The Free Online Aquaculture Dictionary



The device in a pump which provides the pumping action. Impellers are classified as either radial, axial or mixed flow. Impellers are available in a wide variety of materials and it is important to provide information to the pump supplier when specifying a pump exactly what is to be pumped. Impeller designs vary according to the material that they are pumping. Some impellers will for example take quite large solids, whereas others will not. Many impellers are suited to marine conditions, whereas others will corrode very quickly, leading to a loss in pumping efficiency. Impellers also vary in that they may be closed, semi-closed or open. For pumps which need to handle solids, either open or semi-closed impellers are required. In general the more open the impeller, the larger the solids that will pass through. There is however a trade off as a more open impeller results in a loss of efficiency. The more closed an impeller is, the greater it's pumping efficiency, but the greater also the wear rate if used for pumping solids. For seawater pumps where there may be an appreciable amount of solids, semi closed impellers are recommended. Some pumps have what is termed a multistage impeller. These are pumps where a number of impellers are used to boost the water to higher pressures than would be achievable with a single impeller

Impervious Rock

Rock which is not water bearing i.e. Water is unable to pass through the rock. Boreholes may still be sunk into such rock, as long as they penetrate fault lines, along which water can flow, but flows are uoften low, unless the faults are wide. Examples include Granite, Bauxite.

Impulse Pump

See Hydram pump


General term used to describe the mating of fish within family groups. Successive inbreeding can lead to deformities and other problems. Can be avoided by keeping stocks of several different families separate and always breeding between families.

Incubation Jar

See Zoug jar

Incubation systems

see hatching trough, Zoug jar


A chemical that kills insect life (invertebrates). many insecticides are also toxic to other life forms, including vertebrates so care should be taken during use. see also saponin, tea seed cake, piscicide

Insulation ratings

A value given to a material which states it's ability to insulate thermally or electrically.

Intensive aquaculture

All of feed is fed by hand or machine (i.e. no significant natural feed). Fish are generally held at high stocking densities of between 10 and >100 kg / m3. See Extensive

Internal respiration

See respiration


Term used to designate any organism that is not a member of the Vertebrata.  Comprises over 90 percent of the known species of animals (including single-celled animals, colonial forms and those with specific organs or organ systems)

Entry edited by Henrik Kreiberg


One of the halogen gasses and a constituent in seawater. Iodine based disinfectants are commonly used in aquaculture, these are collectively known as iodophores. The iodophores are not as easily deactivated by organic matter (such as mud from boots, if being used in a footbath) as some others, and the brown colouration of the chemical fades as the activity decreases. This gives a good indication of when the chemical is no longer active and needs replacing, unlike some other chemicals such as sodium hypochlorite, which are clear. Iodine base disinfectants do not evaporate as fast as some chlorine based ones, and so remain active for longer if used to disinfect floors etc.


Electrically charged atom or group of atoms.

Ion exchange

The exchange of ions from one medium (usually water or another liquid) to another medium (usually a solid). There are two types of ion exchange material, anionic and cationic. The former will exchange positive ions (as it has negative ions built into it's structure) and the latter will exchange negative ions (as it has positive ions built into it's structure). The most commonly used ion exchange in aquaculture is with the use of zeolite material (an inorganic polymer) in which positive ions are held within a silicate lattice. The ammonia and other negative ions in fish waste water are taken up by the zeolite and bound into it's structure. The zeolite can then be recharged by flushing with a solutions  such as sodium chloride or sodium hydroxide. The use of  ion exchange in aquaculture is limited to small systems due to the cost of equipment and the cost of recharging. Zeolites can maintain and almost 100% effectiveness if recharged correctly. Not effective in seawater, where due to the high levels of potassium (primarily as potassium chloride), this will be removed in preference to the ammonia. 1 gram of zeolite will remove approximately 16mg of ammonia (as long as there are  not other compounds in the water that it can also remove). 

IP rating

The standard ratings for waterproofness and environmental sealing etc. are known as IP ratings and those relevant to aquaculture are given.


Iron is a metal often found in waters. It is particularly a problem in ground water supplies, where the water is acid and has passed through some iron bearing rock. The dissolved iron usually takes the form of ferric sulphate, which at pH values above 3.0 may become hydrolised and form iron hydroxide. It is usually the occurrence of iron hydroxide rather than the iron itself that kills the fish. 

Water with high levels of iron in a soluble ion state: When the oxygen or oxidation potential (REDOX) of the water is increased, either by aeration or the addition of oxidising chemicals such as (ozone or hydrogen peroxide) the metal ions will loose electrons.  As a result, the ferric iron forms ferric oxide (or rust). This is often observed as a brown staining of the tanks and can settle on gills causing respiratory problems. See also Iron Removal

Iron Hydroxide

A precipitate of iron which when precipitated from acid solutions containing more than 3 mg/l iron on to the gills of fish, can cause mortalities when the pH rises above 5.5.

Iron Pyrite

Sulphide form of iron often found in very acid soils (such as the acid sulphate soils found in many of the coastal swamps, mangroves and marshes). In such soils, the pyrites remain relatively stable as long as the conditions in the soils remain anaerobic. However, if drained and exposed to air or water with a high dissolved oxygen concentration, through excavation, the pyrites oxidise which as part of the reaction, forms sulphuric acid. This reduces the pH of the soils and also the pond water above it. For this reason, many ponds only remain viable for short periods, as the acidification effects become too great. Some pond soils are limed to assist in the battle against acidification.

Iron Removal

The removal of iron from water is usually carried out by a series of oxygenation of the water to convert soluble ions to particles of ferric oxide, and then a method of removing the ferric oxide from the water. 0.14 mg of dissolved oxygen is required to oxidise 1mg Fe. The ferric oxide is then passed through a tank with a retention time of approximately 10 minutes, which is filled with plastic media (the type used in biological filtration). The ferric oxide then settles on the filter, forming a film which increases in efficiency as the film develops. Periodically the filter is cleaned of excess deposits.


Exposure to radiation sources. Used for sterilising of food and used experimentally for sterilising of gametes to alter sex see Irradiation - sterilisation

Irradiation - sterilisation

The exposure of young fish to controlled gamma irradiation doses from a cobalt-60 source. Still very much in the experimental stages, but results indicate that there may be potential in the future for this method. The advantage of this method of sterilisation over triploidy is that it can be used on both sexes rather than just females.

Irrigation Pump

The generic name given to pumps which are designed to provide large quantities of water at relatively low heads, typically <8-10m. See also Low head pumps


Marine golden/brown flagellate algae. Used extensively in marine hatcheries due to it's nutritional content and suitable size (5 - 6 microns diameter).