The Free Online Aquaculture Dictionary



The first generation of a cross between two species. F1 hybrids are very often sterile which may result in faster growth rates as there is no energy put into the development of gonad material. F1  hybrids in tilapia are not sterile, they are fully functional sexually, however there is sometimes, depending on the chromosomes that determine sex, a disproportionate number of males, as in the "mallaca" hybrid which is 100% male when the female is pure T.mossambica and the male is pure T. hornorum.

Entry edited by & with thanks to Mike Sipe


The second hybrid generation, obtained when F1-Hybrids are crossed randomly


An organism that is capable of living under a variety of conditions


The solid, settleable wastes excreted by an animal. The faeces of most aquatic animals are enclose in a membrane. The removal or settlement of the faeces, whilst this membrane is in tact, is relatively straight forward as it is a large, dense particle. If the membrane is broken or decays, the removal of the waste form the water becomes much more difficult as the particles are much finer see also mechanical filtration

Fatty Acids

One of the constituent molecules of a fat. See EFA for more detail.


Collective term used to group all animal life


The number of eggs that are obtained from a fish during one spawning cycle. Usually referred to in terms of number of eggs per kg body weight (but sometimes per individual animal). Sometimes refers to the number of viable eggs rather than the total production of eggs. Values are approximations as the true fecundity can vary according to the strain of fish and timing of when it is spawned.

Feed Conversion ratio (F.C.R.)

The ratio of the gain in the wet body weight of the fish to the amount of feed fed. The true F.C.R includes wasted feed and mortalities. The ratio, usually expressed as a true ratio (i.e. 1 : 1.5) is often quoted as a "rate" (1.5). Feed conversion ratios of less than 1 : 1 are possible with commercial diets, as the pellet being fed is a "dry" diet, and a high percentage of weight gained by the fish, is water trapped in the tissues and cells. Feed conversion ratios with commercial "dry" diets are typically in the region of 1 : 0.8 to 1 : 1.5. Ratios with wet diets are higher than this, and can be as high as 1 : 10. See also Feed Efficiency.

Feed Efficiency

A figure used to represent the efficiency of food use. The inverse of the feed conversion ratio e.g. a feed conversion ratio of 1 : 1.5 becomes a feed efficiency of 0.66 (1 / 1.5)

Entry edited by Henrik Kreiberg

Feed formulation

The raw ingredients that go to make up a feed. Many manufacturers use the "least cost" method, where the ingredients of a feed may change regularly according to the availability and price of different feedstuffs, but the final formulation of the feed (in terms of percentage and overall quality of protein, fats etc.) will remain constant. For example it may be more economic to purchase a 50% mix of low quality protein and high quality protein one week, but due to availability and cost changes, the following week it may make more sense to purchase 80% medium quality, 10% low quality and 10% high quality protein.

Feed Rate

The amount of food give to fish over a specified period of time. the most common way of expressing this is as percentage of the animals body weight per day. For example a 1000 gram fish, being fed 20g of food per day would be on a 2% feed rate [(20 / 1000 ) x 100)]


Devices for delivering fish feed to the fish. Fish feeders are defined by their power source, and the method used to deliver the feed. Common power sources are electricity (including solar), clockwork, water, and fish powered (where the fish themselves knock the feeder and food is dislodged into the water, these are called demand feeders). The methods of feed delivery include; vibrating plates, plates turning against a wiper arm, moving belts, rotation drums, water jets (where the food is introduced into the jet of water), air jets and augers. A good feeder is defined a s being one which delivers the feed to as much of the population as possible without causing wastage. Feeders which deliver in a small area (including the demand feeders) can, in many species, lead to the build up of a hierarchy, where the more dominant fish get more food than the least dominant. This in turn can then lead to big differences in the average weights of the stock and stress of the smaller fish.

Feeding Ring

A floating or supported ring on the water surface into which a floating diet is delivered. The ring prevents the feed from floating away. Primarily used in cages, where the ring prevents feed floating out through the cage, although is also used in ponds, especially where there is a fringe of emergent plants where the feed could get blown into and become unavailable for the fish.


Method of direct sex reversal by the treatment of animals with doses of female hormones in the feed during the early stages of development (typically at first feeding). Although the development of female fish is undesirable for species such as tilapia and catfish, it is of benefit to salmonid and flatfish farmers where the females are the more desirable gender due to the later onset of maturity. The use of hormones in fish for consumption is problematical in some countries, where laws prevent the direct use of hormones in this way. Most countries however permit the use of hormones in potential brood fish that are not to be used for human consumption. The most common hormone used for Feminisation is oestradiol-17b, which is used at a rate of between10 -100mg/kg feed for a period of between 20-120 days following first feeding. The hormone is fully dissolved with ethyl alcohol prior to mixing with the feed. A more even mix can be achieved if the feed is spread out and the hormone sprayed onto the feed. After mixing the food is dried to allow the ethyl alcohol to evaporate off before feeding it to the fish. The feed can be stored in a freezer for later use. If the recommended dosage is exceeded, liver damage can occur, especially in salmonids and flatfish. During treatment, many species slow their growth , however, the loss of growth is made up once the treatment has finished. To achieve 100% feminisation, the hormone must be fed over a period of at least 16 hours per day. Doses of the hormone for some species are given in the table available.


(2,3,6 Trichlorophenyl acetic acid) Herbicide used for the control of submerged macrophytes. Applied at 22.5kg/ha.


A once farmed fish (or the progeny of a farmed fish) which is living (not necessarily breeding) in a wild state. Feral fish can pose problems for fish farmers on a number of counts: They may harbour diseases which at any time could re-infect a farm , especially if they are migratory and move between the farm outlet and the farm inlet, they can cause environmental damage by interbreeding with specific wild strains of the same species and also by becoming dominant in an exotic ecosystem. Such an example is the common carp in Australia and USA, where feral fish cause waters to be stirred up, limiting the primary production and food sources for other species.

Fertilisation - Artificial

The manual removal and mixing together of the gametes. Methods of artificial fertilisation are broadly the same for most species with the same ground rules being applied. Eggs and sperm should be kept free from contact with water or any other liquid (with the exception of some saline milt extenders which are occasionally used to prolong the life of the milt (or make it go further when male fish are in short supply). Eggs and milt should also be kept free from direct sunlight. 

Fertilisation Ponds

See Spawning ponds


The level of ability of an individual to produce viable offspring.


Often a result of territorial behaviour which becomes exaggerated when fish are stressed (particularly when overcrowded). Can be more pronounced in some species during breeding times. Insufficient water velocity in tanks and ponds can also induce fighting (e.g. in some tilapia species). Fighting is particularly common in crustaceans when stressed. In addition to leaving open wounds which are easy entry sites for pathogens, the loss of appendages can also lead to a reduction in the value of the animal. In some larger crustaceans such as lobster, rubber bands can be put round the claws, which in addition to protecting the handler, also reduce damage caused during fights. See also  nipping


Strand like. Often refers to species of algae 


A boneless side of the fish cut away from the backbone and rib cage. See also Butterfly Fillet.

Filter failure

See Crashing

Filter Media

A particle or structure which is used for either the mechanical filtration of solid matter (by restricting its ability to pass between particles) or the substrate on which bacterial and/or algal colonies are formed, which provide water treatment as the water passes the colonies. Filter media can be divided into two categories, that which is a solid structure and is packed into a filter in a structured method, and that which is random packed into a filter (i.e. by filling it up without caring how the items of media lie against each other). The latter of these two is the more common type found in aquaculture and includes naturally occurring particles such as sand, gravel, and man made items such as plastic rings, tubes and beads. A general criteria for aquaculture media is that it is non-toxic to fish, and will not breakdown as a result of the water quality that it is immersed in. For the other required specification of media for a specific task see the relevant section (e.g. biological filtration, degassing etc.).

Fin ray

The cartilaginous support rod for the fins. Some fin rays are specialised in that they have spines at the end or can inject poison


The small particles of feed which are as a result of poor pellet binding, erosion of pellets during storage and handling. Such particles, if present in large enough numbers, can cause considerable problems to recirculated water systems, where the mechanical filtration system does not take them out of the water. The fines exert an additional B.O.D. loading on the biofilter, which may compromise it's ability to perform to it's design expectations.


A fish typically between approximately 10 and 40 grams. Often used up to 1 year of age for some species.

First feeding

Term given to describe the period of transition between alevin and fry, when the fish begin to look for food after having exhausted most of their yolk sac. First feeding is critical in many species and if the feed is not properly presented in the short window of time available, the fish may not have the energy left to take feed presented too late.

Fish Pumps

Device for pumping a mixture of fish and water. Used for transporting fish to / from graders, harvesting bins, tanks and ponds. Fish pumps use specialised impellors which eliminate the risk of damage to the fish. They are limited in the size of fish they can handle, usually up to 500g - 1kg is the limit. For larger fish augers are usually used.

Fish Kill

Term used to describe the mass mortality of all of the stock over a short period of time. May be caused by poisoning, sudden changes in water quality or other trauma.


A chemical which is used to preserve the body tissues after death. Most commonly used chemical is formaldehyde.

Fjord and Sea Loch

A long narrow (usually very deep) body of sea water that ingresses into the landmass. Usually formed as a result of glaciers  carving out valleys, and these then flooding with seawater once the ice recedes. Fjords and sea lochs make very sheltered sites for cage farms and shellfish farms. A lack of tidal exchange can however lead to a build up of pollutants from farms.


The tiny hairs used by phytoplankton for movement which may be controlled or uncontrolled.


Single celled plants/animals which are able to move (either in a controlled or uncontrolled manner) by the use of tiny hairs which are also called flagellae.


A pipe fitting used to bolt a pipe to a pump or similar device.

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The action of fish which involves a quick, half-spin movement. During the movement the fish rubs itself against a surface (e.g. tank base or cage side), a flash is caused by the light reflecting off the flank of the fish. Fish typically exhibit flashing when their skin is irritated by parasites. Flashing is often one of the first signs of a parasite infection.

Floating leaved plants

Plants with floating leaves can be divided into two groups; those which have their roots in the sediments, but their leaves floating on the surface of the water such as lilies, however some species are free floating with their roots trailing into the water Such as water hyacinth and duckweed. See also herbicides


The general term given to a colonies of bacteria which are either attached to a surface (e.g. in a biological filter) or free floating in a clump. See also sewage fungus

Flooded Suction

A permanent head of water above a pump which will prevent the possibility of the pump drawing in air during normal operation see also non-return valve, self-priming.

Flow Rate

Flow rate is critical for all intensive aquaculture systems, as it determines the rate at which oxygen and other resources (for example calcium carbonate which will buffer a systems pH) are brought into the system, and also the rate at which excretory products such as faeces, carbon dioxide and ammonia are removed from the system. The more that water is reused or recirculated, the more parameters that flow rate becomes critical for. E.g. in many cases it only needs to be sufficient to supply the oxygen needs of the fish, however once oxygenation is used, and more stock are held per unit of flow, other factors such as ammonia and carbon dioxide concentration become important.

Fluidised bed

These are flooded vessels which are partially filled with a random packed media. Water flows up through the media and the velocity of the water pushes the particles up into the vessel, causing them to swirl around. As the water velocity increases, the particles swirl in higher up the column and become more fluidised. Fluidised beds are sometimes used with ion exchange resins, types of lime and activated carbon as they have the advantage that solids particles can pass through them, whereas they would get caught up in a static bed. They are more commonly used for biological filtration , where sand or small plastic particles are used for the substrate for bacteria to adhere to and grow on. The constant movement of water and particles in the vessel ensures that there are no dead spots, and the velocity of the water is controlled so that as the particles collide, they do so with just enough force to knock off any excessive or dead bacterial floc. The layer of floc is therefore maintained at an ideal thickness. This occurs when the filter is approximately 100% expanded. That is, when the media is occupying a volume twice as great as when the water is switched off. Air is also sometimes used to assist in the fluidising process, especially where water velocities are too low to obtain sufficient fluidising. The use of sand in large biofilters has sometimes been problematical, both in achieving an even bed fluidisation and also from the fact that if the water flow stops, the sand packs down, suffocating the bacteria rapidly. Plastic media filters are easier to fluidise, especially if plastics such as polypropylene or polyethylene are used, due to to their density, which is very close to that of water. Air diffusion is often used to fluidise these filters rather than water.

Fluidised sand filters

see Fluidised bed filters


A method of administration of theraputant chemicals to fish in a tank or pond. The normal flow rate to the pond is maintained during the treatment, and the chemicals administered at the inlet to the pond. To be effective, flush treatments require that the chemical is added at a consistent rate for a period of 30 minutes to 2 hours. If a shorter period is chosen, the efficiency of the treatment will be reduced as the fish will not be in contact with the chemical for long enough. Flush treatments use more chemical than bath treatments, and are only used if it is impossible for the water to be turned off, or the system to be treated is so small that the additional amount of chemical used is not significant ( e.g. such as the use of fungicides on eggs) see also treatment method


The terms given to a process which gives rise to a sudden rush of water out of a tank or pond. Many farmers "flush" their tanks or ponds by removing a pipe or board. It is carried out in order to remove solids particles which have settled on the bottom near to the outlet. Well designed tanks and raceways should need flushing very rarely. There is little to be gained from flushing a tank if there is no build up of solids visible, however it can be useful for occasionally cleaning out any settled waste or bacterial floc building up in the outlet pipes / channels.

Foam Fractionation

Also termed foam floatation and protein skimming, this process removes organic matter and fine solids from water. A foam fractionator consists of three parts, a vessel (usually a column) through which the water flows either up or down, a bubble source ( such as a diffuser or venturi) and a foam collection point. The particulates that foam fractionation removes form the water are known as surfactants, and comprise mainly organic compounds. These compounds have one end which is attracted to water molecules (polar) and one end which repels water molecules (dipolar). As the bubbles rise through the column, the molecules collect around the bubble surface with the dipolar ends in contact with the air and the polar ends pointing out into the water. The molecules are therefore adsorbed into the surface of the bubble. Once the bubbles reach the water surface, the particles in the bubble surface prevent it from bursting and as more bubbles collect behind it a foam is formed. This foam rises steadily out of the water as more and more bubbles build up behind. The diameter of the column is gradually restricted at the foam outlet, which increases the velocity of the foam, it is then fed away from the column and sent to waste. The increase in velocity is important as it reduces the amount of time that the bubbles have to stay in tact for, before they are removed form the column. The amount of foam produced is a result of the air flow rate, the amount of surfactants in the water, and the time taken for the foam to be removed. Foam fractionators are often used in recirculation systems to remove some of the fine solids and reduce the loading on biological filters. If ozone is added to a foam fractionator, it will increase the efficiency, in addition to providing the other benefits that ozone brings (care using ozone as it is very toxic, especially in seawater systems). Although it is felt that a downflow fractionator is more efficient than an upflow one, as the water flow slows down the rate at which the bubbles rise, and therefore give more time for the dipolar ends to fix into the bubble surface, there is no data at the time of writing to prove this theory. See also ozone

Folic Acid

Vitamin, necessary for maturation of red blood cells and used during development of gonad tissues (therefore often included in broodstock diets). Deficiency can result in many symptoms including most notably anaemia.


An inanimate object such as a brush, net or boot which may act as a carrier for transferring disease organisms.

Food Chain

The transfer of energy in the form of food. A typical food chain (although simplified for example) may be phytoplankton - zooplankton - omnivorous fish - carnivorous fish - man. Energy is lost back into the environment from the food chain at every stage due to inefficiency of digestion, heat loss through metabolism and fossilisation (such as the formation of coal where the energy in the body is prevented from returning to the environment - until we humans dig it up and burn it!).

Food Conversion Ratio

See Feed Conversion Ratio

Foot Valve

A valve attached to the bottom of the suction leg of a pump, to prevent water draining from the pipe when the pump is turned off. Essential is pumps do not have a permanently flooded suction and are not self priming. See non return valve.


The act of browsing or grazing to obtain food. In some ponds, the growth of some herbivorous / omnivorous fish is encouraged. These fish are then used as "forage fish" for the higher value, carnivorous species in the pond where they can eat them at will, rather than being fed trash fish or pellets.


Forestation surrounding, or upstream of a farm can impact on the farm in two main ways: Increased forestry (especially conifers) can lead to an increase in the rate of run off from the ground due to he lack of ground vegetation. This in turn leads to larger floods in times of rain, and lower flows in times of drought (as the grounds capacity for holding water is reduced). This is especially so in areas where the trees have replaced moorland areas. In some areas of new forestry, large quantities of fertilisers (phosphates and nitrates) are added to the soil. A percentage of these can wash into the water courses, leading to increased risk of algal blooms and the problems associated with them.

Fork Length

The distance from the tip of the snout to the start of the fork in the caudal (tail) fin. Often used as a measurement in farmed fish, as the tip length (the distance from the snout to the tip of the tail) is often interfered with by fin erosion.


Used for the treatment of many external parasites. Usually sold as a 37% by weight of formaldehyde gas in solution, the most common name given is formalin. Commonly used at 100-200ppm for 1 hour (of the 37% mixture). It is usually a clear liquid, although sometimes a tracer (a coloured dye) is added so that the operator can see that good mixing has occurred in the treatment tank. The chemical has a very strong smell and care should be taken not to inhale. The chemical should be stored out of direct sunlight, as this can cause the formation of a white precipitate, which collects at the bottom of the container called paraformaldehyde. This is very toxic to fish, and any formaldehyde / formalin which shows either this, or a white haze, should be discarded of carefully and not used for the fish. It is often wise to check the concentration of the solution of formaldehyde that you are using first, if it is not clearly stated on the container. As formaldehyde degrades in the water it removes some oxygen, so additional aeration / oxygenation should always be provided in the tanks, and care taken to not reduce the oxygen levels in water courses etc.


See formaldehyde


The addition of nutrients to feed to improve it's nutritional value.


The term given to the build up of plant or animal life on surfaces, where their presence causes a system to operate inefficiently (the term can also be used to describe the build up of uneaten feed or faeces in a tank). Fouling is particularly a problem in cage aquaculture, where marine plant and animal life grows on the nets, increasing the weight of the net (and therefore having an adverse effect of the cage buoyancy system). More importantly however, the net mesh is reduced in size, which reduces the water flow through the cage and in turn reduces the dissolved oxygen supply to the fish, and the removal of waste from the cage. Marine fouling can also cause problems in pipes where the water is pumped in from the sea. Barnacles (e.g. Balanus balanoides) and other animals attach onto the pipe and feed from the water as it passes them. This can be solved either by adding chemicals to the water, such as chlorine which is used in industrial situations is very toxic to fish at concentrations greater than 0.1mg/l, or by periodically filling the pipe with freshwater. The latter is becoming more widely used, with farms often having two inlet pipes, one of which has a small constant trickle of freshwater flowing down it and the other which is used for pumping. Every one or two weeks, the pipes are switched round so that the fouling never gets a chance to become established. Fouling of water quality sensors and levels switches by weed, sewage fungus, dead fish etc. can also be a problem and is primarily overcome by thoughtful location and choice of sensor type to suit the potential fouling hazards.


see foam fractionation

Free living

An organism that is not dependant on a host organism. May be used to refer to specific stages in development of organisms that may spend some of their life as free living and other stages where they require a host.


The distance between the water level and the top of the tank or pond (or the overflow level)


Water with a salt content of less than 0.5 parts per thousand (5000 ppm)


Term used to describe a substance that is easily crumbled or powdered.

Friction Loss - Air

The loss of pressure in a piped system, caused by the drag effect that pipe walls have on the air as it passes through the pipe.

Friction Loss - Water

The reduction in velocity and pressure of water flowing through a pipe or along a channel caused by the drag effect from the surface that the water passes. The rougher and more profiled (shaped) the surface, the greater the friction loss that it imposes. The calculation of friction loss is important in designing systems, to ensure that a pipe or channel is large enough for sufficient water to flow through it. Friction loss is also sometimes called headloss or pressure loss, as it has the effect of reducing the amount of head (or pressure) of the water. 

Fright response

A sudden burst of speed induced by the fish in response to an external stimulus (e.g. sudden change in light). The direction of travel may often be not directly away from the threat or stimulus. The response involves using the White muscle tissue that forms the major part of the "flesh" of the fish.

Fringing Plants

Plants which occupy the littoral zones. These are divided into three categories, emergent plants, floating leaved plants, submerged plants.


The stage of fish development after alevin / larvae. May be applied to any small fish, but generally fish are termed fry up to a size of 5-10grams. Fish slightly larger than this are usually termed fingerlings


Carotenoid pigment, commonly found in bacillariophyta (diatoms). Responsible for giving the diatoms a brown colouration.


Primitive plants, lacking chlorophyll. Reproduction through the production of spores. Some species of fungus are parasitic (such as Saprolegnia). See also sewage fungus


A small area of infection beneath the skin which develops into a raised abscess, forming a lump on the skin typically 10 - 30mm across. Most notable in the disease furunculosis caused by the bacteria Aeromonas salmonicida


Systemic bacterial disease generally of salmonids, but other species such as turbot (Scopthalmus maximus) have also been infected. Caused by the bacteria Aeromonas salmonicida. Was generally regarded as a freshwater disease but migratory salmonids may contract the organism in freshwater and break out in the disease when in seawater. Many seawater outbreaks have however occurred without any association with freshwater (e.g. in fully marine species such as Turbot). May be vaccinated against with some success. Occurs primarily following stress and in Springtime when temperatures begin to rise. Symptoms include furuncles on the skin, haemorrhaging of body organs, loss of appetite. Treatment by antibiotics is usually successful but as the disease has been in aquaculture for several decades, there are some resistant strains reported. Loss of appetite sometimes makes antibiotic treatment difficult unless caught very early on. Farms are often infected by migratory salmonids traveling upstream from other infected areas.

Fusion Welding

Process using heat to melt two surfaces (usually of the same material) so that they combine together to form one complete surface. Used for welding plastics (primarily polyethylene and polypropylene) in two ways; the first of these uses electrodes and heating coils in pipe fittings. When an electric current is applied to the fitting it heats up and fuses the fitting and pipe inside together. This method is usually the choice method for smaller pipe diameters i.e. > 4". The other method and that of choice for larger pipes is butt welding.