The Free Online Aquaculture Dictionary



Tumour like pocket where blood escapes the blood vessels (usually as a result of physical damage) and collects. Has the appearance of a fluid filled sac which may be floppy or quite hard, depending on the state of the damage and internal healing that has taken place.

Haematopoietic tissue

Area where blood and blood cells are formed. In fish this is mainly in the anterior end of the kidney.


A protein in the red blood cells which carries the oxygen and carbon dioxide in the bloodstream. The protein has four "haem" groups, each of which can bind one oxygen or carbon dioxide molecule. When the haemoglobin is rich in oxygen it is sometimes called oxyhaemoglobin. At the tissue site where oxygen is absorbed for respiration, carbon dioxide takes the place of the oxygen molecule. High ambient carbon dioxide concentrations in the environment, can lead to an inability for the haemoglobin to release all it's carbon dioxide across the gills. This in turn leads to a lower take up of oxygen in the blood as one or more of the haem sites is occupied by carbon dioxide, preventing the oxygen from binding. See also Bohr effect.


Bleeding in the body tissues. Caused by damaged capillaries and other blood carrying vessels. Common in many systemic bacterial diseases where the vessels become damaged and/or blocked by attack from the bacteria.

Hand Pumps

Hand pumps are sometimes used for supply of water for very small operations (eg to fill small ponds), or for the extraction of chemicals from drums etc. 



Small (typically 1-5m2), fixed, net enclosures sited in ponds. Usually pegged by a number of sticks/posts with the net strung between them. Often used in ponds in tropical areas for fry and broodstock. Enables cost effective method of control of broodstock and fry within large rearing ponds.


A cell with a a single set of unpaired chromosomes. reproductive cells  formed as a result of meiosis are haploid, fusion of the cells (i.e. fertilisation) results in the normal diploid number of chromosomes (i.e. two pairs). Diploid organisms generally produce haploid gametes.


The amount of cations of the earth metals (mainly calcium and magnesium) in the water. In most waters, the hardness is similar to that of alkalinity, as calcium and magnesium are usually bound to the main alkalinity bases (bicarbonate and carbonate). Alkalinity tends to be used more as a measurement than hardness. Water is often referred to as "soft" or hard" see Table.

Hatchery Constant

A figure used in some countries to determine the correct feed rate when temperature, food conversion and growth rates are constant. Percent of body weight of feed per day  = (3 x food conversion x daily length increase x 100) / length of fish. This figure is sometimes useful for new species, or when using diets where the correct feed rates are not specified. It does however require some estimation, as the food conversion and growth rate are likely to change with feed rate, until the optimum feed rate is achieved. Therefore the desired figures of feed conversion and growth rate are often used. This method of calculation is declining as commercial diets with recommended feed tables are available more countries, and for more species.

Hatching Jar

See Zoug jar


See Human chorionic gonadotropin


High density polyethylene. Plastic, often used for items such as tanks, floats etc. Has a high degree of resistance to chemicals and is easy to keep clean. Very difficult to adhere to with glues etc. silicone is one of the only adhesives that will give some bonding. Easily welded, but note that high density, medium density and low density polyethylene's will not weld to each other


Expression of pressure in height of water. The height of one body of water above another at the first place where it is open to the atmosphere. For example, if the water is piped from a lake in the hills down to a farm in the valley and the surface of the lake is 100m above the surface of the fish ponds, the water has a head of 100m at the level of the fish ponds. If however, the pipe is stopped halfway, the water allowed to form another lake and then piped the rest of the way, the head at the fish tanks will only be 50m see also friction loss for calculations for air and water loss in pipes

Headloss - air

See friction loss - air

Headloss - water

see friction loss-air, friction loss-water

Heat Exchangers

A device for transferring heat from one fluid to another, without the two fluids coming into contact with each other. Heat exchangers are generally one of two types. Type 1 is the simplest and is a series of tubes through which one fluid flows, enclosed in a container in which the other fluid flows in the opposite direction. A variant on this is using a tube within another tube. Type 2 is a series of plates packed together. One fluid flows through the gap between every other set of plates whilst the other fluid flows, in the opposite direction through the remaining gaps. This type gives a very large surface area for heat exchange, and is generally more efficient than the tube type. In both types, the materials should be made from a material that is a good heat conductor (e.g. metal) rather than a poor one (e.g. plastic). A recurring problem with all heat exchangers, especially if the water passing through them contains solid or dissolved organic particles is that of fouling. Over a period of time, the gaps or tubes in the exchanger will get a coating of floc and solid particles which will restrict the water flow. It is therefore essential that heat exchange systems can be backflushed. Backflushing is often carried out with the addition of a chemical such as Sodium Hypochlorite, which assists in the removal of organic matter. A good heat exchange system can typically get one fluid to within 1.5 - 2oC of the other fluid. This is possible because the two fluids flow in opposite directions to each other (The diagram below shows how this is possible). Heat exchangers are therefore only useful if the temperature gain/loss required is greater than about 3-4oC, otherwise the capital cost of the exchanger, combined with the addition friction loss and subsequent pumping power required, makes their use uneconomical. See also Heating systems

Heat fusion welding

See Butt Welding, Heat gun, Plastic Welding

Heat Gain

Heat gain is a term used to describe two conditions. Its first use in describing the amount of heat produced by an animal as it metabolises it's food, the second use is to describe the increase in heat of any system, from all sources of heat and energy (such as solar gain, heat from pumps etc. which have an impact on the system. 

Heat Gun

Hand held, controllable, hot air blower used for plastic welding. The hot air melts the plastics. Guns usually have different heat settings for different plastics.

Heated Effluents

Power stations and other industrial plants use high volumes of water to cool machinery (including nuclear reactors). This cooling water is then usually discarded by the plant, back into the environment at elevated temperatures. Many attempts have been made to utilise this effluent water to grow warm water species n temperate regions or to accelerate the growth rate of temperate species. Although some systems have succeeded, the majority have failed. These failures have been largely as a result of inconsistency of water supply and water quality. A fish farm attached to a heated effluent is often good for public relations and may bring in a small additional income to the company. In general however, these benefits are far outweighed by the need for the plant to carry out it's business and so the company will ensure it's own security of production first, even if this means making problems for the fish farm. Coal, oil and gas fired power stations tend to have a more consistent temperature of outlet water than nuclear power stations. This is due to the fact that nuclear power stations require additional safety measures that result in the reactors shutting down and temperature fluctuations. The dosing of chemicals such as chlorine in to the intake pipes to reduce fouling (especially in marine supplies) can also cause problems for the fish farm. Water recirculation is generally regarded as a more reliable method of maintaining consistent elevated temperatures.



Heating systems are used in many hatcheries to boost growth rates, in recirculation systems to maintain temperatures and in broodstock units to control spawning. The table available gives details of the different types of system available. Calculations to work out the energy required to heat water are also given. See also heat exchangers

Heavy Metals

A general term used to describe metals with high relative atomic mass. Such metals include copper, lead and zinc. When dissolved in the water, such metals are toxic to most aquatic life. Heavy metals are leached from the ground into water supplies when the water is acidic (generally below pH5.5). Problems do not therefore readily occur in alkaline waters. Toxic levels of the metals can be cumulative, e.g. if copper is present it increases the toxicity of zinc. Heavy metal poisoning can also be a result of industrial pollutants. A feature of heavy metal poisoning, is that mortalities often do not occur until one or two days after the exposure. This can make detection of the cause of death a problem. 


Type of Spring


See Haematoma

Hematopoietic tissue

see Haematopoietic tissue


See Haemoglobin


See Haemorrage


Of female gender. Opposite - "Cock" - of male gender


A chemical used to kill unwanted plants. The use of herbicides in an aquatic environment must be carried out carefully. Some chemicals used as herbicides on land, will, if allowed to come into contact with the water, also kill aquatic life. Care must also be taken when killing off aquatic weeds that only small areas are treated at any one time. This is because the decaying, dead plants in the water can have an impact on the water quality (in particular, low dissolved oxygen concentrations). The table gives the recommended dose rates for some commonly used herbicides, with their target plants listed. See also Herbicide susceptibility and Herbicide withdrawal periods

Herbicide Susceptibility

Most herbicides will be effective only against certain species or types of plants, the table gives susceptibility information for four of the main herbicides used in aquaculture. See also Herbicide withdrawal periods

Herbicide Withdrawal Periods

Some herbicides require that water where herbicides are used in not used for irrigation for a specific time, otherwise the water may damage crops. See also Herbicide susceptibility


Animal that consumes vegetable and decayed organic matter as the principle constituent of diet


Animal that posses both sperm and ovaries. Able to release both eggs and sperm into the water for fertilisation. Eggs and sperm are released at different times to prevent self fertilisation. Many molluscs are hermaphroditic. Most plants are hermaphroditic as they contain both the stamens and the carpels (the equivalent of sperm and ovaries in plants).


Term referring to a forked tail (caudal) fin see also Homocercal


See Hybrid vigour


An organism which is termed heterotrophic (e.g. heterotrophic bacteria), is one which gets it's energy from the intake and digestion of organic substances. All animals get their food this way, through the consumption of plants and other animals, however the term is more commonly used to distinguish between the different types of bacteria which are found in ponds and biological filters. The heterotrophic bacteria are those which lower the BOD of the water by consuming the dissolved and particulate organics from the faeces of the fish. The organic matter is oxidised to carbon dioxide, ammonia and water. These bacteria are often distinguished form the nitrification bacteria (nitrosomonas and nitrobacter) which use the ammonia and nitrite respectively for their energy source. The heterotrophic bacteria are more dominant than nitrifying bacteria and so colonise surfaces first. In a fixed bed biological filter for example, the heterotrophic bacteria will often dominate the first stages. Another example of a type of heterotrophic bacteria is the sewage fungus commonly found in areas of high organic loading.


The arrangement of populations into a graded order. In animals, a hierarchical system results in a graded population from very dominant individuals down to very passive individuals. The dominant animals can monopolise the feeding stations which results in their becoming larger and even more dominant. This perpetuates and accentuates the hierarchy which can lead to poor feed conversion rates and a percentage of the population becoming poorly conditioned. through starvation and bullying.


The study of body tissues. Histological examination of tissues from dead or dying fish can be used to determine the cause of death.

Hofer Ponds

See Spawning ponds


Something that people working in more sensible industries have.


Warm blooded. Body temperature is independent of the temperature of the surrounding environment.


A tail fin with a single lobe (i.e. not forked) see also Heterocercal

Horizontal transmission

Term used to describe the transfer of a disease agent from one individual to another. Does not include the transmission of disease agents from parent to progeny through reproduction (Vertical transmission)

Hormonal sterilisation

The feeding of high doses of 17a-methyl testosterone to young fish as they are undergoing sexual differentiation with the aim of making them sterile. This has been achieved successfully with several species. Sterile salmon still migrate to sea as normal but do not migrate back again, some rainbow trout will begin to develop gonad tissues after 2 or more years. The use of hormones in fish which are to be consumed by humans, receives a lot of consumer resistance which is why in many countries, it is not practiced.


Naturally occurring (produced in endocrine gland cells) or artificially manufactured chemical, that affects the function of organs.


An animal on or in which a parasite lives.


Highly Unsaturated Fatty Acid, essential for some species such as carnivorous marine larvae. see EPA, DHA, EFA

Human Chorionic Gonadotropin

A hormone produced by the developing human blastocyst and placenta. In humans this serves to prolong the production of oestrogens and progesterone without which menstruation would begin. It's presence in the urine is a sign of pregnancy in humans. Used in aquaculture to induce ovulation and spermiation Available commercially as a semi purified extract, (SPE). Also available as a purified SPE, known as SGG100


The concentration of water vapour in the atmosphere. The relative humidity is the ratio, expressed as a percentage, of the amount of moisture in the air to the amount that the air could hold if it were saturated at the same temperature and pressure. High humidity can occur in poorly ventilated fish farming buildings, caused by the evaporation of water into the air through water surfaces and waterfalls. As many fungi etc. thrive in humid conditions, high humidities can cause spilled food to quickly become covered in fungus, which then release spores into the air, causing a health risk to farm workers. Very humid buildings also often result in poor fish husbandry as the workers try to complete their task in the minimum time, so that they can leave the building (and the unpleasant conditions) as soon as possible.


The occupation of the care of animals

Hyamine (3500)

Trade name of a Quaternary Ammonium Compound (amine salt) used for external infections such as bacterial gill disease. A colourless liquid. The chemical is generally more effective than others as it also "washes" off a lot of the excess mucus which has grown on the gills, in response to the bacteria. Without the removal of this mucus, the chemical would not be able to effectively penetrate the mucus layer to kill all the bacteria. Treatment is sometimes followed 24 hours later by a Chloramine T  treatment to ensure eradication of any remaining bacteria and additional parasites that have taken advantage of the weakened state of the gill area. Hyamine 3500 is typically administered at 2 - 4ppm for 1 hour, with the lower concentration used in poorly buffered waters (<100mg/l CaCO3 alkalinity)


An organism that is cross bred between two species. See also F1-Hybrid, F2-Hybrid and Hybrid vigour

Hybrid Vigour

Term referring to the occurrence where hybrid offspring perform better than the parents. Also called Heterosis. See also Hybrid, F1-Hybrid, F-2 Hybrid

Hydram Pump


Also called a hydraulic ram, or impulse pump, this is a device which uses the energy of falling water, to lift a lesser amount of water to a higher elevation than the source. A ram should be considered when there is a source that can provide at least seven times more water than the ram is to pump. The siting of the ram must be at least 0.5m below the water source. The main advantage of rams is that they require no electrical or mechanical power to operate. The situations where they can be located are however, very few.


Combining with water.

Hydrated Lime

See Quick lime

Hydraulic Ram

See Hydram pump

Hydrogen ion

(H+) The ion responsible for acidity. The more hydrogen ions in the water, the greater the acidity and the lower the pH. Hydrogen ions come from many sources including mineral deposits and biological processes such as nitrification which produce hydrogen ions as a result of the conversion of ammonia (NH4-) to nitrite (NO2).

Hydrogen Peroxide

Chemical used for treatment of sea lice in cages. Advantage of the chemical is that is breaks down into water and oxygen

Hydrogen Sulphide

Hydrogen sulphide is produced by bacteria in anoxic waters, silts and muds, especially where organic loadings are high, such as beneath cages and in heavily stocked ponds. Hydrogen sulphide exists in two forms in the water, HS- (ionised sulphide ion) and H2S (unionised hydrogen sulphide); the H2S form is toxic to fish. Analytical methods for the detection of hydrogen sulphide measure total hydrogen sulphide and so the table given (which shows how the proportion of H2S varies with pH) should be used to determine the proportion of this which is in the toxic form.
In well oxygenated waters, sulphide is rapidly oxidised to sulphate. The best way to stop hydrogen sulphide being formed, is therefore to maintain a well oxygenated and mixed system, especially close to the sediments. This can be achieved by using aeration or oxygenation techniques.

Hydrological cycle

A continuous process by which water is transported from the oceans to the atmosphere by a process of evaporation, then to the land by clouds and precipitation (rain), and back to the sea again through rivers and underground waters.


A chemical reaction of a compound with water. The hydrolysis of certain feedstuffs can cause them to breakdown and eventually become rancid


Device for measuring the density of fluids. Available pre-calibrated for salinity measurements. Can be difficult to use in the the field due to fragility and the need for a level, undisturbed water surface. inexpensive. Some hydrometers read in S.G. (specific gravity) the table gives conversion of S.G. to salinity. See also refractometer.


The growing of plants, which usually grow on land, without soil, in a nutrient enriched water solution. Some fish farms grow plants as a secondary crop in their waste water. The nutrients from the fish waste feed the plants. This water is sometimes recycled back to the fish. There are some designs of recirculation systems which do not use biological filters at all, but use hydroponics to purify the water. Such systems are mostly laboratory scale or still in their early stages of development.


Anaesthetic used for fish as a dip / bath treatment. Generally used at 10mg/l.


An increase in the amount of blood in the body causing the blood cells to stretch


The production of excess mucous on the gill. The mucous is produced as a response to an irritant, with the intention of protecting the gill. May be triggered by physical damage from suspended solids in the water (esp. some bacillariophyta (diatoms), which have sharp edges and so lacerate the gills), infections by pathogens or the water chemistry. The mucous around the gill will often cause the opercula to be pushed outwards. Treatment is usually a combination of quaternary ammonium compounds to clear the mucous, and the removal of the irritant to prevent it recurring.

Hypobromous Acid

HBrO, Bromic Acid - Produced as a result of the oxidisation of seawater with very strong oxidising agents such as ozone. Highly toxic to all life. Seawater contains high levels of bromine which is usually bound to other chemicals such as potassium or sodium (total elemental composition approx. 65mg/l). Following ozoneation (for sterilisation or maintenance of general water quality) the bromide forms hypobromous acid. Hypobromous acid is itself a strong oxidising agent (like ozone) and it's life before degradation is largely dependant on the organic loadings of the water and also the pH. If organic loadings are high, the acid will have a short life. If however loadings are low for example in systems with high REDOX potentials (>300), the acid will have a long life and could build up in a system, leading to mortalities.


See Sodium hypochlorite


The lower layer of a thermally stratified lake, see also stratification


The use of hormones to promote maturation of gonads. Hypophysation techniques are widely used in aquaculture, as their use enables the spawning process to be more predictable and reliable. The process is especially used for fish such as cyprinids, where maturation is dependant on a number of factors and not as predictable as other fish such as salmonids.


The part of the brain which controls many of the internal body functions and also the activity of the pituitary gland which responds to releasing hormones (such as GnRH) produced by the hypothalamus.