The Free Online Aquaculture Dictionary


The lowest point attainable. Opposite= Zenith


Marine unicellular algae. Very small <2 microns diameter. Due to it's small size it is often used for culture of rotifers and other zooplankton. Quite difficult to culture compared to other larger algae.


Collective term used to describe the species of crustacean that are swimmers, such as the Penaeids (shrimps). Opposite: Reptania


Term given to free swimming newly hatched larval stage of some animals (especially crustaceans).


The process during which body cells die, leading to dead (necrotic) areas of tissue.


Dead. Usually used to refer to dead tissue (cells). Necrotic tissue is often caused as a result of infections or abrasion, and provides an easy entrance for pathogens to enter the body. Large areas of necrotic tissue on the skin of the fish can lead to difficulty in osmoregulation. Areas of necrotic tissue can grow as the dead areas prevent the flow of blood and nutrients to other healthy cells, causing the gradual increase in the size of the necrotic area.


The collective term for passively floating life forms (plankton), including animal and plant life see also phytoplankton, zooplankton


Condition in which calcium deposits build up in the kidney. Particularly associated with the rearing of fish in waters with a high alkalinity and high carbon dioxide concentrations. Typical scenario is a farm, where an oxygenation system is used in order to gain more production form a given flow of water, without degassing the water for carbon dioxide between ponds.. Severe cases can lead to mortality, but more often it is a problem of the chronic stress caused by poor kidney function


Animals showing signs of nervousness (startling, hiding etc.) are usually under some sort of stress. The most common of these is stress through poor water quality such as low dissolved oxygen concentrations.

Net bagging

The distortion of a net pen (see cage), usually by currents or being towed at too high a velocity. The distortion of the net leads to a reduction of the volume of the net enclosure and can damage fish by forcing them to make contact with the rough net surfaces (see also fouling). The problem can be overcome by the addition of sufficient weight around the bottom of the cage.


See Ammonia nitrogen


Biting of one fish by another. Not necessarily associated with cannibalism, nipping is often a result of territorial behaviour which becomes exaggerated when fish are stressed (particularly when overcrowded). Nipping is usually associated with the fins or eyes. Some farmers report that by increasing dissolved oxygen levels when nipping is evident, the problem can be stopped. This is probably a direct result of reducing the overall stress level on the fish. It may be assumed that any measure to reduce stress levels may also have a beneficial effect. The effect of nipping can lead to wounds which are easy entry sites for pathogens, the loss of sight can also lead to starvation. Nipping is usually more apparent in the early stages of development, where territorial behaviour is more pronounced. Some species (e.g. some of the Barb family) nip fins as a source of nutrition, and can be a problem in mixed populations, especially where ornamental fish are kept or reared, as the fin damage results in a loss of value. see also overcrowding,


Chemical symbol NO3 - Formed as a result of the breakdown of ammonia to nitrite and then to nitrate by bacteria (see nitrification). May also be present in watercourses through run-off from the addition of nitrate as fertiliser to agricultural land. Nitrate, along with phosphate form one of the critical chemicals that all plants require. The growth plants in natural conditions are more usually limited by the availability of phosphates rather than nitrates, although in some areas, the addition of nitrates alone can cause algal blooms etc. if there are sufficient phosphates naturally available. Relatively harmless to fish, toxicity levels range from 3-400mg/l for Salmonids, to >1000mg/l for Cyprinids. For every 1g of ammonia converted to nitrate, 4.57g of nitrate are produced. Nitrate can build up in recirculation systems with a small make up water rate, as it is only removed from the water by special denitrification filters ( there are some zeolites which can remove nitrate but their cost generally makes them un feasible for aquaculture). In a recirculation system where commercial diets are used (protein approx. 40%), the make up water required to keep the nitrate levels below 250mg/l is approximately 40 lpm per 100kg feed / day.


The oxidation of ammonia to nitrite and then nitrate by bacteria. See also biological filtration.


Chemical symbol NO2 - Toxic chemical formed during the oxidation of ammonia to nitrate by bacteria (se nitrification). As the conversion of ammonia to nitrate occurs in biological filters, most of the nitrite is converted to nitrate before the water exits the filter. There will however be a residual amount of nitrite left in the water that will usually be converted to nitrate once the water re-enters the biological filter on it's next recirculated cycle. In slow water turnover systems and ponds, bacteria in the ponds/tanks and pipes may convert the nitrite. The addition of ozone or other strong oxidising chemicals to a system will also convert the nitrite to nitrate. This can cause problems if the addition stops (e.g. through machinery breakdown), as insufficient bacterial floc in the biological filter will have grown to accommodate the sudden increase in the nitrite loadings (i.e. the nitrite which was previously being removed by the ozone). The result can be the build up of nitrite to toxic concentrations before the bacterial floc multiplies in response to, and has the ability to oxidise the additional loading. Toxic concentrations vary between species but general guidelines are between 0.01 and 0.1mg/l for freshwater species and 2-8mg/l for saltwater species. 


Species of aerobic bacteria which converts nitrite to nitrate. One of the critical bacteria in biological filtration. Optimum pH range between 6.0 and 9.0, temperature 10oC - 34oC. Will acclimate to changes in water quality, but activity is reduced during acclimation which can lead to a build up of nitrite. See also nitrosomonas, nitrification


Drug for the treatment of bacterial diseases. Relatively cheap when compared to some other antibiotics, but can have palatability and resistance problems. 



Gas. Chemical symbol N. Atomic weight - 14.0 Dissolved nitrogen gas is the primary cause of supersaturation as it is the gas in most abundance in the water. In stabilised waters the percentages of dissolved gas in the water are the same as those in air. Nitrogen forms a significant part of the waste from fish farms. This is primarily in the form of ammonia and nitrate. The amount of nitrogen added to a fish farming system can be calculated from the protein level of the feed. Boiling point -195.8oC, melting point -209.9oC. 1 m3 nitrogen gas  = 1.2506kg.  For every gram of protein added to the system, 6.25g of nitrogen will be produced in one form or another. This includes the fishes body tissues that contain nitrogen locked up in compounds in the cells. As a rule of thumb, approximately 90% of the fishes nitrogen excretion is in the form of ammonia


Species of aerobic bacteria which converts ammonia to nitrite. One of the critical bacteria in biological filtration. Optimum pH range between 6.0 and 9.0, temperature 10oC - 34oC. Will acclimate to changes in water quality, but activity is reduced during acclimation which can lead to a build up of ammonia. See also nitrobacter, nitrification


Used to describe a process where 100% accuracy is not guaranteed. For example sand filters are usually sold to filter to a nominal 10 microns, which means that they will filter most particles of 10 microns  and larger, but not all. A filter which is guaranteed to filter all particles of 10 microns would be termed "absolute" rather than nominal.

Non return valve

Pipe fitting which permits the flow of water through it in only one direction. Used for applications such as the end of suction pipes, where, when the suction pump is turned off, the pipe must remain full of water. Also often used where multiple pumps are used, positioned on the discharge side of the pump, the non return valve prevents water flowing back through the pump, when it is switched off and other pumps in the system are still operational. Non-Return valves are often spring loaded which ensures a better seal, but in low pressure systems can result in a high head loss and high pumping costs.  Sometimes abbreviated NRV or called foot valves. Shown in drawings in a variety of ways but usually using the same symbol as an electrical diode, a vertical line and a triangle. the direction of flow is indicated by the point of the triangle. See drawings for more details.


Order of fish (part of the superorder Elopomorpha) which contains deep sea eel like fish.


Anaesthetic drug. Must be injected, which limits it's use in aquaculture.

Nutritional Gill Disease

A condition affecting the gills resulting in the wasting of gill lamellae. When observed under a microscope, the lamellae appear to be thinner towards the end. See diagram for comparison to healthy gill and one infected by bacterial gill disease. Caused by a lack of Pantothenic acid in the diet.


Organophosphate pesticide, originally developed for use against external parasites of sheep and other livestock by dipping.  Added to water to treat pathogens such as sea lice (lepeophtheirius), typically used at a concentration of 1 mg/l for 1 hour. Most common trade name is Nuvan. Very toxic to humans and also shellfish, both beneath and in the vicinity of cages. Now banned in many countries due to fears about health and also damage to local environment esp. crustaceans.