The Free Online Aquaculture Dictionary



Abbreviation for Granular Activated Carbon 


Order of fish (part of the superorder Paracanthopterygii) includes fish such as Cod (Gadus), Haddock (Melanogrammus) and Hake (Merluccius). Mainly temperate and arctic in distribution.

Gall Bladder

Small sac, usually yellowy / greenish in colour, used for storing bile after it has been produced by the liver. The bile is then released when food (especially fatty acids) enters the duodenum and assists in the breakdown of the food. With some systemic diseases the gall bladder becomes pale in colour and sometimes also distended. The gall bladder is usually located in the region of the heart and liver.


The process which coats iron and steel with a layer of zinc to protect it from corrosion. The metal is usually galvanised by immersing it in molten zinc. Galvanising is essential for any equipment to be used in a marine environment, and where the use of stainless steel and plastic is either too expensive or not strong enough. Galvanising is also recommended for any steel or iron work used in freshwater farms as it increases the lifespan of a piece of equipment by many years. Most areas will have companies who specialise in galvanising and usually charge by the weight of the item(s) that are being galvanised. It is important to note that when equipment is galvanised it should already have been pre-cut and welded as any cuts or welds made after the process will lead to corrosion in these areas, as they are not protected. When the zinc is scratched, the iron underneath is still protected since zinc is above iron in the reactivity series and so the zinc will slowly corrode instead. See also Sacrificial protection 


Common term for eggs / sperm


The opening of the mouth. A large gape indicates that the cross sectional area of the open mouth is large.

Gas bladder

See swim bladder

Gas Bubble Disease

Caused when supersaturated gases in the water, escape from the water into the body fluids of the fish. Gasses in supersaturated water will try escape out of the water into any medium where the gas saturation level is lower (e.g. into the air, or in the case of gas bubble disease, into the blood and other fluids of the fish). Most commonly seen in the yolk sacs, gills and eyes (and occasionally the fins) as these are the areas of the body which have the most gas permeable membranes. The gas then causes bubbles to form inside capillaries and under the skin, restricting the blood flow and forming haemorrhages and clots. Fish often show signs of swimming upside down or vertically, sometimes looking as if they are gasping for air at the surface. The condition affects small fish more, as the membranes that the gas has to permeate through are thinner. Large fish can be affected, but only at higher supersaturation levels. Gas supersaturation levels need only to be >103% to put very small fish at risk.

Gas Stripping

see air stripping, degassing

Gas Supersaturation

See supersaturation


Order of fish (part of the superorder Acanthopterygii) includes fish such as stickleback (Gasterosteus)


Relating to the stomach


Inflammation of the stomach


Inflammation of the mucous which lines the stomach and intestines


One of two small stones, present in the stomach of some crustaceans (e.g. crayfish), consisting of stored calcium carbonate. After moulting the calcium carbonate is released into the blood stream to assist in the formation of the new shell.


Common term for animals in the Class Gastropoda


Part of the phylum Mollusca. Marine, freshwater or land dwelling with a single shell e.g. abalone, limpet


A chemical code contained within the cell, responsible for a particular characteristic such as eye colour. See also allele


A machine that converts mechanical power into electrical power. Usually powered by diesel (or petrol is smaller), but can be powered also by gas, water (water wheels), wind and any other fuel/energy source. Generators are rated in kVa. 1kVa = 1kW. When sizing a generator for a farm, one needs to take account of the starting currents of the machines on the farm. Starting currents are typically between 4 and 8 times the normal current. Calculations should be made to the maximum possible current if the mains was to fail, and the generator start up. The start up of machinery can be staggered by timers in the control panel, which can spread the load. Automatic switching gear (AMF Automatic Mains Failure) can be installed to start the generator automatically when the mains power fails. Wind generators are often used to recharge batteries used to power feeding and alarm systems in remote locations, where there is no mains electricity. The use of fuel sources other than diesel/petrol (such as gas) is preferred in some circumstances (esp. cages), where the spillage of fuel could jeopardise the stock. See also turbines.


Relating to the reproductive organs

Genital Papilla

The orifice (may contract and extend) through which eggs and sperm are ejected.


The genetic make up of a fish i.e. the inherited genetic make up see also phenotype


Scientific classification which includes one, or several, closely related species. For example the genus Penaeus includes many different species including Penaeus japonica, Penaeus vannemei etc. The first word of the scientific name for usually relates to the genus and the second the species. For example when a reference is given as Penaeus sp., this means all the species of the genus Penaeus.


The geology of an area has a bearing on the water quality and quantity of a site. The main examples of how geology affects aquaculture are given in the available table.

Germinal disc

The area of an egg cell (disc shaped when viewed through the shell) in which cell division first starts.

Gill Arches

The bony structure which supports the gill lamellae. Attached to the branchio cranium.

Gill Cover

See Operculum

Gill filament

See Lamellae

Gill lamellae

See Lamellae

Gill Rakers

The bony, comb like structures which prevent large objects from entering and damaging the gills

Gill Rot

Fungal infection of the gills, sometimes also called Branchiomycosis. Caused by the fungus Branchiamyces sp.


The "lungs" of the fish. The gills consist of many feathery like filaments which comprise a large surface area. Blood flows through the filaments (lamellae) and gasses are passed through the fleshy tissue to and from the blood into the water as it passes across. The gills are covered by a film of mucous, which, with the tissues forms an ideal surface for pathogens to attach to as the water passes the gills. Damaged gills are one of the main areas of infection. Fish will often produce an excess amount of mucous on the gills in reaction to chemicals (such as high concentrations of unionised ammonia) or pathogens detected at the gills. This is done in an attempt to protect the gills and prevent contaminants form entering the body, see Hyperplasia. See also Respiration


Sticky substance secreted by some crustaceans (e.g. crayfish) used to attach eggs to the swimmerets of the female, where they incubate and hatch. This enables the animal to carry on life as normal, with it's eggs secure and protected.


see Glycerol


Colourless sweet tasting viscous liquid (a trihydric alcohol), soluble in water. Found in all living organisms as a constituent of the glycerides. Sometimes used for binding chemicals such as antibiotics to feed pellets, although not as effective as oil as the coating is quickly washed off.


A starch (carbohydrate) used as an energy store - see respiration


Herbicide used for the control of emergent and floating leaved plants. Applied at 1.2 - 2 litres / ha, (Dose rates are for the active ingredient. Formulations may contain diluted amounts. Adjust dose accordingly) See Herbicide susceptibility, Herbicide Withdrawal


see Gonadotropin releasing hormone


see Gonadotropin releasing hormone 


Suborder of fish (part of the order Perciformes) including mostly marine fish of less than 10cm such as Mudskipper (Periophthalmus)

Goblet Cells

Glands located in the external tissues which are responsible for the secretion of mucous


The term used to describe the sexual organs in which sperm and eggs are produced. 

Gonadotropin (GtH)

A hormone, produced by the pituitary glands. Stimulates sexual maturation. Produced as a response to other hormones see Gonadotropin Releasing Hormone. Sometimes spelled Gonadotrophin.

Gonadotropin Inhibiting Hormone (GIH)

Hormone which prevents the precocious development of ovaries.  

Gonadotropin Releasing Hormone (GnrH)

Hormone used in the hypophysation (artificial spawning of fish). Stimulates the fish to release it's own gonadotropin (GtH) (maturational hormone), and the subsequent onset of maturation of the ovaries. Sometimes an analogue (a copy) of the hormone is used which is referred to as GnrHa, the "a" standing for "analogue". 


Order of fish (part of the superorder of Ostariophysi) includes fish such as Milkfish (Chanos)



Gallons per minute, unit of flow measurement. May sometimes be expressed as Gpm(US) or Gpm(imp) which relate to American gallons per minute and imperial gallons per minute respectively. See flow rate for conversion tables. When not stated whether "US" or "Imp." (or "Imperial") care should be taken to ensure that the correct value is used to avoid miscalculations.


The process of sorting for size. Animals often have to be sorted to avoid the formation of a hierarchy, to enable smaller, less dominant individuals to get access to food. It can also prevent cannibalism. The process of grading is usually used before fish go to market in order to supply boxes of fish of consistent size. Grading is usually either carried out manually where the animals are netted onto a table and sorted by hand, with grids, where the animals are netted into a grid which allows animals of below a certain size to pass (such systems still require considerable manual input to agitate the grid or fish to encourage passage) whilst holding back others, or by grading machines where the fish are netted into a trough and rollers or belts carry live fish along an ever widening slot. The fish fall through into troughs underneath.

Gram Negative

see Gram's staining

Gram Positive

see Gram's staining

Gram's stain

A method of staining bacteria to differentiate between them.. Named after H.C.J.Gram who developed the method. The bacteria are stained with a violet dye, treated with acetone - alcohol (a de-colouriser) and finally restained with a red dye. Bacteria either fall in to the gram negative or gram positive category. Gram negative bacteria do not retain the violet dye when it is washed out but retain the red dye (and appear red under the microscope), whereas gram positive bacteria retain the violet dye (and appear violet under the microscope). Other modifications of the original Gram staining technique are also used. Most of the bacteria pathogenic to fish are gram negative, however there are exceptions such as Corynebacterium (the causative agent of bacterial kidney disease).

Gravitational constant

Acceleration constant which is used in some flow calculations = 9.81 ms-2


Animals which eat continuously. Bites are well defined but often small. Includes mostly herbivorous and omnivorous species such as mullets. Grazers are sometimes used in cage nets with the main species as a method of reducing fouling on the nets. See also Browsers

Green Eggs

Unfertilised eggs. i.e. those that have been stripped or removed from the female, but not yet had contact with the sperm. Eggs in their green state can usually be moved about without too much damage. It is best to move the eggs at this stage than  when fertilised. see also storage

Green Water System

A system in which fish are reared with phytoplankton and zooplankton. Many favour the green water system as a more natural method of rearing some species, especially marine fish and crustacean larvae which require live feed in the early stages of development. The system usually involves a static tank, with aeration to give water movement and prevent the tank from becoming stagnant. Algae is seeded into the tank from cultures and nutrients for the growth of algae may be added. Live feed animals are also added, which feed on the phytoplankton, and become prey for the fish. The system is largely unreliable with regard to accurately predicting end survival rates. Some believe that the presence of so many organics in the water has a probiotic effect which can enhance survival rates, but this is largely unproven. The term "Green water System" is not usually applied to ponds which are fertilised for extensive aquaculture, but this application may found in some texts. See also activated sludge, a type of green water system where all the nutrients are kept in suspension.


Greenhouses have largely been overtaken in their use by polytunnels due to the increased capital cost and risk of damage, especially in exposed areas where many fish farms are sited. 


A salmon which natures after only one winter at sea. Due to the short time spent in the sea (where the over 90% of the fishes final weight is gained by the fish), grilse tend to be small, and give a poor return for the farmer after he has put all the effort into hatching and rearing the fish. A fish which stays in the sea for two sea winters or more is called a salmon. The difference in weight of a fish which has been in the sea two winters over one that has been in the sea for one winter can be more than 200%. Most strains of salmon give rise to some grilse and much effort has been spent through selective breeding to reduce this percentage. The average percent for good strains, is now in the region of 4-6%. Farmers usually grade the fish by in mid to late Spring, when the skilled worker can tell a grilse by the beginning of the secondary sexual characteristics. These fish are harvested and sold. If the sorting of grilse is left too late, the secondary sexual characteristics and the development of gonads (and subsequent reduction in flesh quality) can make the fish unmarketable. Some farms satisfy niche markets for small salmon by harvesting all fish prior to grilsing. See also smolt

Gross Pathology

Symptoms etc. which are apparent by viewing the body and tissues with the naked eye.


Water found beneath the surface of the earth. Generally of constant quality and temperature. Quality can range from very good to very poor (caused by mineral deposits etc.). Generally high in dissolved gasses such as nitrogen and carbon dioxide and requires degassing before use for fish (especially fry). Often used for hatcheries as a disease free water supply, however, many ground water supplies are from underground streams which may at times rise to the surface where they can become contaminated. Some pathogens such as viruses have been found in groundwater supplies close to fish farms, where seepage from ponds has contaminated the ground water supply. see Springs, borehole, artesian well


Abbreviation for gonadotropin hormone


This process involves the irradiation of the sperm before fertilisation. This means that only the female parent contributes to the genetic make up of the offspring. This process then results in the offspring being all female, a desirable attribute in some species, such as salmonids, where the females mature later than the males. Still largely in the experimental stages.


An animal that has been subject to the process of gynogenesis


Offspring produced through gynogenesis

Entry submitted by Shmuel Rothbard


Abbreviated term used to describe trematode parasites, especially Gyrodactylus but sometimes also applied to others.


Trematode parasite of the gills. In extreme circumstances is also found in skin scrapings.