above is the most common used in aquaculture. These filters, are primarily designed for high water flow rates on farms where space is at a premium and a drumfilter is too large a structure to be sited, the solids removal efficiency is about the same a as drumfilter, however maintenance (such as periodically removing and pressure washing screens is slightly more time consuming.) see also See also  conveyor/belt filter


Any condition which is results in a fall in the level of health of an animal. For a disease condition to exist, there must be three predisposing factors, 1. A disease agent, 2. A host for the disease agent, 3. The environmental conditions which make the host susceptible to the disease agent. Most of the time, animals (the host) live in a state of equilibrium with the pathogens (the disease agent) in their environment. A change in environment can stress the fish, and weaken it's defences against the pathogen, giving it the opportunity to multiply and result in a disease situation. Other environmental changes may include an increase in stocking density in cages which leads to abrasions as the fish rub the netting. This causes lesions which provide an easy entry for the pathogen into the body.

Disease agent

A factor (may be chemical such as ammonia, biological such as a parasite, or physical such as handling damage) which has the potential to result in a disease condition. Agents may be present without the presence of a disease. For example harmful bacteria may be present in the water and the fish, but may only cause a disease when the fish is stressed. See also pathogen


A process of killing some or all life forms in an attempt to achieve sterilisation. See sterilisation for example of difference between the two meanings. Common disinfectants used in aquaculture include hypochlorites and iodophores.

Dissolved Oxygen

Oxygen gas which is in solution (i.e. not that which is present in bubbles) see oxygen, oxygenation, aeration

Dissolved Solids

The residue of all the dissolved materials left behind when all the water from a sample has evaporated. e.g. If seawater is left to evaporate, the dissolved salts will remain


The part of an organ which is farthest away from the point of attachment. The remote or extreme end of a structure


An agent who sells equipment manufactured by other companies


Daily or during the daytime. see also Diurnal Rhythm

Diurnal rhythm

Term used to describe a regular cycling of a factor throughout the day. For example, dissolved oxygen concentrations of natural waters fluctuate through the day as a result of photosynthesis. This rhythm exhibits a gradual rise in dissolved oxygen concentrations through the day and a gradual fall through the night.


Developmental stage of molluscs, so called as the shell of the larvae resembles a capital "D". 


Chemical symbol CaMg (CO3)2. Type of lime rarely used in aquaculture as it has a low solubility rate. Alkalinity of 46meq which is nearly the same as Calcium carbonate. May have it's uses if it is a very local source and easily abstracted.


Chemical found in the brain which inhibits LHRH and LH secretion. The injection of chemicals such as Pimozide which block the effects of the dopamine allow better control of artificial spawning of some species.


The top side of the body. Commonly called the back

Dorsal Fin

The fin present on the dorsal side (the back / top) of the fish. In species with an adipose fin, it refers to the fin(s) in front of the adipose fin. Some species have more than one dorsal fin which are often numbered (Dorsal # 1 usually being the front fin followed by dorsal #2 etc.)


A measure of medication given at one time

Dose calculations

Method of calculating amount of chemical to administer to fish or water

Dosing Pump

A device for supplying a continuous, measured quantity , of a (usually liquid) substance. Dosing pumps are available in three main types; 

1.Simple devices with a fixed speed, where the user must dilute the substance to be dosed to a level where the correct dose rate will be achieved. For example: if a dose rate of 1 ml/second is required, and the fixed rate dose pump doses at a rate of 5ml,second, then the active ingredient must be diluted by 5 times. 

2. Dosing pumps with a speed controller, these units generally mean that the active ingredient does not have to be diluted at all and the control knob is turned until the correct dose rate is achieved. 

3. More sophisticated dosing pumps are those which include on-board software, which can provide functions such as starting and stopping the dosing automatically, providing detains of the amount of active ingredient dosed, operate using a feedback signal from a controller (for example a pH controller, with the dosing pump adding lime to a system). 

Dosing pumps are fully independent of pressures of both the substance being dosed and the water into which the substance is being dosed into. Therefore a container of substance when full, will be dosed at the same rate as when it is almost empty. This is not the same case as a dripping tap put i n the bottom of the container, as when the container is full, the Tap will drip faster than when it is nearly empty because of the greater head /pressure. Dosing pumps are used in applications such as flush treatments of chemicals and theraputants, adding substances for water quality maintenance such as lime and are often used for research purposes, to guarantee that processes are fully repeatable. 


Refers to the methods of draining tanks and ponds, or surface water run-off. One of the keys to long term success of any farm, is the ability to completely empty tanks and ponds. This is often overlooked at the design stage, which can lead to problems of maintenance and sterilisation at a later date.


The process of slowly lowering the level of a tank or pond. The slow drainage of ponds is important for the harvesting of many species, as fast lowering may result in animals becoming stranded in isolated puddles in the pond.


A device for mechanically removing or disturbing layers of sediment. Primarily used to clear channels and ponds of excessive sediments a, but also used to harvest some shellfish species (such as clams) form the sediments.


Term used to describe the preparation of a product for market. May include processes from basic cleaning to cooking and other preparations. An example is "Dressed Crab" which refers to crab meat that has been cooked and flaked.

Drug Resistance

See Resistance #2

Drug sensitivity

See Sensitivity


Item of equipment for mechanical filtration. Drumfilters are often found in fish farms and are used for three main applications 1. The polishing of inlet waters 2. Removal of faeces and uneaten food in recirculation systems 3. Removal of faeces and uneaten food from fish farm outlets. The filters use a fixed mesh on a rotating drum. A pressure jet washes the cloth as it rotates. Drumfilters are the most common method of filtering water in the range of 20 - 500 microns. See also disc filter, conveyor/belt filter

Dry Feed

Term given to most commercial pelleted diets, as opposed to wet diets that consist of fish and other unprocessed materials.


See air dryer

Dubisch Ponds

See Spawning ponds


Ducks are sometimes reared in association with fish (esp. cyprinids etc.) in extensive ponds. The ducks are fed as per usual and allowed to defecate in the pond. This has the effect of fertilising the pond, providing nutrients for phytoplankton, which in turn provide the food source for the zooplankton. The zooplankton are then eaten by the fish. In this way, two crops (the ducks and the fish) can be harvested from just feeding one of them (the ducks). Ducks are also responsible for predation of shellfish (particularly Eider Ducks).


The first section of the small intestine


Waters where pH is so low (or through other water parameters ) that no life can exist. As a result branches, leaves and other organic matter is not biologically broken down. Such conditions are found in some lakes (e.g. in Scandinavia) where a combination of acid rain and geology combine to make such conditions.