The Free Online Aquaculture Dictionary



Abbreviation for Enteric Red Mouth

Earth metals

The general term given to elements such as calcium and magnesium.


A biological community and the physical environment associated with it. An eco-system can refer to a very specific area, such as a small stretch of a stream, or a very large area such as an ocean. In all eco-systems, the nutrients and energy moves around the system in loops or cycles. Sometimes these loops and cycles are simple, involving only a few species which impact on each other, but more often a complete eco-system is very complex involving many hundreds or even thousands of species (such as in some coral reefs). In such systems, very small changes can have very large impacts. Often eco-systems can seem very basic, but if one tries to recreate an ecosystem artificially, it's complexities are soon apparent.


Externally. Prefix usually used to describe a process that occurs on the skin or outer surface of the body (e.g. ectoparasite refers to a parasite that is attached to the external surfaces of the body), rather than externally (Endo)


A parasite that affects the external surfaces (including external surfaces of the gills) of an organism


see Oedema


Essential Fatty Acids. These are required in the diets of fish mainly to maintain the structure and integrity of the phospholipid cell membranes. If there is a deficiency, the cell membrane weakens which can lead to a reduced resistance to disease, problems with osmoregulation and a weakening of the red blood cells. EFAs fall into two categories, the linoleic acids and the linolenic acids. They are written in a code form which tells a lot about the fatty acid for example 18:2 n-6, this shows that the fatty acid has 18 carbon bonds, 2 double bonds and the -6 shows that the position of the first double bond in the molecule. Fatty acids with an n-3 at the end are grouped together as linoleic acid, and those with an n-6 at the end are termed linolenic acids. In general the more the number of double bonds, the more unsaturated the fat is. A saturated fat has no double bonds, whereas one with more than 1 double bond is called a polyunsaturated fat. The fatty acids in the carcass of fish are almost always unsaturated, whereas those in land animals are saturated. Different species require different requirements of EFAs. In general as temperatures are lower, salinities are higher and carnivorisity is higher then the fats required by the fish must be more polyunsaturated. The EFA requirements of some species are given in the table. As can be seen from the table, the total amount of EFA required in the diet is only about 0.5 - 2%. Most commercial diets contain between 10 and 30% total lipids (fats). Having more than this in the diets is not harmful to the fish but other lipids are much cheaper, and so these are used to make up the balance. Two more terms that are used to describe some of the EFAs are HUFA and PUFA (Highly unsaturated polyunsaturated acids and Polyunsaturated acids respectively). HUFAs (EPA = 20:5n-3 and DHA = 22:6n-3) are essential in the rearing of marine carnivores as these fish cannot convert the n-3 linoleic acids. In the early rearing stages, HUFA enrichment compounds are fed to some live feeds such as rotifers which can accumulate them. These are then fed to the fish. Artemia can also be enriched but cannot accumulate DHA and are therefore just used as a vehicle to get the HUFAs into the fish.


The discharge of waste liquid. Used to refers to the discharge of water and wastes from fish farms back into the environment or into a treatment system.

Egg Baskets


Boxes of fully or partially perforated material, in which ova are incubated. An egg basket has to meet several criteria; 1. That the hole size in the basket prevents eggs from falling through or becoming jammed in the holes 2. That the design of the basket is such that the water flow is directed across the eggs, this is sometimes achieved by forcing the water up through the perforated sheeting beneath the eggs to guarantee that the water passes by the eggs 3. That the baskets are manufactured from a material that is non toxic to the fish, baskets are commonly manufactured from PVC , polyethylene, polypropylene or aluminium, the latter of these only being suitable if water is not too acidic. A number of egg baskets are usually housed in a common tank/vessel such as hatching troughs, stacked in a Californian hatching system. The number of eggs each basket can hold is dependant on the size of the baskets, the size of the eggs, the stage of incubation (e.g. once Salmonid eggs have reached the eyed stage, mortality rates are very much lower and so they can be stacked in a double layer without much additional management , as long as the water flow is good and the water quality and egg quality. Poor water quality may lead to increased numbers of dead eggs and a single layer of eggs is easier to manage in such circumstances than a double or treble layer). Typically 1.5 litres of eggs can be housed in a 300mm x 300mm egg basket and flow rate should be approximately 2 litres per minute per tray held in the hatching trough. Where water is short, egg silos or Zoug jars are used, as these systems require less water / volume of eggs. 

Egg Pickers

Device used for the manual removal of dead eggs from baskets. Pickers range from large plastic tweezers, to bulb pipettes and siphon tubes. The ideal solution is one where a minimal number of viable eggs are removed with the dead ones, and where the viable eggs are disturbed as little as possible, especially before the eyed stage.

Egg Picking

Process where dead eggs are removed from viable ones. The main reason for this is that the dead eggs will quickly become infected with fungus, which can rapidly spread to adjacent viable eggs. Eggs may be picked by hand or using machines. Other methods include immersing the eggs in a saline solution (or other solution denser than water). The dead eggs often have a different density than viable ones, and they can be separated by flotation.

Egg Sorters

A piece of equipment sometimes found in large Salmonid hatcheries which will automatically sort dead eggs from viable ones. Used after shocking, the units usually use a light which shines through the egg, dead eggs become opaque and so the light does not penetrate. Egg sorters will typically sort between 100 - 200,000 eggs per hour. see also egg picking

Egg Trough

See Egg Basket

Egtved Disease

Another name for Viral Haemorrhagic Septicaemia. Sometimes called "Egtved" after the region in Denmark where the disease was first discovered

Eider ducks

Somateria mollissima. Eider ducks can cause losses of line grown shellfish (e.g. mussels (Mytilus sp.)) through predation. The primary problem through duck predation is not the numbers of shellfish that they eat, but rather the number of shellfish that are knocked off the lines or damaged during the feeding process. Other diving ducks can also be a hazard, but eiders remain the main predators.

Electrical Phases

Typically two different phase types are encountered, single phase and three phase. Single phase electricity uses a live, neutral and earth wire and is only usually found in items up to 2.2 kW, a 220 Volt single phase supply will have 220 volts flowing down the live wire, some items such as heating elements are available with a greater wattage, this is because unlike pumps etc. heating elements do not draw a large amount of current on starting up . Three phase involves the use of three wires with a total voltage of 415 volts and an earth wire. Most equipment is available with a three phase supply, as the starting currents can be much higher much larger motors etc. can be operated. In three phase electricity the live and neutral wires constantly change, so that there is only 1/3 of the total voltage flowing down any wire. 

Electrical Wiring

See wiring

Electrocution in fish

Has been used to slaughter fish during harvest. It is however an expensive (both capitally and operationally) method of harvesting, and in addition to providing a health and safety risk to the operator, electrocution also sometimes causes blood spots in the flesh which lowers the value of the end product, it is rarely used in aquaculture for slaughter. It is also used in electrofishing for the removal of unwanted species form a water course or lake. 


Order of fish (part of the superorder Elopomorpha) which contains primitive fish such as Tarpon (Megalops)


Superorder of fish, includes orders such as Anguilliformes, Elopiformes and Notacanthiformes


A juvenile eel which is in the process of making the transfer from a marine environment to a freshwater one. Elvers are semi transparent (hence often called glass eels) and begin to develop pigment once in freshwater. Elvers enter rivers in the Spring when temperatures begin to rise, using the surge of tides up rivers to push inland. As the tide recedes, the elvers move to the sides of the river, where the water velocity is less and begin the migration upstream. Typical weights of a European eel (anguilla anguilla) elver is 0.3g, with a length of 100mm.


The wasting away of body tissues due to starvation. Often brought about by other causes even when feed is available. E.g. Eye fluke (Diplostomum spathacaeum) causes blindness in some species of fish, which, especially if they are sight feeders (such as salmonids) will result in starvation and emaciation.


Refers to any foreign body carried around in the blood stream (including clots, lumps of fat etc.), but more typically is used to describe gas bubbles trapped in the blood vessels and tissues. See also gas bubble disease.


An organism that is developing, but has still to hatch or be born.

Emergent Plants

Plants which rise out of the water for the majority or all of the year e.g. Grasses, sedges, rushes etc. Dependant on sediments for nutrients. Most photosynthesis takes place in the atmosphere so these plants have little impact on the carbon dioxide and oxygen concentrations of the water. Of little value for nutrition for farmed fish, with possible exception of species such as grass carp (Ctenpharyngodon idella) which have been known to graze emergent plants in the absence of other food


The process where nutrients are "wrapped" in a membrane to make them into what are effectively small pellets. Used for some larval and fry diets, especially where live food production has not proved viable. Material used for encapsulation vary. At present most farms still feed live feed up to the point where the fish can be weaned onto normal pelleted diets. This is in part due to the cost implications of using encapsulated feeds, but is mainly due to the uncertainties surrounding the nutritional needs of many species in the larval stages. Established methods of providing the correct nutrition, such as using live feeds are therefore preferred.


Within. Prefix usually used to describe a process that occurs within the body, rather than externally (Ecto)


A gland which secretes hormones directly into the blood stream, (such as pituitary and thyroid).


A parasite that affects the internal organs and / or tissues of the body


The inner bony and cartilaginous skeleton of vertebrates (as opposed to the external skeletons (exoskeleton) found in invertebrates such as crabs)


Defined as vigour, force or activity. Can be applied to electrical supplies, nutritional components and fuels. Many different methods of expressing including calories, Joules, watts etc. Use the convert software to convert between one type of energy and many others.


Relating to the intestines. Enteritis = bowel inflammation

Enteric Red Mouth

(E.R.M.) A systemic bacterial disease. Primarily associated with salmonids. Caused by the bacteria Yersinia ruckerii. Symptoms usually include loss of appetite, usual effects of systemic bacterial diseases on internal organs (pale liver, enlarged spleen), characteristic pink / red tongue (caused by many small haemorrhages) and pinpoint (petechial) haemorrhaging of the belly. Vaccines available in many countries, responds well to antibiotics unless resistant strains have formed.


Refers to an inflammation of any part of the intestine (including the stomach)


The surrounding conditions that have an impact on survival and growth

Environmental gill disease

The excessive growth of mucus on the gills caused by an environmental factor such as a chemicals or particles in the water (as opposed to a pathogenic factor - see bacterial gill disease). The build up of mucus can lead to suffocation of the fish, resulting in the need to exert more energy to meet it's oxygen requirements. The stress caused by these additional exertions can make the fish more susceptible to pathogens. Often results in infection of the gills (bacterial gill disease). Once the irritant is removed, the excessive mucus can be removed with quaternary ammonium compounds. These treatments are often followed by Chloramine treatments to kill off any bacteria that has become established on the gills. The mucus must be removed first, or the Chloramine will often not reach the bacteria, leading to a short term recovery followed by rapid re-infection.


A disease organism that is present in a population at all times, but only manifests itself as a disease in a few individuals at a time.


20:5n-3 highly unsaturated fatty acid (HUFA). Essential for some species such as carnivorous marine larvae. See EFA


The surface (upper) layer of a thermally stratified lake. A warm layer of less dense water which lies on the top of the surface. Can also refer to the top layer when a body of water is stratified by other characteristics. e.g. in some sheltered sea lochs and fjords, the upper layer of water is freshwater, which has flowed in from rivers and lies on top of the more dense seawater. 

Equivalents per million (meq)

see Milli-equivalents


Totally remove from the environment.


The removal of all the individuals of a particular species (animal or plant) form a given environment.

Eradication program

Attempts to eradicate all the individuals of a particular species (animal or plant) form a given environment.


The wearing away of a surface by physical means. Can be used to refer to the wearing away of geological features by wind, rain and wave action. Also used to refer to the damage done to fins by wearing on tank sides, nets etc. A certain amount of damage attributed to fin erosion is caused by nipping and subsequent local infections. Fin erosion is more prevalent a higher stocking densities, and can reduce the market value of a fish, especially if sold for the restocking or ornamental markets. Erosion of mouth parts is often a sign of underfeeding or poor sight (caused by cataracts see Eye Fluke), as the fish search around for food. Erosion of the fins can lead to bacterial and fungal infections.


Antibiotic, little used in aquaculture as it is much used for treatment of humans.


see Oesophagus

Essential Amino Acids

See Amino Acids

Essential Fatty Acids

See E.F.A.


"Of the estuary". Term given to describe organisms that live in estuary areas.


Tidal area where rivers meet the sea. Also sometimes used to describe an area where a river flows into a lake. Estuary area is generally regarded as the area over which waters from the two sources mix to form a changing environment. I.e. in the case of a river flowing to the sea, the area where the water is between freshwater and seawater salinities due to mixing (e.g. by tides).


The preparation of a smooth surface by chemically or physically roughening it, so that glues and cements bond more tightly to the surface.

Ethylene Glycol

Cryoprotectant chemical used in cryopreservation.


See Aetiology


Ability to tolerate a wide range of salinities. Such species are usually estuarine such as grey mullet, bass etc. see also stenohaline


Fish that can withstand a wide range of temperatures e.g. carp (Cyprinus carpio)


Term used to describe an environment which has an excessive concentration of nutrients. Eutrophic waters usually have dense plant (usually algae) populations. Typical of lowland lakes in agricultural areas, where nutrients are washed into the water, or farmed ponds where manure and other nutrients are added to increase productivity. See also Mesotrophic , Oligotrophic


The change of state of liquid, from a liquid to a vapour state, below the boiling point of the liquid. Has the effect of cooling the liquid as it is the molecules with the highest kinetic energies which escape into the atmosphere through the liquid surface. This results in a drop of the average kinetic energy of all the molecules in the liquid, and consequently a fall in temperature. The calculation of evaporative losses is very complex. Most areas have a meteorological office who can provide approximations of evaporative losses for the area.


To remove the internal organs (the viscera) from a fish.

Exclusive economic zone

The area up to 200 miles form a countries shores, reserved to that country for exploitation and management of resources.


The discharge of metabolic wastes from the body. Can be used to include faeces, urine and gaseous exchange across the gills or skin.


The bulging of the eye. Usually as a result of physical damage or infection. Symptom found in many diseases, especially the systemic bacterial diseases.


The outer skeleton (shell) characteristic of crustaceans such as crabs


An animal that is not native to the area.


Term used to describe culture methods where there is no artificial food, theraputants etc. (and usually no artificial fertilisers, only natural ones such as manure). Low water exchange. Characterised by very low stocking densities and slow growth.

External Respiration

The exchange of oxygen and carbon dioxide between the body and the environment see also respiration

Extinction coefficient

The depth at which light penetration is no longer sufficient for photosynthesis


A pellet which is pushed through a die to give it a certain, consistent shape. Extrusion methods are used in the production of floating pellets

Eye Fluke

Causative agent; Diplostomum spathacaeum. Parasite which enters the fishes body and works it's way up to the eye. Life cycle involves two other hosts, a fish eating bird and certain species of snail (Limnea peregra). Will affect all species of fish, however those native to areas where the disease is present have a degree of resistance. Infection can lead to total blindness and mortality. In extreme circumstances, the number of parasites entering the fish can cause such stress that the fish dies before the parasites have infected the eye. Prevention is by ensuring that ponds and channels are free form snails and netting of ponds to reduce predation by birds. Once infected, the disease is incurable. If the parasites are being released from snails in the water course, mechanical filtration to 30 - 40 microns, will eliminate the cerceria from the water and make the water safe for the fish. The cerceria are buoyant and some success has been had by taking water from depth to feed the fish farm. One snail can produce many hundreds of cerceria.


An egg that has reached the stage of development where the black spot of the eye is clearly visible. In most species, this indicates that the egg is less sensitive to movement and can be handled safely (e.g. for transportation).