The Free Online Aquaculture Dictionary

pvc pipe supplies



See S  


A salmon smolt that has been produced in a period of approximately six months from hatching (rather than the usual 12 or 24 months for S1 and S2 respectively). This is achieved through a combination of heated water in the early stages of rearing leading to increased growth rates, and changes in the light regimes to trick the fish into thinking that a whole year has passed in a period of only 6 months. The advantage of producing out of season smolts is that the farmers are no longer confined to stocking just once a year and manipulation of the daylength period can result in smolts being produced at any time between October and July, giving the ongrowing farmer more stocking options.


A salmon smolt that runs to sea in the spring after one summer of growth in freshwater. Most well farmed salmon fry will result in S1, with only about 10% as S2 dependant on strain. Many S1 fish are now being replaced by the production of S1/2 fish where the smolting time can be manipulated and therefore allow greater flexibility of stocking and cropping programs.


A salmon smolt that runs to sea in the spring after two summers of growth in freshwater. Largeley undesirable in farming due to the time that fish require to be looked after for in comparison to S1 and S1/2 fish. S2 fish are also more likely to produce grilse.

Sac fry

The life stage of a fish when the yolk sac is still present. Also called alevin. the time that the yolk sac remains for is dependant on temperature and the activity of the fish (i.e. the speed at which it uses up the energy contained within the sac). Salmonids have a very long yolk sac phase of 30 days or more, whereas cod have a relatively short period of only 3-5 days.

Sacrificial anode

See sacrificial protection

Sacrificial Protection

The protection of iron or steel against corrosion by using a more reactive metal. Pieces of zinc or magnesium alloy are attached to pump bodies and pipes. These items are known as sacrificial anodes and "attract" the corrosion to them rather than the iron/steel. The sacrificial anodes must be replaced periodically as they corrode.


Pipe fitting for attaching to the outside of a pipe to enable a branch (usually smaller that the diameter of the mounted pipe) to be taken off. Often cheaper than a TEE fitting. 


The increase in salinity of fresh surface and ground water supplies. A result of saltwater intrusion by pumping of seawater boreholes and wells, and the building of inland seawater ponds structures. Can have a serious effect on local agriculture, especially paddy fields.


The term used for the measurement of the total amount of salts in the water. Expressed in terms of the weight of salts per volume of water, common units are parts per million (abbreviated to ppm) mg/l and parts per thousand (abbreviated to ppt, also sometimes shown as o/oo) or grams per liter (see concentration). Distilled water has a salinity of zero, whereas full strength seawater has a salinity in the region of 34-36 ppt. The salinity of seawater varies from sea to sea and there are exceptions such as the Dead Sea which has a salinity of approximately 200ppt. The salinity of seawater near to coastlines can fluctuate due to the amount of freshwater from rivers and run-off that is entering the sea. In calm weather, sheltered bays with a freshwater river flowing into them can sometimes exhibit stratification where the freshwater (which is less dense) which lies on top of the seawater and does not mix. The freshwater layer can sometimes be up to 1m deep. The salinity of water is either measured using an electrochemical cell which gives a reading through a probe and back to an electrical meter, by a hydrometer which uses the changes in the density of the water (higher density = higher salinities) to give a reading from a scale, or by a device called a refractometer. The probes and meters are very accurate but for occasional readings can be expensive. Hydrometers and refractometers are a cheap and reliable method of taking readings, and some hydrometers can be purchased which are calibrated to read over the salinity range that aquaculture requires (for example winemaking hydrometers are unsuitable). Given a choice between the two, refractometers are the more efficient to use.  The relation between salinity and density is also temperature dependant (as the water expands with temperature and therefore the density changes). Variations in salinity can cause stress and acute or chronic mortalities amongst fish stocks. This is largely brought about by the fishes inability to regulate it's body salt and water content (see osmoregulation). Some species, whose natural habitat is estuarine, such as bass and mullet, are more tolerant of changes in salinity (see euryhaline) than others whose habitat is confined to the ocean, such as cod. See also stenohaline


Common name given to a group of anadromous fish which spend part of their life in sea.  Incorporating some species from the genus Salmo and Oncorhynchus. An exception to the rule are some "landlocked" species which no longer run to sea e.g. Danube Salmon (Hucho hucho)


Family of fish (salmonidae) which includes many commercially farmed species such as the salmons, trouts and chars. In general Salmonid fish are found in temperate climates and in clean, oligotrophic waters. All of the species breed in freshwater, however, many of them are migratory and spend a period of their lifecycle in the sea. Includes genus of Salmo, Oncorhynchus, Salvelinus.


Order of fish (part of the superorder Protacanthopterygii) includes fish such as Salmon and trout (Salmo and Oncorhynchus), Whitefish (Coregonus), Ayu (Plecoglossus) and Pike (Esox)

A compound which has been produced by the reaction of an acid with a base, in which the hydrogen ion of the acid has been replaced by metal or other positive ions. Typically salts are crystalline compounds such as Sodium Chloride. See specific salt for more information. Common salt (sodium chloride) can also be used as a bath treatment against external pathogens at salinities of 10-20 parts per thousand.


Sand is used for filtration (see sand filtration), however certain types of sand, termed "coral sand" have a high calcium carbonate level and are used for pH buffering in very sensitive systems, where the user is trying to exactly mimic the natural environment. The coral sands are however very expensive compared to other types of lime used for pH buffering. 

Sand Filters


Used for the mechanical filtration of solids, not to be confused with fluidised sand filters, which are used for biological filtration. Although the size of particle that the sand filters remove is dependant on factors such as the size of the sand particles, the depth of the bed of sand and the flow rate through the bed, sand filters are usually regarded to filter water to a nominal 10 microns. Sand filters are designed in tow distinct ways; the first is a simple box structure which operates with a very low pressure across the filter. The water flow rate : cross sectional area ratio of such filters is very low, and the filters tend to rapidly block in the first few centimetres, with the rest of the filter staying clean. Such filters are only of use in applications where the use of pressure filters is impossible or the water is generally very clean and there are only a few particles that need removing. Such an example may be a ground water supply which is thought to be contaminated with pathogens through seepage into the spring / borehole. Very large filters of this design are very difficult to clean effectively, usually resulting in the bed being periodically dug out and replaced by fresh sand. Pressurised sand filters are in very common use in many aquaculture applications. They consist of an enclosed vessel which is typically half to two thirds full with sand. See diagram Water is pumped into the top of the filter under a pressure of approximately 1-2 bar and is forced through the sand to a water collecting device at the bottom which allows the water through, but not the sand particles. The flow is then reversed to back flush the filters. Pressurised sand filters are very expensive to use for high flows due to the cost of pumping the water through them. They are however used extensively in hatcheries and also some recirculation systems, where they are either plumbed in for all the water or as a side stream, where only a percentage of the water flows through. Their limitations in recirculation systems is that, in addition to the operational costs, they use a lot of water for backflushing (a typical sand filter in a recirculation system will require back flushing 4-6 times a day for 5 minutes each time. The water flow rate whilst backflushing is the same as the flow rate when filtering). This is exacerbated by the fact that sand filters in recirculated water will also act as biological filters, and a layer of heterotrophic and nitrification bacteria will build up on the sand, causing channeling and increase back washing frequency. The back wash process is insufficient to eliminate all the bacteria which soon multiply and block the filter again. A way round this is to add ozone or other disinfectant chemicals to the water when back flushing (the advantage of ozone here is that any residual amounts after backflushing will quickly be neutralised by the organic compounds in the water). Now that self cleaning mechanical screen filters are available with screens of less than 10 microns, the use of sand filters is becoming less common. 


Chemical contained within tea seed cake with insecticide and piscicide qualities


Organism (such as fungi and bacteria) that feeds by absorbing dead organic matter


Comprises a group of closely related fungi which are found in freshwater, especially at low temperatures. Appears as a pale brown / cream coloured cotton wool like mass. Will rapidly infect dead tissues such as dead eggs, from where it can spread to affect healthy eggs. Will also affect any wounds on the fish, especially at spawning time, when fighting in male fish leads to appearance of wounds at a time when energies are directed towards production of gametes rather than fighting infection. Once it has a hold on a wounded area it can spread to living cells of the fish. Control is by the removal of infected eggs in hatcheries and use of fungicides such as malachite green or other branded chemicals on the market. Saprolegnia will often be seen on wild adult salmon migrating back into freshwater to spawn. this is especially so when that salmon are affected by U.D.N. which leaves areas of necrotic tissue for the fungus to grow on.


Used to describe animals which have been fed to the limit i.e. they will not eat any more.

Saturated - 1

Term used to describe the state of a chemical or compound in a solution or vapour which is at it's maximum equilibrium amount at the given temperature and pressure of the solution/vapour. E.g. when water is 100% saturated with oxygen, it is holding the maximum amount of oxygen that is naturally can at the temperature and pressure of the water. See also supersaturation

Saturated - 2

A fatty acid with no double bonds. As land animals have a high percentage of saturated acids, animal meal is generally used a source for commercial diets. Cheaper than polyunsaturated fats, and are often used to make up a large percentage of the fishes total fat diet. Used for energy. See EFA


Instruments which measure the total gas saturation level in the water. Usually used to determine whether the dissolved nitrogen levels are supersaturated, which could pose a threat to fish stocks. Probes can take up to 20 minutes to stabilise and accuracy is often a problem, especially in aquaculture, where nitrogen saturation levels as low as 102 -103% can cause problems for small fish. 

Scale reading

See Annulus


The bony plates that cover the surface of most species of fish. There are many types of scale, those of Teleosts are either  Ctenoid and Cycloid. Ctenoid scales, typical of fish such as perches and basses, have a serrated / spiky edge, whilst the cycloid scales, typical of carps and salmonids, have smoother rounded edges Scales can be used to read the life history of a fish in the same way that rings can be read on trees. The rings are closer together in winter months, when food is scarce and disturbed areas in the rings denote times of spawning, when the calcium was used for gamete production rather than scale building. See also Annulus


Bivalve. Beginning to be farmed commercially in some areas. One of the highest values of all bivalves.


The "paddle" that is used by crustaceans to provide a water current across the gills.


Suborder of fish (part of the order Perciformes) including fish such as Tuna (Thunnus), Mackerel (Scomber). Fast swimming marine predators.


Order of fish (part of the superorder Acanthopterygii) includes over 1000 fish such as Marine redfish (Sebastes) and Gurnard (Trigla). Head and body usually spiny or bony.

Scouring Velocity

The scouring velocity is the water velocity required to lift solid particles and keep them moving . If velocities are too low particles will settle in the channel, reducing the flow rate. One should be aware that velocities at the bottom and sides of a channel, will be lower than those at the top, due to the drag/friction effects of the bed. Supply and discharge channels should ideally be faster flowing than the scouring velocity for the majority of particles that will travel down the channel. This will avoid the additional management involved in cleaning the channels manually. There is however a trade off with increased headloss.


Used for the removal of unwanted particles for water (see mechanical filtration) and to prevent fish from escaping from tanks and ponds. Slotted screens generally give better performance over holed screens as the ratio of open area to screen material is usually greater. Slotted screens, in particular those with D-ended slots, are also easier to clean, especially when the slots are aligned in the direction of the cleaning brush. Mesh and net screens have a very large ratio of open area to screen material, but are difficult to clean effectively due to particles becoming wedged in the angles of the hole. The stiffer the screen material or screen the easier manual cleaning is. There are generally three types of screen 1. "Cylindrical", "Top Hat" or "Tube" where the screen stands upright, with the mesh around the circumference 2. "Flat" - where the screen lies level with the base of a tank or pond, 3. "Vertical", where a screen is built into the side of a tank or pond. The former two types are generally used in tanks and the latter in ponds for ease of installation. Screens are generally cleaned by hand, however some screens with automated brushes are available. 

Seal Finger  (Crayfish Handlers Disease)

The condition known as "crayfish handlers disease" or "seal finger", is well known in the fishing industry. It can be caused by various bacteria, but Erysipelothrix insidiosa and various species of the Vibrio genus have been specifically incriminated. The bacteria gain entry to the skin via abrasions, lacerations or fissures and cause a painful itching or burning sensation. Swelling then occurs and the adjacent joints become stiff and painful. If left untreated the injury will heal in three weeks although a relapse at a later date is possible.

Symptoms :An itching, burning sensation at the site of the injury followed by swelling. Stiff painful joints. Lymph node enlargement and inflammation may occur, but is rare.

Treatment : Apply a local antibiotic powder or cream four times a day. Can be treated with various antibiotics including penicillin, tetracycline, erythromycin, chloramphenicol and novabiocin.

Prevention : Thoroughly clean all marine cuts and injuries with antiseptic and keep dry and clean. If handling marine animals, wear thick protective gloves. Wear thick boots when wading or fishing. Use a knife to touch things when diving, not your hands.

Seasonal Control

The provision of out of season light and temperature regimes to induce or delay spawning and other physiological processes. See also photoperiod

Seawater fouling

See fouling 

Secchi Disc

A circular disc, divided into quarters and painted alternately black and white. The disc is lowered on a rope of chain in to the water and used as a measure of water clarity. The depth at which the disc can no longer be seen is noted and used as a measurement value. See also extinction coefficient

Secondary consumer

Animal which feeds on other animals (and also sometimes the primary producers).

Secondary Lamellae

See Lamellae

Secondary Sexual Characteristics

These are the features that some species exhibit which are not directly related to the spawning process. Examples include the production of tubercles (raised areas of skin which make the fish feel rough to the touch) of cyprinids, the development of new and exaggerated colours (e.g. in some crustacean species) and the development of bony structures such as the kype developed by male salmonids.


The administration of chemicals to the fish to produce a calming effect without making the fish fully anaesthetised.

Self cleaning

see tank design, mechanical filters

Self Priming

A water pump which his able to automatically pump air out of the pipes and draw water up the suction pipe. True, reliable, self priming pumps are usually very expensive. Submersible pumps or the design of a system to incorporate a flooded suction are usually chosen as a more economical and reliable option.


Generally used in aquaculture to describe the effect of drugs on pathogens especially antibiotics on bacteria.

Sewage Fungus

A mass of filamentous bacteria (primarily of the species Sphaerotilus) which grows in response to organic nutrients in the water. Sewage fungus builds up on almost any surface where there is a flow of water and the necessary nutrients (such as organic wastes form feed and faeces). Whilst generally not a direct threat to fish (although poor water quality can lead to sewage fungus building up on the gills of some species of fish post hatch, when they are static, and lying on the bottom of a tank), sewage fungus can build up in pipes and channels, occasionally breaking off, causing valves and smaller pipes to block. This can in turn limit the flow of water to the fish and cause suffocation or toxic levels of wastes. The density of the sewage fungus is a result of the velocity of water that it is growing in. In fast flowing pipes it will form a dense, almost white, layer, whereas in tanks and sumps it is often very diffuse in it's structure. In systems where the water is not reused, sewage fungus is rarely a problem. In systems where water is reused or recirculated (without the use of water treatment) sewage fungus can be a problem. The growth of the sewage fungus can be limited by careful feeding (waste feed provides a much higher level of nutrients than faeces), mechanical filtration before reuse, the inclusion of biological filters and designing sumps etc. where natural sunlight can enter (as this inhibits the growth).

Sex Reversal

See feminisation, masculinisation and sterilisation


The process of identifying the sex of fish. Performed where the sex of the fish to be farmed is of substantial importance, as, e.g., in semi-intensive and intensive tilapia farms where a mixed sex population will lead to energy being put into population growth through breeding, rather than the growth of individuals.

Entry provided by Menakhem Ben-Yami  

Sexual differentiation

The period when, after hatching, the fish determines whether it will develop into male or a female.


General term for marine decapod crustacean such as Penaeus spp.


Of the same parents.

Side Stream

General term used to describe how a percentage of the main flow is taken away and then reintroduced to the main flow. This is usually to allow a process to take place on a percentage of the main flow. Some processes (such as measurement, oxygen injection, pH buffering) do not require the entire flow of water, and to pump the whole flow of water through is usually more expensive than operating on a side stream. Side streams also allow individual pieces of equipment to be isolated from the main flow for maintenance. Where two different pieces of equipment are used on a side stream, by passes are usually built in to allow maintenance of one without interrupting the other.


Naturally occurring compound of silicone. Used by bacillariophyta (diatoms) for cell structure.


Conical or straight sided vessels which operate on an upwelling or downwelling of water through a bed of eggs. Similar to the Zoug jar method, except that the eggs are not hatched in the vessel. Commonly used as a method of storing excess eggs, or holding eggs whilst parent fish are tested for diseases. Usually leads to higher losses through fungal infections due to the proximity of the eggs to each other and the inability to remove the dead eggs. Channeling of water flows, leading to isolated low oxygen areas have been held responsible for deformities (such as "piggy eyes" in salmonids). Typical flows for salmonid eggs are in the region of 0.3 litres per minute per litre of eggs.


Order of fish (part of the superorder of Ostariophysi) includes over 200 species of fish such as catfishes - Clarius, Ictalurus, Arius

Single stage

Term used to describe pumps which use only one impeller to drive the water, i.e. only a single pumping action, see also multi stage.


The process of removing surface debris and oily films. Of particular importance in marine fish hatcheries where an oily film on the surface can prevent proper development of the fish whilst in their larval stages.

A gate that can be raised or closed to vary the flow of water. Often used to control large volumes of water in channels where pipes and valves are impractical. The flow through a sluice is dependant on the open area of the sluice and the drop (head) between the inlet side and outlet side.


A (Salmonid) fish which is ready for, or actively migrating to the sea from freshwater. Due to the different physiological requirements of marine to freshwater fish. The main changes that the fish undergoes are related to osmoregulation. These are in the form, of changes to the kidney operation, which changes from producing large amounts of urine low in salt content to a small amount of urine, which is high in salt content ( as the fish in seawater tries to preserve it's freshwater), the covering of the fish in a "silvery " layer called "guanine", which acts as an extra waterproof barrier, preventing freshwater loss from the body tissues into the seawater, and the activation of "chloride cells" which excrete salts across the gills. If fish are moved to seawater before the smolting process is finished, there is a risk of high mortalities. The smolting process is governed by the size of the fish and the seasonal light variations. Most salmonids naturally smolt in early to late spring, ready for the migration in late Spring, however manipulation of temperatures ( to speed up growth) and light, have lead to the ability to produce smolts at almost any time of the year. A smolt which travels to sea after only one summer in freshwater is called an S1, fish which travel to sea after two summers are called S2 and so on. S1 fish are generally preferable for aquaculture as they have a lower tendency to become grilse. S1/2 or S0 are now produced by many farms by controlling the temperature and light regimes. See also Grilse


Common term used to describe the physiological process which some Salmonid species undergo as they prepare for migration from freshwater to seawater. See also Smolt

Sodium amobarbitol

Anaesthetic in occasional use for fish. Used only by injection at a dose rate of 85mg/kg fish body weight. 

Sodium amytal

Anaesthetic in occasional use for fish. Used only by injection at a dose rate of 50 -80mg/kg fish body weight 

Sodium Bicarbonate

Chemical symbol NaHCO3. Type of lime sometimes used for raising the alkalinity and pH of water. See table for more details

Sodium Carbonate

Chemical symbol Na2CO3  .Type of lime sometimes used for raising the alkalinity and pH of water. See table for more details

Sodium Chlorate

Herbicide used to control Macrophytes. Applied at 300kg/ha. Applied to pond when dry as a preventative measure. 

Sodium Chloride

Common Salt, the type that is usually used on your food. Can be used as an effective treatment against external fungal and parasite infections but the amount required usually makes dissolving the salt difficult and time consuming. The salt works through causing the pathogens to take in water due to the osmotic pressure and eventually explode or be damaged to the extent that they die off.

Sodium Hydroxide


Chemical symbol NaOH - Chemical sometimes used to raise the alkalinity and pH of the water. Easy to overdose due to the high alkalinity of the chemical, resulting in a very high pH and a fish kill through rapid pH change. Chemicals such as Sodium bicarbonate  are usually preferred as they are safer (with a lower alkalinity)

Sodium Hypochlorite

Strong oxidising agent, used for disinfection of surfaces such as floors etc. Can be neutralised by sodium thiosulphate at the rate of 7 mg sodium thiosulphate to 1 mg sodium hypochlorite. Can also be used to disinfect water at a rate of 10 mg/l. Chlorine must then be removed (either by chemical addition or extensive degassing). 

Sodium Nitrite

NaNO2 Chemical sometimes used for the starting of biofilters as a supply of nitrite for the bacteria.

Sodium Thiosulphate

Na2S203 - chemical used to neutralise chlorine and remove ozone  from water. Can also be used to remove oxygen from the water, for instance, before entering denitrification filters which must be anaerobic

Soft Shell

Term used to describe the stage of moult in crustaceans immediately after the shell has been discarded. Calcium and other minerals are absorbed into the flesh of the animal and are then re-deposited in the new shell. Until this re-deposition is completed, the shell remains soft. Animals are very prone to predation and damage during this period. Crustaceans in their soft shell state command much higher prices as they can be cooked and eaten whole. Some facilities hold crustaceans (especially crabs and lobsters) until they moult and become "soft shell", after which they are immediately harvested. This is widely practiced in the USA with the Blue Crab (Callinectes sapidus) and is gaining popularity in the Far East for the Mud Crab (Scylla serrata).

Soft Water

See Hardness


See geology

Solenoid (Valve)

A valve which is electrically operated and has two states, fully open and fully closed. Solenoid valves use a coil of wire which when energised by an electrical current, opens or closes the valve. If the current ceases, the valve automatically reverts to its non-energised state. The choice of whether to use a normally open or normally closed solenoid is dependant on the process it is being used for. For example a solenoid valve which supplied emergency oxygen would be required to be normally open. When energised it would be closed but as soon as any alarm condition was detected the current to it would be switched off by a controller, and it would open supplying the oxygen. The valve would also open on a power failure making it fail-safe. See also Modulating Valve


see mechanical filtration, suspended solids


The amount of one substance that can be dissolved in another. The solubility of substances can change with alterations in the physical properties (such as temperature and pressure) of either or both substances.


A substance dissolved in a solvent, forming a solution.


One or more substances which are uniformly mixed and will not separate or settle out. 


A liquid that dissolves another substance (or substances) to form a solution.

Solvent cement

A glue compound which contains chemicals which melt or attack the surface of the item to be glued, in order to form a strong bond. e.g. upvc and abs pipe cement. see also solvent welding

Solvent welding

The use of a chemical to bond two pieces of plastic together. The solvent cement is brushed on to both surfaces after they have been cleaned and keyed. The solvent in the cement melts a thin layer of the plastic and when the two pieces are joined and allowed to set, the melted plastic of both pieces and the cement combine to form an amalgamation which bonds the plastics together. A very strong method of joining plastics where consistent and even pressure can be applied (e.g. with pipes and sockets). Solvent welding is limited to only certain types of plastic such as ABS, PVC and UPVC. ABS may be solvent welded to UPVC.


General term for all cells, except reproductive cells in an organism. Thus the mutation of a somatic cell is not heritable.


Abbreviation for "species" where the species is singular see also spp.


The stage in a molluscs life when it changes from free swimming to a stage where it has the ability and the desire to become attached to a substrate. Many mollusc farms (such as mussel farms) rely on wild spat settlement for their seed stock. Ropes or other substrates are hung in the sea during the spring and the wild spat attach to the ropes. The ropes are then either cultured in place, or moved to culture areas. The timing of spat collection is critical as if the ropes are put in too soon or too late, species other than those desired can colonise the ropes.

Spawning cycle

A spawning cycle refers to a period when the animal is producing viable gametes which are ready for the fertilisation process. This can be in a single batch or spread out over a longer period / season see also spawning spread 

Spawning ponds

Special ponds or tanks which are used for short periods to hold brood fish prior to and during spawning. Brood fish are usually removed from the ponds soon after spawning to prevent damage to the eggs. Where spawning tanks are used, the brood fish are encouraged to spawn on a removable media such as spawning ropes or kakabans to which the eggs stick. Some species such as the catfishes will lay their eggs in a sticky mass in pipes and other artificial nests that can be  removed. These are then removed and incubated in a suitable environment, free from predators etc. Spawning ponds take on many forms, however one of the more common types is often called the Dubisch pond, after a farmer in the mid 19th Century of the same name. These ponds were then modified by Hofer in the 20th Century and are sometimes called by his name instead. The typical Dubisch pond is small (approx. 100m2) and shallow (approx. 0.2 - 0.3m deep). They are positioned in a sunny spot and are often surrounded by reeds or a 0.6 - 1.0m high fence to avoid disturbance. A ditch of 0.5 - 0.6m depth is dug around the periphery of the pond, leaving a "table" in the centre. This "table" area is given a slight slope towards the peripheral ditch of about 1 : 15. The Hofer pond is of similar size, but does not have the peripheral ditch and table. Instead, the pond has a sloped bottom (from upstream to downstream) of the same incline, with a ditch dug across the bottom end of the slope, ending at the outlet of the pond. Both types of ponds are used in a similar fashion. For much of the year the pond is left dry and is cultivated to promote a thick growth of grass (such as ryegrass). The grass is mown if it becomes too long. The grass cuttings must be removed, or they will rot and foul the pond when it is filled with water. The ditches are maintained to be free of any vegetation. The ponds are filled when the temperature of the water will be above 18oC at all times and the risk of cold spells has passed. The broodfish are immediately introduced into the pond. The shallow pond heats up quickly during the day, and as long as temperatures are not allowed to fall at night to below 16oC, the fish will be encouraged to spawn. If the ponds are filled too early, it gives opportunity for the establishment of the fauna of the pond, which can included invertebrate predators of small fish such as dragon fly larvae (Family Odonta) and water boatmen (Notonecta glauca). If the pond is filled too early by mistake, it must be treated with an effective insecticide and piscicide prior to stocking. Prior to being introduced, the fish are given a dose of 2.5% sodium chloride (salt) for 15 minutes to rid them of any parasites. Following spawning, the eggs stick to the submerged vegetation. Between spawning and hatching the brood fish are removed by lowering the level of the pond and forcing them into the ditches. This makes their capture easy and avoids disturbing the eggs. About a week after hatching the pond level is very slowly lowered again and the fry (now approximately 9mm) are removed form the ditches and transferred to nursery ponds. Fry are most easily caught on a sunny day, when they remain near the surface of the water. Immediately after removal, the ponds should be treated with quicklime and left dry until the following year. In temperate regions, where high water temperatures cannot be guaranteed, the spawning is often encouraged with the use of hormone injections see hypophysation.

Spawning ropes

Several lengths of unwound ropes, bound together and weighted. Placed in tanks or ponds when fish are ready to spawn, they enable the removal of the egg mass (which in many species is sticky and bonds to the rope) and incubation under controlled conditions.

Spawning spread

Some species (such as Sea bass), spawn over a period of days or weeks rather than releasing all their gametes at once. The period between when they start to spawn and finish spawning is called the spawning spread.


Smallest classification unit used. The group whose members have the greatest mutual resemblance. 

Specific Gravity

Measurement used for density of fluids. Refers to grams per cm2. See also hydrometer, refractometer, salinity

Specific Growth Rate

The average percentage of body weight added per day over a period of time. Used as a means to express growth rate. Useful for growth predictions and comparisons of stocks.


Device used to measure water quality samples by the addition of chemicals to the sample which make the sample take on a colour. The spectrophotometer passes a beam of light through the sample and the amount of light that passes through is converted into a concentration reading of the chemical being tested for (See Beer's Law). Some spectrophotometers will have many in built programs for a variety of water quality parameters. Very cost efficient if a large number of samples of parameters where reliable metering (such as use of oxygen meters etc.) is not applicable (such as for alkalinity, ammonia etc.), are being made. 


The development and/or maturation of the sperm cells


A capsule of sperm which is produced by the male and transferred to the female, where it is usually held on the abdomen. Process used by some crustaceans, the packet of sperm can then be used for a single or multiple spawning by the female.

Sphaerotilus natans

Bacteria that comprise most of the Sewage "fungus" found in pipes and channels. Most is not actually a fungus at all, but a filamentous bacteria


A sudden rise, followed quickly by a sudden or gradual fall. In aquaculture this terminology is often applied to sudden changes in the concentration of a compound (such as ammonia) in the water.


Term given to a reproductive cell that can develop into an individual without the need to fuse with another reproductive cell. May develop into an organism resembling the parent or another stage in the life cycle. Produced by fungi, plants, bacteria and some protozoa


Abbreviation for "species" where the species are multiple see also sp. E.g. the text "Salmo spp." Refers to all species names with the genus Salmo (i.e. those which begin with Salmo) such as Salmo salar, Salmo trutta etc.)


Place where ground water naturally rises above the ground, usually to form a stream, although in some areas such as limestone, the water may return through fissures in the rock to the ground. In areas where there is no place for the water run off (e.g. in a depression in the ground) the spring forms a static boggy area or pond. Springs come in three forms, "Rheocrene", "Limnocrene" and "Helocrene". A "Rheocrene" is a spring which literally flows directly out of the ground, the water is often being forced out under pressure (see also artesian well) and directly forms a stream. A "Limnocrene" is where the water table is higher than the land and forms a pond in a depression in the land, this then overflows to form a stream and a "Helocrene" is a marshy spring, where the water seeps up through an area of soil and/or permeable rock and forms a large marshy area. Water quality from springs is usually very constant, though not always suitable for aquaculture. In addition to mirroring the surrounding geology of the area, spring water is often low in oxygen and high in carbon dioxide. If it has been released from being under pressure, it may also be high in nitrogen gas. Whenever spring water is used it is advisable to aerate or degass the water prior to use because of this. In some areas,. spring water is not necessarily disease free. In some geological formations, underground streams come ot the surface in pools, which may contain fish and other life, before going underground again. See also borehole, degassing, supersaturation

Spring Tide

A tide of maximum amplitude giving rise to the highest high tides and correspondingly, the lowest low tides. Occurs when the sun, moon and earth are in a straight line, which results in the maximum amount on gravitational pull. Such alignment occurs at a new moon and full moon. Spring tides can give rise to greater currents at sea and subsequent, variations in water quality as the sea bed is disturbed. Pump ashore systems will also see a greater variation in the amount of water supplied by the systems pumps. This is as a result of a maximum difference in pumping/suction heads between high and low tides.

Stainless Steel

Stainless steel is available in two forms, 304 and 316. The 304 stainless steel is not as pure as the 316, and can corrode in marine environments, although it is suitable for freshwater applications. 316 stainless steel should be used whenever the metal is to be in contact with corrosive environments. Welding techniques for stainless steel have improved now, but older machinery may be susceptible to fractures and some corrosion on on the welds.

Standard Oxygenation Test Conditions

The temperature and dissolved oxygen conditions that many efficiency figures, given for aeration and oxygenation systems, are quoted under. The standard conditions are for "tap water" at 20oC and 0 mg/l oxygen. This is largely a misleading figure, as it does not reflect the efficiency of the system under normal aquaculture conditions where, as the oxygen concentrations are usually >5mg/l, efficiencies are much less. This is due to the fact that  the greater the difference between the gas and the dissolved oxygen concentration, the easier the gas dissolves into the water. The conditions do however offer a ready comparison between systems, as those that are more efficient at Standard Oxygenation Test Conditions, are also more efficient at higher concentrations. As a rule of thumb, the efficiency of an aeration device at 20oC and 6 mg/l, will be approximately 4.75 times less efficient than it is at Standard Oxygenation Test Conditions. Therefore is a aeration device is quoted as delivering 4.75 kg of oxygen per kW hour under Standard Oxygenation Test Conditions, it will deliver 1kg / kW hour at 20oC and 6mg/l. To properly compare systems that use pure oxygen, the efficiency of use of oxygen (i.e. the percentage that is made available for the fish) must also be considered. The table shows typical oxygen transfer efficiencies for several devices.


A polymer of glucose


Starfish can cause predation problems in shellfish systems. The starfish work their way up the lines or posts from the sea bed and predate on the shellfish as they do so. With lines, this can be avoided if they are kept well clear of the sea bed, as the starfish cannot then become as readily attached. Predation by starfish is not only a problem through the numbers of shellfish that they can consume, but also by the numbers that can become dislodged form the line whilst the starfish is feeding.


A sudden swimming / darting response by the fish when they are suddenly stressed by something. Similar to someone making you jump. Fish often become more easily startled when they are under stress from other sources. Often associated with low dissolved oxygen or other respiratory problems as the fish are at the surface and the startling is easily noticeable as they dart away from the water surface.


Common name given to a strain of rainbow trout (Oncorhynchus mykiss) which migrate to sea.


An organism which is intolerant of changes in salinity, the opposite of Euryhaline.


Fish that can only withstand small differences in temperature, the opposite of Eurytherm


Unable to reproduce. 

Sterilisation 1

The treatment of water so that it is completely devoid of any life forms. This is different to disinfection, which is the process by which one tries to achieve sterilisation, by methods which are not guaranteed. For example a surgery heat treats all it's instruments in a process where it is impossible for anything to survive; this is sterilisation, whereas the washing down of surfaces with chemicals is not 100% guaranteed (even if it is 99.99% guaranteed) to kill all life and this is disinfection. 

Sterilisation 2

The process by which organisms are made incapable of reproduction. See triploid, hormonal sterilisation, irradiation


General terms for a hormone produced either by the adrenal glands or by the gonads.

Stocking Density

The weight or number of fish held per unit area or volume. Stocking densities depend on the species and it's tolerance to the stress of increased overcrowding. For example crustacean species will begin to fight, leading to the loss of limbs and reduction in market value if over stocked. Many finfish species will bite each other (particularly on the fins - see nipping) if overstocked. The effects of overstocking can be alleviated by attention to the water quality and also the feeding regimes. The accumulative effect of stresses on the fish, has lead to the realisation that high stocking densities can be tolerated by the fish if other stresses such as low dissolved oxygen, poor diet etc. are eliminated. It is by using this methods that some recirculation systems, which have total control over their water quality, can achieve efficient production at stocking densities of over 100kg/m3.

Storage - of eggs and milt

The short term storage of green eggs and sperm is carried out for a number of reasons; in order to assess the disease status of parent fish,  to enable movement from one site to another and to ease management of stripping of large numbers of broodstock. More commonly associated with sperm rather than eggs. The object of sperm storage is to maintain the sperm in an immotile state. Salmon sperm is only motile for 30 seconds, tilapia for 2 minutes and marine fish for 1 hour. The sperm is usually maintained in an oxygen rich environment at a depth of 2-3mm. Extenders and microbial agents such as Oxytetracycline are sometimes added to improve survival.

Storm surge

A rise in the height of a tide beyond it's predicted level. Generally caused by wind blowing on-shore, driving more water in with the tide than would other wise be present. Storm surges can overwhelm ponds and other systems built close to the sea, and in particular in funnel shaped estuaries, where the effect of the water surge driving up the estuary, is compounded by narrowing of the estuary, forcing the water higher. Prior to building flood defences, investigation should always be carried out into the history and height of storm surges in the area. Storm surges can also case seawater to intrude further upstream than usual, and can result in seawater entering, what would otherwise be freshwater farms. This can result in fish kills in extreme cases.


A group of organisms of the same species, that either come form the same area (such as the same catchment area of a river) or are the result of a particular breeding program. A strain of fish will be sufficient in number to exist as a self perpetuating population, without the need for introduction of fish from other strains. A particular strain of fish is often distinguished by certain properties (such as colouration, disease resistance etc.) over other strains of the same species.


The separation of different "layers" of water by distinct boundaries. In freshwater the boundaries may are often thermal, where warmer water overlies colder water. In thermal stratification, the upper layers are termed the "epilimnion" and the lower layers the "hypolimnion". In times of low winds and low currents, the stratification of lakes can result in a lack of transfer of dissolved oxygen to the lower layers, which can then lead to an anaerobic zone developing at the bottom of the lake. This can result in the build up of toxic chemicals such as hydrogen sulphide at the bottom of the lake and the entrapment of nutrients, in deep, dark areas, where they cannot be utilised by the algae and other plants in the higher layers, where light can easily penetrate. When winds and currents increase at the end of the summer or during a monsoon, the layers become mixed and the nutrients/toxins mix with the rest of the lake. This can result in algal blooms and fish kills through lack of dissolved oxygen or poisoning by other toxins, especially where fish are held in cages at high stocking densities. Stratification also occurs in sheltered and calm seas, where freshwater entering the sea from rivers, lies on top of the more dense seawater. This can have a detrimental effect on fish (especially stenohaline species) kept in cages, if the freshwater layer becomes too deep.  The use of simple aeration devices such as a few small medium bubble diffusers in the bottom of the lake, will provide sufficient water movement and can prevent the occurrence of stratification and guard against it's potentially harmful effects.


Can be defined as "A stimulus acting on a biological system and the system reacting to that stimulus" Stress changes a biological system from one that acquires energy to one that uses it. Stress can be divided up into two types, acute (such as handling)  and chronic (such as continued poor water quality. Stress is cumulative, i.e. a fish that is suffering from low dissolved oxygen stress will increase stress levels if it is then handled. This is because all the stresses act in the same way which results in the release of plasma corticosteroids from the kidney. In addition to changing the body to one that starts using energy, rather than storing it, this also results in a suppression of the immune system and therefore the animal is at greater risk of disease. The level of Plasma corticosteroids in the body can remain high for longer than the period when the stress occurs, leading to effects from the stress several days after it has occurred. Stress is of paramount importance for a successful aquaculture system, stressors should at all times be kept to a minimum.



The artificial removal of sperm and eggs from fish by gentle pressure applied to the abdomen of the fish.

Sub Littoral

Name given to the zone of shallow areas where macrophyte plant growth is possible. The sub littoral zone ends when there is no longer sufficient light penetration through the water to enable plants attached to the bottom of the water body to grow. See also littoral, profundal


Beneath the skin. Often used to describe the injection of a substance beneath the skin, but not into the muscle tissue. 


A situation which may cause injury or stress but will not cause death. Several sublethal situations may however combine to cause death.


Any solid object or collection of objects. Very broad term.


The area below the average (mean) spring low tide mark.


Group of antibiotic types which includes chemicals such as sulphamerazine


General term for a tank or chamber used as a reservoir, stilling chamber or water collection point.


The liquid that is removed from a tank, once the solids / wastes have settled. The supernatant from solids waste tanks can be very high in B.O.D. and C.O.D. and usually needs some biological treatment (see biological filtration) before release to the environment. Some recirculated farms use the supernatant water (which is also usually low in dissolved oxygen) to supply denitrification filters, which require water low in dissolved oxygen and high in C.O.D. (the carbon in the C.O.D. providing the necessary source for the bacteria). Such systems are largely experimental and not yet in common use.


Powder herbicide used to control Filamentous algae (except Chara sp.) Applied at 600 kg / ha. 


The term given to a body of water which contains more than the "natural" amount of a particular gas or gases. The sum of all the gasses dissolved in the water is called the total gas pressure of the water; under normal conditions this is 100%. Under some conditions however, the gas pressure can increase above 100% and it can then cause problems to fish (such as gas bubble disease). As nitrogen and oxygen form over 99% of the earth's atmosphere, these are the gasses that we are primarily concerned with. Nitrogen can cause fish health problems at anything greater than 100%, whereas oxygen can be safe up to and over 200%. See gas bubble disease for more details. The solubility of a gas under normal conditions is a function of the pressure and temperature of the water. A body of clean water, left open to the atmosphere will become 100% saturated, however if the is water is heated, it's capacity to hold gasses is reduced. If the heating is done in a sealed pipe or other vessel, the excess gas now in the water gas cannot escape, and is retained within the water causing the water to hold more gas than it naturally can at that temperature. The water is now said to be supersaturated. The reverse occurs if the water is pressurised. The water when pressurised (e.g. by a pump), can hold more than it's capacity under normal pressure. This is not a problem unless there is air being sucked into a pipeline, which then readily dissolves. When the water leaves the pressurised pipe and enters a fish tank, it is suddenly back to normal atmospheric pressure. All the extra gas that is now in the water is now excess to the natural amount that the water can hold and the water becomes supersaturated. Oxygen is rarely a problem as in addition to the fish have an ability to withstand supersaturated concentration of up to 200%, the fishes consumption of oxygen, immediately begin to reduce the amount in the water. Nitrogen however can only escape out of the water through the water surface, and a reduction from 110% to 100% can take up to several hours. A tank with supersaturated water can sometimes be detected by observing bubbles forming on tank surfaces and any items put in the water. Often however these signs are lacking as the saturation level is not high enough. The removal of supersaturation is carried out using degassing techniques.

Surface Skimming

See skimming

Surface tension

Denoted in calculations by the symbol "g". The property of a liquid that makes it act as if it's surface was the formed from an elastic skin. Occurs because a molecule below the surface of the liquid is being acted upon by other molecules equally from all sides, whereas one on the surface is only attracted by molecules below it in the liquid. See also rafting for effects on some marine larvae


A substance such as a detergent which when added to a liquid, increases it's spreading or wetting properties, by reducing it's surface tension.

Suspended Solids

Particles larger than 0.45 microns which are found in the water column. The maximum size of suspended solids is dependant on their specific gravity and the velocity and turbulence of the water. In turbulent waters large and heavy items can remain in the water column, however in slower moving and still waters, these items may settle to the bottom and are then not regarded as suspended solids.

Swim Bladder

An air filled sac that the fish is uses to control it's buoyancy.


A piece of cotton wool, mounted on a stick or held with forceps, tweezers etc. Used for cleaning wounds, applying localised chemicals for treatment and taking wet samples for laboratory examination. 


Small paddle like appendages that some crustaceans use for prolusion and also to supply fresh well oxygenated water to the egg mass.


Term given to fry that have finished absorbing their yolk sac and are ready to start feeding. Many species swim to the surface to gulp air to fill the swim bladder. 


The existence of two organisms which live in partnership, in which the mutual advantages, are greater than the disadvantages. An example is the symbiotic relationship between xoothanthellae and giant clams (Tridachna spp.). The xoothanthellae (species of algae) live within the tissues of the clam, this provides a substrate for the algae where there is plenty of light (the clams lie open in shallow, well lit waters). It also renders the algae free from predation by filter feeders etc. The algae impart nutrients into the clams tissues, which in turn provide nutrients for the survival and growth of the algae. Therefore both organisms benefit from the partnership.


A group of symptoms which, even without the evidence of a disease causing agent, define a disease. Many new diseases often start out being regarded as syndromes, until the causative agent is discovered.


In combination with. For example some heavy metals act synergistally with others meaning that the presence of the other metal increases the overall toxicity effect to the fish.


Order of fish (part of the superorder Acanthopterygii) includes fish such as sea horse (Hippocampus) and Cornet Fish (Fistularia)


Affecting the whole body. Applied to diseases to indicate that they are not localised infections or infestations.