The Free Online Aquaculture Dictionary

T

Tagging

Tagging of organisms is used in aquaculture primarily to enable the tracing of specific animals and family groups etc. whilst rearing them as part of a larger mixed group. Many farms use tagging techniques for broodstock, enabling easier management as they can be reselected should their offspring perform well. There are many types of tag, including some which can cause a lot of physical damage to fish, such as those which penetrate deeply into the muscle. More modern tagging techniques use microchips (PIT tags), which are inserted just under the skin. The microchips can then be read with a special scanner much like a barcode. Such modern techniques are less damaging to the fish, which is especially important where the tagging is being used for trials, as the ability of some fish to cope better with the tag, will influence the result of the trials. A third type of tagging involves using a device (Panjet) to inject a measure of dye into the skin or fin of a fish. These dye markings are limiting in their lifespan (which can be up to 1 year) and are also limited in the number of variations that can be used, to identify individuals. 

Taints

Flavour that can be imparted to the flesh of a fish as a result of it's environment. Can render the fish unmarketable. Often associated in fish reared in earth ponds, where the fish can take on a muddy flavour, which is thought to be due to certain types of algae which are ingested. Fish reared in recirculation systems can also carry taints, the cause of which is uncertain, but is related to compounds that can be broken down by the addition of ozone to the water. In general, if sufficient ozone is being added to a recirculation systems to keep the water clear, taints will not be a problem. Pollution incidents (such as diesel spillage) can also impart taints to the flesh. Taints can be flushed out of fish by holding them in a clean, taint free water supply. The time taken for taints to be removed is dependant on the type and intensity of the taint. Muddy flavours and those associated with recirculation systems, can in general be removed by a few days in such holding facilities. Taints such as diesel, can take many weeks to disappear.

Tambak

Indonesian name given to ponds constructed in coastal, brackish areas such as mangroves, and used for milkfish and mullet culture. 

TAN

Total Ammoniacal Nitrogen See Ammonia nitrogen

Tank design

There are three main tank designs used in aquaculture, Round tanks, D-ended tanks, Raceways (although there are many variations to the basic types). Due to the cost of construction, tanks are usually only used for hatcheries and the intensive culture of high value species. See also screen design, scouring velocity 

Tapeworms

Parasites that infect the gut of the fish. Of commercial importance in some cyprinid species (especially Caryophyllaeus fimbriceps and Khawia sinensis) and occasionally others (e.g. salmonids - Diphyllobothrium sp. which has been connected with outbreaks of P.K.D.). Often associated with other animals such as oligochaete worms (see also Tubifex tubifex) which ingest the eggs excreted by the adults.

Taura Syndrome

Disease affecting shrimp species. Thought to be a virus which is complicated by the use of certain pesticides / herbicides in banana plantations. Responsible in the past for the decimation of many shrimp farming areas. Some farms now experimenting with the use of filtration (including ozone systems) to remove the pesticides / herbicides from the water before entering the farm.

Taxon

Organisms which comprise of a particular taxonomic entity e.g. a particular class, family or genus. See also classification

Taxonomic category

A category used in classification

Tea Seed cake

A residue that remains after the oil has been extracted from the seeds of certain plants of the Camellia family. The residue is compressed into a cake and contains saponin which is toxic to most aquatic life as it reacts with the blood. Used widely to eliminate fish and insects from ponds prior to stocking. The saponin detoxifies quickly in the water and is not toxic to cattle or humans who use the water.  Generally administered at 10-25mg/l as a piscicide.

Teleosts

Group term for bony fishes (i.e. those with vertebrae). Derived from the word Teleostei, a taxon of uncertain category (e.g. suborder, class). Falls within the Subclass Actinopterygii. Of the fish in the group, the soft rayed fish such as salmonids, herring and carps are considered relatively primitive, but the spiny rayed fish such as perches are considered more progressive

Telson

Tail of the abdomen in crustaceans

Temperate

A region where the weather is neither extremely hot or extremely cold. Sometimes defined as the region between the Tropic of Capricorn and Antarctic circle in the Southern Hemisphere and between the Tropic of Capricorn and the Arctic circle in the Northern Hemisphere.

Temperature

   

Measurement of heat. As fish are cold blooded their metabolism is temperature dependent. A fishes metabolism increases with temperature. All fish have an optimum temperature for growth and food conversion rate. This can vary between species and sometimes between strains of species. Fish will become increasingly stressed at temperatures outwith their normal growth range and will cease to feed. In general oceanic marine species are more stenothermic than freshwater and coastal fish, as they have evolved to withstand the temperature changes of the ocean which are slower and less extreme than other environments. For species where the limits are unknown, it is wise to look at their natural environment and the temperatures that they regularly experience, and use these as a guideline. Sudden changes in temperature (even as low as 2oC) can stress fish. When fish are being transferred to water of different temperature, every effort should be made to reduce temperature shock to a minimum. Growth rates and egg development rates are governed by temperature (see degree days). Optimum temperatures for some species are given below, however these should be regarded as guidelines only, as they can change with different developmental stages and acclimation: See also heating, chilling

Terbutryne

 Herbicide used for the control of Submerged plants and algae.  Susceptible plants - 5kg / 100m3, moderately resistant plants - 10 kg / 100m3. See Herbicide susceptibility, Herbicide Withdrawal

Tertiary amyl alcohol

Anaesthetic used at a rate of 1 - 10 ml/l in water.

Testosterone

Hormone secreted by the testes. Responsible for the controlling of the production of sperm and, along with ketotestosterone, the development of secondary sexual characteristics. Included in feed given to some species during early developmental stages to produce all male stocks for future spawning use see masculinisation.

Tetracycline

Group of antibiotic drugs including Oxytetracycline

Tetraodontiformes

Order of fish (part of the superorder Acanthopterygii) includes fish such as Triggerfish (Balistes), Pufferfish (Tetradon) and Boxfish (Ostracion)

Tetraploid

A cell with four sets of chromosomes (double the normal amount, see diploid)

Tetraselmis

Marine flagellate green algae. Used often in marine hatcheries. Larger than most other cultured algaes, (8-16 microns diameter), it is much easier to culture than some of the smaller algaes such as Isochrysis and Nannochloropsis

Theraputant

Chemical which serves to heal or cure

Thermocline

The layer of water between the epilimnion and hypolimnion in a stratified body of water. The layer where there is a large difference in temperature per unit depth.

Thermocouple

A device consisting of two different metal wires, for temperature measurement. One of the wires is exposed to the temperature to be measured and the other is maintained at a known temperature. An electro motive force (e.m.f.) is developed as a result and this can be read by an instrument in millivolts and converted into temperature units.

Thermoplastic

A plastic material that can be repeatedly softened through heating and hardened again on cooling. An example is UPVC, which when heated, softens to enable moulding and welding, but on cooling hardens. If the finished product is not correct, the material can be heated, and manipulated again.

Thermosetting

A plastic that is initially soft, but once changed to a rigid form (e.g. by heating), will not reverse back to the soft form on cooling. An example are the resin based compounds that are used as adhesives, where once the reaction (involving heat) has taken place, they cannot be softened back to the liquid glue form again.

Thermostat

A device which controls the heating or cooling of a substance, by turning machinery on or off,  in order to maintain a constant temperature. 

Thunderstorms

Can affect fish farms either through lightening strikes, or more commonly through sudden changes in air pressures which can reduce the amount of dissolved oxygen available in the water.

Tides

Rise and fall of the sea occurring twice each lunar day. See software link (freeware) for tide predictions for all areas of the world. Tides can affect aquaculture systems in a number of ways:

1. As tides rise and fall in sea lochs and fjords, currents can be formed which will allow good water exchange rates in cages, supplying dissolved oxygen to the fish and taking away waste products. If water velocities are too high, the cage net can be pulled at an angle (see net bagging), reducing the volume of the cage and damaging the fish.

2. Pump ashore systems can experience changing water quality  between the tides. This is especially so with temperature and salinity. Where the high tide floods sand or mud flats that have been warmed by the sun, as the water draws back to low tide, temperatures on the farm can rise. Similarly where a river enters the sea near an inlet, low tides can result in lowering of salinity.

3. Tides are essential for the maintenance of some shellfish growing techniques. The window of low tide allows the farmer to inspect and work with the stock.

Tilapia

Follow this link to the main Tilapia species page

Tip length

 The distance from the snout to the tip of the tail. Not often used in farmed fish due to interferences associated with erosion of the tail. Fork length is more commonly used, as this is largely independent of fin erosion.

Tissue respiration

See respiration

Titration

The gradual adding of controlled and measured amounts of a substance (often a reagent) into another (often the sample) to elicit a response. The measured amount added can then be converted into a value that denotes the concentration of a substance in the sample.

Toxic

Poisonous

Torpor

A state of apathy. Animals appear sluggish and unwilling to exert energy.

Torsion

The twisting of the body in development of gastropods

Transpiration

The loss of water vapour by plants to the atmosphere. Occurring mainly from leaves through pores, whose main function is the exchange of oxygen and carbon dioxide. Water lost through the leaves is replaced by a continual flow of water (and nutrients) up from the roots. Also sometimes refers to the cycle of the uptake of water by plants and subsequent loss to the air as water vapour through the leaves

Trash Fish

The feeding of low value species such as mackerel, anchovy and sand eel in a whole, chopped or minces form. The feeding of trash fish is carried out in many marine farms where either it represents a cheaper option than using pelleted diets, or the species will not feed on pelleted diets (eg yellowtail (seriola quinqueradiatia). Trash fish is losing popularity as a diet as commercially prepared diets become more readily available. The feeding of trash fish involves problems such as increased risk of introducing a disease organism, storage difficulties, consistency of quality / supply and fouling of the water.

Treatment Method 

The way in which chemicals are administered to stock depends on the chemical, the dose rate and the type of holding system they are in.

Trematode 

Common term for flukes in the Class Trematoda. Comprises of external (monogenetic) and internal (digenetic) parasites of vertebrates. Main species of concern for aquaculture are the eye flukes, see Diplostomum.

Tri butyl tin oxide

 (TBT) Chemical used in paints and other solutions to give an antifoulant property. Once often used in aquaculture for the treatment of nets, the chemical also affects wild populations of molluscs and crustaceans, living in the vicinity of cages and so is banned in some areas as a result of pressure form environmental groups and fishermen.

Trioxygen

See ozone

Triploid

A cell that has three sets of chromosomes. Such organisms with triploid cells rather than the usual diploid (2 sets) are sterile. The production of triploids in some species is desirable as there is no loss of condition caused by the maturation processes. The process of turning fish into triploid involves either heat, cold, pressure or chemical shocks during the period just after fertilisation. The process only works on female fish; the male fish are only partially affected. The treated male fish are unable to produce sperm, but still produce testes and all the secondary sexual characteristics which accompany this. Male parent fish for eggs to be subjected to triploidy treatment are not true males, but a result of feminisation techniques which ensures that there are no male chromosomes in the parents. There are various method of inducing triploidy in Rainbow trout and Atlantic Salmon and some of these are detailed in the table. Triploidy has also been used successfully with flatfish, common carp, channel catfish and tilapia (Oreochromis aureus). Triploid flatfish, catfish and tilapia have been shown to perform better, however in rainbow trout, this is unproven and in mixed populations of triploid and normal diploid fish, the triploid fish are not as aggressive feeders as the diploid which outweighs most of the advantages gained. 

Trochophore

The first free swimming stage of mollusc larvae. Followed by veliger and pediveliger.

Trophic level

Classification of organisms according to their feeding relationships

Tsunami

Tidal wave produced as a result of earthquakes and volcanoes. Such tidal waves are caused by relatively small wave heights in the deep oceans, which when hitting shallow reefs and continental shelves, increase to great heights. 

Tubifex tubifex

Commonly called the "freshwater bloodworm" or "sludge worm". Species of oligochaete worm found especially in polluted waters. Indicated as intermediate hosts in some diseases such as tapeworms. Used by some aquarists as a live feed for their fish but have little value as feed in aquaculture.

Turbidity

The ability of light to penetrate through the water. Turbidity is caused by the sum of suspended solids particles and any dissolved chemicals in the water which may restrict the passage of light through the water. The most common method of measurement is by using Secci disc. Electronic meters are available for measuring, but these are only of use in aquaculture in green water systems as the sensitivity of the probes is generally not high enough for other waters. The turbidity of the water can affect aquaculture in a two ways ; 1.By limiting the feeding capabilities of sight feeders (such as salmonids), 2. By restricting the light available to algae and plants in the water, causing them to die off or limit growth. The treatment of turbid water is dependant on the nature of the particles or chemicals that are causing it. 

Turbidostat

Device which maintains a constant cell density of algae by continuous harvesting of algae and dilution of remaining biomass, and constant addition of nutrient chemicals etc.

Turbines

A machine in which a fluid or a gas is used to produce a rotational motion. Most commonly used for generation of electricity. The most common turbines found in aquaculture are powered by wind or water. Wind turbines are useful for providing electricity in remote locations, especially for recharging battery packs for feeders, alarms etc. Wind turbines are flexible and can be easily relocated. Water powered turbines are used in some mountainous regions where farms have a supply from a body of water much higher than itself. The power generated by a turbine = head x flow (P=H x Q). On normal farm sites, water power is not commercially viable, for example a water turbine with a flow of 22,500m3/day with a 4.5m head will only generate about 7kW if used as a generator. If used as a direct drive for a pump it will produce the equivalent of 9kW. Small hydroelectric generators typically take between 3 and 5 years to pay for themselves, but this does not include the additional management and maintenance time of such a system.

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