The Free Online Aquaculture Dictionary


Swelling of tissue through an increase in the fluid volume of the tissues. Can occur during inflammation through the blockage of lymph system, preventing the return of lymph fluid.


The gullet.


Hormone produced by the ovary.


A synthetic hormone which is used for the manipulation of the sex of fish see Feminisation


Hormone produced by females, principally by the ovaries. Promotes the onset of secondary sexual characteristics


General terms for the release of a dissolved gas into the air. See also degassing

Oligochaete worms

Class of hermaphrodite annelid worms, including the freshwater bloodworm (tubifex)


Term used to describe waters where there is a very low concentration of nutrients. Characterised by very little algal growth, clear waters and some macrophyte growth. Typical of mountain streams and lakes. See also Mesotrophic , Eutrophic

Omega Acids

Method by which lipids used to be classified. The Omega symbol has now been replaced by "n" in the method used to describe the fats. e.g. "Omega3" is now "n-3".


Animal that eats both vegetable and animal matter as part of it's diet


Prospective egg cells contained within the ovary.


The production, growth and maturation of the ova in the ovary.

Operculum - (Opercula)

Covering the gill slit. May vary from a bony plate, to a flap of skin depending on the species. Primary purpose is to protect the gills from damage and also to assist with the buccal plate in providing a water flow through the mouth and across the gill.

Opportunistic Zone

Area immediately around cage farm , but not directly below it or directly in the main area of waste deposition (see azoic zone). Characterised by some downward deposition of food and faeces, few original species remaining, high quantity of tubifex worms and other species which thrive in such areas of high pollution. i.e. they are "opportunistic" species, which, without the pollution from the cage farm, would not thrive in such an environment.


General term, loosely used (although not necessarily always correctly) to refer to any material which consists of live or dead cells, or carbon containing material which is capable of being broken down by biological means. A material containing carbon which is derived from a life form.


Group of chemicals sometimes used for the treatment of external parasites. Used extensively in land animal farming, especially for dipping sheep to eliminate parasites on the skin.  Very toxic to humans and shellfish beneath, and in the vicinity of cages. Available under trade names such as Nuvan, Dichlorvos. Most widespread use is for the treatment of sea lice, however environmental and health issues have lead to the chemicals being banned in many counties.


The process by which an animal maintains it's correct balance of salts and water in it's body. For example, marine fish are unable to osmo-regulate correctly in freshwater; in seawater their osmoregulation system discharges salt from the body and keeps water, but in freshwater they need to reverse this operation as salts are scarce and freshwater is plentiful. See diagram for processes in fresh and saltwater. The natural salts concentration in a fishes body is 10 parts per thousand. Some aquaculturists who farm euryhaline species, claim that maintaining their stock in systems at 10 parts per thousand salinity, reduces stress and promotes growth rates.


"The passage of a solvent through a semi-permeable membrane, separating two solutions of different concentrations". The semi-permeable membrane is one which the molecule of the solvent can pass but the molecules of the solute cannot. In the case of aquaculture the "solvent" is usually water and the "solutes" usually salts dissolved in the water. In such a case where water on each side of the semi permeable membrane are of different salinities, fresh water (i.e. salt free) will pass from the side with the lowest salinity to the side with the highest salinity until the two sides are of equal salinity. The pressure required to stop the flow of pure water into a liquid through osmosis is called the osmotic pressure. In fish terms this means that as the body of a fish contains approximately10 parts per thousand salts, a fish in freshwater will experience water passing from the environment, through it's skin and into it's body. A seawater fish experiences he opposite with water constantly trying to escape from it's body, through it's skin and into the higher salt environment that surrounds it. see also osmoregulation


The turning of cartilage to bone. Occurs at various stages of development depending on the species. see also whirling disease.


Super order of fish including the orders Gonorhynchiformes, Cypriniformes and Siluriformes


Order of fish (part of the superorder Osteoglossiformes) includes fish such as the Featherback (Notopterus) and bony tongues (Arapaima)

Osteoglossiformes (2)

Super order of fish containing the orders Osteoglossiformes and Mormyriformes. Tongues bear large teeth, all freshwater


A gelatinous mass containing a high level of calcium carbonate. Forms part of the inner ear. Can be cut after dissection and used to tell the age of a fish through it's rings (see also scales). Some of the life history of the fish can also be attained from studying the spaces between the rings, and disturbances in the rings.




Female sex organ where eggs develop and are held until when released during spawning


A subjective term relating primarily to the level of stress suffered by the fish when in too close a proximity to others. Can often lead to physical injuries such as lesions from fish being forced to rub on the sides of bottom of the enclosure (e.g. tank, cage), fin and eye nipping by other fish, loss of appendages (in crustaceans), cannibalism and a greater than normal deviation in the fish sizes in the enclosure, due to the inability of small fish to get near to the feeding point leading to a hierarchical situation. Also increased risk to disease etc. through the stressing of the fish (see stress)


The process of releasing the eggs from the fish.


A reaction which results in the addition of oxygen to a molecule. For example when nitrite (NO2) is oxidised, it becomes nitrate (NO3). Oxidation is the cause of rust on iron (Fe2), where iron combines with oxygen to from Iron Oxide (Fe2O3). Use is made of he oxidation process in many aquacultural situations. The oxidation process can cause the breakdown of organic molecules, and can therefore be used to sterilise water (as the molecules in bacteria and other pathogens break down, the pathogen dies) see ozone. The oxidation process is also used to break down complex molecules that the bacteria in the biological filter are unable to. In the simpler, broken down form, the bacteria are then able to break them down further. The process of nitrification is one of oxidation, as it adds oxygen molecules to the nitrogen.



Odourless, colourless gaseous element. Chemical symbol O. Atomic weight 16. Most abundant element in the earth's crust, forms 20.95% of earths atmosphere (by volume). Essential for all forms of aerobic life. Accelerates combustion in the prescience of a fuel source. It is therefore essential that items used near or in oxygen are non-combustible and grease free. Available "pure" (percentage purity usually quoted by supplier) in most countries, as either compressed gas or liquid. 1 m3 oxygen = 1.429kg at 20oC and atmospheric pressure. Boiling point : -183oC, melting point -218.4oC. Used in many farms to supplement the oxygen available in the water to enable more fish to be held in a given flow of water than would otherwise be possible. See also respiration, oxygen injection, dissolved oxygen, oxygen demand, oxygen debt, oxygen depletion, oxygen generators, oxygenation

Oxygen debt

The state that exists in aerobic animals when not enough oxygen is available in the blood stream for the functions required by the animal, e.g. during a period of strenuous activity. The body degrades stored glucose (by an anaerobic process) to supply additional energy. This results in the formation of lactic acid in the bloodstream which then requires oxygen to oxidise it (in the liver) back to glucose. When the oxygen to do this is made available by the fish again (i.e. when it's metabolic activity has reduced to a level where it can breath in more oxygen than it requires to fulfill it's necessary functions) the lactic acid is converted back, thus repaying the debt. Excessive build up of lactic acid can be toxic to the fish. Presence of high levels of lactic acid in the bloodstream at the time of death can accelerate the decomposition rate of the fish, reducing it's "shelf life". Therefore, the more stress and activity free the slaughtering process, the longer the shelf life of the end product.

Oxygen Demand

The amount of oxygen that an animal must consume to maintain a given level of metabolic activity. In farmed fish this is roughly equivalent to 0.23 x the feed rate (assuming dry commercial diets with energy levels in the region of 18-20Mjkg). For lower energy feeds, the oxygen demand is roughly equivalent to 0.012 x feed weight for every 1MJ/kg energy level of the feed. Thus a feed with an energy level of 10MJ/kg will result in an oxygen demand of 0.12 x feed weight. Small fish require more oxygen as a result of higher feed consumption rates in relation to their body weight. Nitrification bacteria require 4.5g of oxygen to convert 1g of ammonia to nitrate. To reduce B.O.D. and C.O.D. requires the value of the B.O.D. / C.O.D. e.g. Heterotrophic bacteria will require 1g of oxygen to oxidise 1g of B.O.D. Oxygen demand of fish is dependant on the distribution of feed throughout the 24 hour day. For example; a fish fed at a steady rate for 24 hours will tend to exhibit the oxygen demand rates shown above all the time, however a fish fed all it's daily ration only once per day, will tend to exhibit a cyclic oxygen demand, with the peak demand typically being 30-40% higher then the average demand (above). The time taken for feed to exert an oxygen demand is dependant on the metabolic rate of the fish, and as fish are cold blooded, this is largely dependant on the water temperature. During high temperatures, the feed will tend to exert it's demand after 4-8 hours, but during low temperatures, the demand may be spread over a period from 8-24 hours post feeding. For these reasons, the feeding strategy and likely peaks in demand must be calculated, rather than using a base figure. The diurnal rhythm of dissolved oxygen concentration in natural waters results in a zenith in the late afternoon and a nadir in the early morning (caused by biological demands form flora and fauna in the water). When temperatures are high, and oxygen short in the supply water, some farmers feed only in the morning so that the peak oxygen demands occur in the afternoon, when there is the most dissolved oxygen available in the supply water.

Oxygen depletion

The lowering of dissolved oxygen concentration through consumption by animals, plants other than those being cultured. May also be caused by the addition of some chemicals to the water (such as formalin).

Oxygen Generators

Devices that divide the oxygen gas fraction of atmospheric air from the nitrogen fraction. This is achieved through  process of either pressure swing adsorption (PSA) or, for very large (the size that the gas manufacturers use) systems, cryogenic fractionation. Oxygen generators in themselves are quite efficient in terms of cost of energy per kg of oxygen produced, but all generators also require a compressed air source (typically at between 60 and 100 psi). The cost of energy required to produce this compressed air can often be very high. The suitability of an oxygen generator to a site is dependant on the cost of electricity in the area, the cost of oxygen (if purchased in other forms) and also the cost of replacement of the filters. These costs often mean that in accessible areas where a bulk oxygen supply is readily available, that oxygen generators are not viable. It should also be remembered when installing an oxygen generator that a back up supply needs to be in place in case of breakdown or failure of electrical supply. The purity of gas from PSA oxygen generators varies between 75 and 95% according to the manufacture and also the level of operation of the system. e.g. a 5kg/hr system will produce 95% pure at a production rate of 1 kg/hr, but when it is running at full rate, the purity will fall to 75%. This can be overcome by purchasing a machine capable of more than is actually required, but there are additional capital costs associated with this. it should also be noted that when using oxygen of less than 100% purity, that pressurised oxygen injection systems will result in supersaturation of nitrogen.

Oxygen Injection

The addition of gaseous oxygen to water under pressure. Oxygen injection systems generally use a pump to create a high water pressure. Oxygen is then added to the water, often with a venturi. At the higher pressure, the water is capable of naturally holding more oxygen than at atmospheric pressure, so the oxygen dissolves much more efficiently. Also the high pressure reduces the size of bubbles and so increases the interface between the oxygen gas and the water (many small bubbles have a much higher surface area than a few large bubbles), this also increases efficiency. Typically injection systems operate at around 90-95% efficient. See also oxygenation, aeration

Oxygen Reduction Potential (ORP)



The addition of pure, or very high purity oxygen to the water, to increase the dissolved oxygen concentration of the water. Common devices used are diffusers, injection systems and venturis. The use of pure oxygen systems is, in general confined to intensive aquaculture, where oxygen saturation levels of above 75-90% are required in the tanks to maintain the oxygen demand of the stock. In systems where oxygen levels lower than this are satisfactory (such as in more extensive ponds), aeration is usually a more efficient option. Care must also be taken with the use of pure oxygen in hatcheries, where highly supersaturated concentrations of oxygen may cause harm to the fish.


Antibiotic drug. probably one of the most used antibiotic drugs in aquaculture due to it's broad band of effectiveness and good palatability qualities. See also antibiotic resistance



Colourless gas (O3). Sometimes also called trioxygen. Made by passing oxygen (either as air or as pure oxygen) through an electric discharge. Small quantities may also be produced by certain ultraviolet light wavelengths. Very strong oxidising agent, highly toxic to all forms of life and corrosive to many materials. Boiling point -111.9oC, melting point -192.7oC. Liquid ozone is dark blue in colour. Can be measured directly (with ozone probes) but such equipment is very expensive and the REDOX level (which changes with the amount of ozone in the water) is commonly used instead. Control is essential, as over-dosing can result in concentrations which will harm the fish, and also the off-gassing of ozone in to the air, which in an enclosed area can cause long term harm to humans. Uses in aquaculture are in sterilising systems, where relatively small quantities of water need to be sterilised (such as inlets to hatcheries etc.), and in recirculation systems where the ozone has several beneficial effects such as the breakdown (through oxidisation) of long chain molecules into simpler forms which can then be broken down further in the biological filter. It is through this process that ozone eliminates the yellow / brown colourations, that build up in recirculation systems. See also ozone generators, ozone destruction

Ozone destruction

Ozone destruction systems are required after ozone contact systems to destroy residual ozone (that which is remaining after the process) in the water before it comes into contact with the stock. Ozone is very toxic to all life and so systems must be properly maintained and controlled. Ozone destruct systems commonly use one of the methods given in the table.

Ozone Generators

Devices which convert a percentage of the oxygen gas passing through them into ozone gas. there are two types : 

1.Ultra Violet lights in the wavelength range 100 - 120 nm

2.Corona discharge. 

The use of UV lamps to produce ozone is generally confined to small aquarium and test systems. This is due to the fact that they are usually cheaper than corona discharge units at the low dose levels that are required. Dry air or oxygen is passed across the surface of the tube, causing a percentage of the oxygen to be turned into ozone. Corona discharge units are more common in commercial situations and consist of two electrically charged plates, across which a current flows. Oxygen or dry air is passed between the plates and the ozone is produced. Corona discharge units typically convert between 5-15% of the oxygen passed through to ozone (if using oxygen as feed gas), depending on the quality of the machine. Air is rarely used as the feed gas as in addition to a larger ozone generator being required to produce the same amount of ozone, an additional compressor and air dryer is required to supply the generator. Air supplied ozone generators are in general, less reliant than oxygen supplied ones, as they have the additional compressor and dryer that can fail. Most ozone generators require a cooling water supply (freshwater) and in many, the temperature of the cooling water has a bearing on the percentage of ozone produced, per unit of oxygen passed through. The lower the temperature, the higher the ozone percentage.