The Free Online Aquaculture Dictionary



See Chemical Oxygen Demand


See table for specifications of different cable types. See also chain, rope


Metal, falls into the category of heavy metals. Very toxic to fish, safe levels are 3.0mg in well buffered water, 0.4mg/l in poorly buffered water. 0.2mg/l in seawater. 


A fixed or floating enclosure used to contain fish. There are many designs of cage. Fixed designs are usually referred to as Hapas and tend to be limited to an area of several square metres. Larger systems involve a net suspended from a floating and supporting collar. 


The build up of calcium carbonate deposits in the tissues of the fish, often without harmful effects see also nephrocalcinosis

Calcium carbonate

Chemical symbol CaCO3. g/eq - 50, Solubility - moderate. Most common naturally occurring form is chalk and limestone. Used for increasing the alkalinity of water. See Lime for more details.

Calcium Cyanamide


Herbicide, used to control macrophytes. Applied at 750 kg / ha to filled ponds with no stock in. Pond should be fully drained and refilled prior to stocking. 

Calcium Hydroxide

Chemical symbol Ca(OH)2 . g/eq - 37, Solubility - high, has a high degree of alkalinity and therefore is often used as a method of buffering systems. Applied to ponds at 1000-1200kg/ha See Lime for more details

Californian hatching systems

Hatching system consist of a number of stacked hatching troughs or trays, with grids or baskets to support the eggs. The troughs then flow from one onto another, often with water cascading to aerate between the troughs. Used extensively for salmonids and other species with large (>4mm eggs).


Unit of energy. The amount of heat required to raise 1 gram of water by 1 degree centigrade. See Energy for conversion tables

Canister Filters

Enclosed housings usually containing fine mesh baskets, into which filtration or other media ca be put. Often used in conjunction with activated carbon and zeolite materials. Generally designed for small flow rates, although larger units are sometimes custom built.


The eating of ones own kind. Cannibalism can most often occur as a result of overcrowding, starvation and lack of grading (through there being a size difference in the fish great enough to enable the larger of the fish to eat the smaller fish). Some species which are left to spawn relatively naturally (e.g. tilapia) have been known to eat their young if severely stressed. More prevalent in carnivorous fish such as eels (Anguilla anguilla) see also nipping


Canthaxanthin is a naturally occurring chemical in the family of carotenoids. It is a pigment that is sometimes used to give salmonids their pink colour. Synthetically produced canthaxanthin is available under the name of Carophyll pink, EEC no.E161. Astaxanthin has largely replaced the use of Canthaxanthin in salmonid diets due to adverse publicity.


The hard exoskeleton shell of crustaceans


General term for compounds such as sugars, starches and cellulose. Contain carbon, oxygen and hydrogen. Most carbohydrates can be used as an energy source by animals, although some are difficult to digest (especially by carnivores) and so release only small amounts of energy to the body


Chemical symbol "C" - The essential building block of all organic compounds. Often needs to be added to the water (often in the form of methanol) to supply bacteria in denitrification filters. See also Activated Carbon

Carbon Dioxide


Produced as a result of respiration by the fish and other aerobic organisms (including plants) in the system. The amount of carbon dioxide produced is directly proportionate to the oxygen consumed. For every 1g oxygen that is consumed, 1.4g carbon dioxide is produced. Carbon dioxide has the effect of increasing the acidity of the water, it is present in three different forms in the water : CO2 (free carbon dioxide, which is the toxic form), HCO3- (bicarbonate ion, and CO3-- (carbonate ion). The concentration of each is dependant on the pH of the water. The table shows the effect of pH on the proportions of different forms of carbon dioxide in freshwater. As free carbon dioxide is the form that is toxic to fish, high toxic concentrations are only normally found in neutral or acidic waters. Most surface waters, in their natural surface state contain low concentrations of carbon dioxide (<6 mg/l). There are situations however, where toxic levels may be reached : 1. Acidic groundwater can contain very high carbon dioxide concentrations and require degassing prior to use 2. Ponds with large phytoplankton populations can be the cause; carbon dioxide may reach high concentrations during the night or during cloudy weather, as the plants consume oxygen as part of their natural respiration, 3.Fish transportation - carbon dioxide build up can occur in transport tanks, especially when in enclosed tanks or bags containing oxygen. Open tanks and aeration is much more preferable as it gives the carbon dioxide a chance to escape, this is especially important when transporting fish in acidic water for long journeys. 

High carbon dioxide concentrations are harmful to fish as the blood becomes less efficient in getting rid of it's carbon dioxide across it's gills. This means that some of the carbon dioxide stays in the blood, taking up a space where an oxygen molecule would otherwise have occupied. For example a carbon dioxide concentration of 25mg/l equates to almost a 50% drop in the fishes carrying capacity for oxygen. This will limit growth and make the fish more susceptible to any diseases. General guidelines for carbon dioxide concentrations are : <15 mg/l acceptable for most species, 15-30mg/l sublethal effects including respiratory stress and the development of kidney stones (nephrocalcinosis) in some species. >30mg/l lethal to many species with prolonged exposure. Carbon dioxide can be removed / controlled by using aeration or degassing techniques, or adding lime to a pond (this should be carried out very slowly, as in acidic waters, small additions of lime can lead to large increases in the pH, which will stress the fish), approximately 1meq of hydrated lime can remove 2.25mg/l CO2--. Careful planning of stocking, feeding, fertilising and use of oxygenation can also help prevent high carbon dioxide levels occurring. Shallow ponds are less likely to suffer carbon dioxide problems than deep ponds.

Carbonate minerals

A group of minerals containing the anion CO32- as the fundamental unit of their structure. Carbonate minerals are used for buffering water to increase the alkalinity. See also lime


Any substance that either produces or accelerates the development of cancer


An animal that feeds solely on other animals.


A group of yellow, orange and red lipid soluble pigments. A form of vitamin A. See also Astaxanthin, Canthaxanthin


An animal that harbours a disease without showing symptoms. see also asymptomatic carrier

Carrying capacity

The amount (either expressed in weight or number) of animals that a given system is capable for supporting. The carrying capacity is limited by a factor which on farms is usually oxygen then ammonia and carbon dioxide / B.O.D.


A firm, flexible connective tissue. In vertebrates, the cartilage forms the skeleton in the early stages of development, after which it is largely replaced by bone. Some cartilage remains at the joints to give flexibility and support. Some fish, such as sharks remain with their skeletons made of cartilage, these are termed "cartilaginous". Some parasites (such as Myxosomas cerbralis) can only enter the fish and complete their life cycle, if the skeleton is still cartilaginous. For this reason, if there is a risk of infection, fish are often kept on spring or borehole water supplies until their skeletons have hardened.

Cartridge Filters

Enclosed vessels which house a replaceable filter membrane. The filter membrane is generally of a pleated or a woven material. Cartridge filters are used primarily for filtration of water of 20 microns and below. Disadvantages of using cartridge filters include the cost of replacement filter elements, the cost of pumping the water through the filters ( the finer the membrane, the greater the headloss across the filter) and the maintenance of large numbers of filters, should large flows be required. Cartridge filters do however have their applications in situations such as marine hatcheries, where pre-filtration is often used and very low flow rates are required. see also  mechanical filtration


A fish which spawns in the sea and lives most of it's life at freshwater e.g. European eels (anguilla anguilla) See also Anadromous


An irreversible clouding of the lens of the eye. Appears as a grey / white colour in the normally black lens of the eye. May be due to either physical damage sustained whilst handling, irritation from disease organisms such as Diplostomum spathacaeum, or a result of incorrect nutrition. The cataracts impair the vision of the fish. In some species, such as sight feeders like salmonids this can lead to a wasting away and eventual mortality. Some fish will however adapt and feed from the bottom of a tank / pond. This will usually result in very poor FCR and lesions to the mouth parts which can in turn lead to other infections. Poorly sighted fish are normally removed and killed to prevent them becoming stressed and risking the development of disease conditions which could threaten the rest of the stock.  


see Watershed


A positive ion

Caudal Peduncle

The name given to the area where the tail of a fish joins the body. Sometimes also called the "wrist". Some operators will use this area to hold the fish, although excessive struggling by the fish can cause damage to the spine and body tissues if it is not also supported elsewhere. This can result in poor fillet quality.


see Channel Catfish Viral Disease

Centrifugal Pumps

The workhorses of most applications, centrifugal pumps use a rotating impeller that draws water into it's centre and through centrifugal action, throws it outwards through vanes in the impellor and out of the pump. Efficiencies are generally in the 55 - 80% range depending on how closely the application fits the optimum curve of the pump. 

Cell division

The process by which cells split into two copies of the original. Forms the basis of development of ova once fertilised. The single cell divides into two, these two then divide resulting in four and so on.


Common term for animals belonging to the Class Cephalopoda, including squid and octopus


Class of animals, under the phylum Mollusca. Regarded as the most advanced of all the Mollusca classes. Mostly marine. Possess a well developed head surrounded by a ring of tentacles. Includes squids, octopuses and cuttlefish. 


Larval stage of flukes such as eye fluke (Diplostomum spathacaeum). The stage at which the flukes are released from their mollusc host (usually a snail) and infect fish.


Cubic feet per second. Measurement of flow. see Flow or click on software link to get conversion tables program.


See table for specifications of different chain types. See also cable, rope


An open water course i.e. not enclosed as a pipe would be. Several design factors must be taken into account when designing a channel these are :
1 Material for construction - this has a bearing on the flow which will be achievable down the channel. Smooth surfaces will allow greater flows than rough ones. See Friction loss. The material used must also be of sufficient structure that the water flowing down the channel will not erode. The velocity at which a material erodes is known as the limiting velocity.
2 Gradient - The greater the gradient, the faster the water will flow down the channel
3 The scouring velocity. This determines whether silt and other particles will settle in the channel or be carried along with it.

Channel catfish viral disease

Disease affecting primarily channel catfish (Ictalurus punctatus), but other species of catfish have been infected experimentally. Confined to USA and Honduras.
Symptoms include high mortality among fry; outbreaks associated with high temperatures; high head posture; erratic swimming; hemorrhages at fin bases; swollen abdomen; exophthalmia; pale gills.  
Control of the disease is by using an available vaccine,  selection of resistant strains (don't use wild fry), reduction of stocking densities, maintenance of temperatures below  25C and oxygen concentrations above 4 ppm Ponds should be drained and/or chlorinated between batches of fry.


The term used to describe the concentration of flow through some areas of a filter, whilst other areas become blocked. This particularly applies to sand filters and some biological filters and degassers. The media in the filter gradually becomes blocked with solids and bacterial floc, eventually forming clumps of bound up media. Unable to pass between the individual pieces of media in the clump, the water flows around the clump. Channeling can dramatically reduce the ability of a piece of equipment to perform to it's design specification. Once channeling has started, the only way to remedy the situation, is to backflush or wash the filter media to remove excess floc/particulate matter.


Suborder of fish (part of the order Perciformes) including fish such as snakehead (Ophiocephalus)


Common term used to describe fish of the order Characoidei


Suborder of fish (part of the order Cypriniformes), includes fish such as Tetras and Pacu (Colossoma). Usually have adipose fin and well developed teeth for cutting. Mainly tropical Africa and South America

Check Valve

Another name for a non-return valve

Plastic PIpe Shop Ltd - Suppliers of quality plastic valves and fittings


The term used to describe legs that have been adapted for feeding. Many crustaceans such as crabs, shrimps and lobsters have a well developed set of pincers on the front pair of legs, these are used for catching prey and also for feeding.

Chemical Oxygen Demand

(COD) A measure of the amount of chemicals in the water that can be oxidised by oxygen. It is therefore a measure of the oxygen consumed by these chemicals. Used more in the waste water treatment industry than aquaculture, where the biological nature of the wastes (faeces and feed) make the biochemical oxygen demand a more representative measurement. The COD can be measured much more quickly than the BOD.


A device which constantly doses chemicals into a vessel to maintain stable conditions in the vessel whilst a process is occurring. In aquaculture chemostats are some times used for algal culture. Rather than producing algae by the batch method, a chemostat provides a continuous harvest of algae. Water entering the chemostats is dosed with the nutrient chemicals that the algae requires to grow. See also turbidostat


Chilling of the water is necessary in may aquaculture applications including broodstock conditioning, larval rearing, recirculation systems and hatcheries. Two main types of cooling are used in aquaculture, electrical and air cooling. To calculate how many kilowatts of cooling power are required, use the following calculation where DT = the difference in temperature required, and Q = the flow rate in litres per minute kW = (Q x DT) / 14.4. This is the cooling power required to cool the water, it is not however the electrical power required. Because of the way that air cooled chillers operate, the required amount of electrical input to the chiller is less. The percentage of electrical input energy required to achieve a given amount of cooling is dependant on the ambient air temperatures i.e. in tropical regions it is possible to get more cooling per kW electrical input than in temperate regions.

Chinook Salmon

A species of Salmon (Oncorhynchus tschawytscha) which is native to the West coast of North America. Not extensively farmed for food, but there are many hatcheries which are used into restock into the wild.


A polysaccharide material very similar in structure to cellulose. Naturally occurring material which is used to strengthen the supporting structures (e.g. shells) of many invertebrates such as shrimps, crabs etc.

Chloramin T


Chlorine based chemical for the treatment of external parasites. Usually used as a bath or flush treatment, at concentrations of 2.5mg/l (at water pH of 6) to 20 mg/l )at water pH of 8) for one hour. Care should be taken not to mix chloramine T with formalin / formaldehyde, and to avoid the use of metal containers. Appearance - fine white powder, inhalation can cause respiratory problems and irritation.



Antibiotic drug. Widely banned for use in many countries, as it is one of the primary antibiotics used for humans. Improper use could therefore have consequences to human populations through the build up of resistant strains of bacteria in the wild.


Marine algae, often used in the production of live feeds esp. for feeding as part of diet to rotifers

Chlorinate Polyvinyl Chloride

CPVC or PVC-C - Also referred to by some brands as Corzan. Modified PVC used in pipes and designed especially for high temperature applications. Working temperatures for fluids up to 100oC. Solvent welded, can be welded directly to normal PVC pipework. General higher resistance to chemicals than ordinary PVC

Nationwide delivery of CPVC Corzan Pipe and Fittings

Chlorinated water

Tap water, used for some small scale hatcheries and research facilities often has chlorine added to it at the treatment source, to kill any bacteria etc, in the water and also to keep the water supply pipes clean. For removal methods see Dechlorination. Some chlorinated tap water fluctuates in the amounts it contains so random checks are not sufficient. If in doubt - filter, aerate or confirm with your water supplier that chlorine is not added. See also Heated effluents. Toxicity is dependant on the species, but in general, concentrations of above 0.05 mg/l may cause problems. Municipal water supplies typically contain between 0.5 and 2 mg/l chlorine, and effluents from power stations and other industries, where the chlorine is added to keep fouling of pipes to a minimum is typically 0.1 - 0.25mg/l. Some power stations and industrial sites will periodically flush with higher concentrations.

Chloride test

Test used to determine whether a fish is ready for transfer to the sea. For example it is used to test salmon smolts (especially when they have been produced out of their natural season) for their ability to osmoregulate correctly in seawater prior to being transferred.


Chemical symbol Cl. Added to potable water by many countries for sterilisation and antifouling purposes. Must be removed from water prior to use with fish. Toxicity levels for fish are in the region of 0.1mg/l. Also added to industrial (and power station) cooling water systems to reduce fouling in pipes.


Anaesthetic sometimes used in aquaculture. Used at 1000 mg/l as a bath / dip


The green pigment that is essential for photosynthesis in plants


Animal phylum. Contains  vertebrates (subphylum Vertebrata) and also invertebrate subphyla Urochordata and Cephalochordata. 


Metal, falls into the category of heavy metals. Recommended safe concentrations : 0.05mg/l (freshwater and marine) however may act synergistically with other metals.


Thread like structures found in all cells. The chromosome carries the genetic information which is essential for development and growth.


A disease or other process with a long duration. Chronic does not imply anything about the severity of the condition see also acute.


Common term given to a fish which belongs to the family Cichlidae


Family of fishes. Containing genus such as Oreochromis (tilapia) and Cichlosoma. Typically ctenoid scales, single dorsal fin. Natural habitat typically in tropical regions  (inc. Africa and South America) but spread throughout tropical and sub tropical regions throughout the world (esp. tilapia) through aquaculture and ornamental trade


Small organs that take the form of beating hairs and are used to create a current or for movement. Cilia are possessed by many species of zooplankton and also some parasites.


A single celled animal that possesses cilia.


The growth rings found on fish scales and in otoliths, plural circuli, also called annulus / (pl) annuli


Sometimes called Natural Classification. The arrangement of organisms into a series of groups based on physiological, biochemical, anatomical or other relationships. In general the higher up the hierarchy, the greater the difference between the members of that that particular group. 


The division of cells, by which a single cell egg, is transformed into a multi-cellular body through the division of the cells.


See Ion exchange


The culture of a single cell into an animal that is physically identical to it's parent. Cloning techniques are relatively new to science and are not used at present in aquaculture

Clove Oil

A natural oil extracted from cloves and related plants, which
has anaesthetic properties useful in fish and crustaceans (as well as human
toothaches).  The raw oil may be used, or  either of its main active
ingredients eugenol (80-95%) and isoeugenol (up to 5-10%) (often the latter
are supplied as synthetic forms not directly derived from the oil).  All
forms require use of 3-4 volumes of solvent such as ethyl alcohol to make
them water-soluble.  Typical published effective dose rates range from 40-50
mg/l to 100-150 mg/l depending on species, temperature and objective. The
drug appears to be equally useful for light sedation (e.g. fish transports),
full immobilization and humane killing.  Fish exposed to clove oil products
respond in a manner similar to popular anaesthetics like TMS (MS-222) or
benzocaine, however the physiological effects and the fate of drug residues
are not as well studied yet.  A commercial product, Aqui-S TM, contains
isoeugenol and is in use in the salmon culture industries of New Zealand and
Australia.  Other sources are common and often inexpensive, as clove oil and
its ingredients are used in making perfume and as human food additives.
While regarded in many jurisdictions as uncontroversial compounds, clove oil
and its components have a characteristic strong scent and still-unknown
potential for health concerns which aquaculturists should consider carefully
before using.  Shelf-life of clove oil is not as long as some other
anaesthetics, especially when stored exposed to air, and there may be
considerable variation in quality  depending on source.

Entry provided by Henrik Kreiberg


Order of fish (part of the superorder Clupeomorpha) which includes species such as herring (Clupea) and Shad (Alosa)


Superorder of silvery, compressed fish containing the order Clupeiformes


A group of eggs laid at the same time. Most fish lay eggs in clutches. Clutch sometimes, more specifically is used to relate to groups of eggs that remain together, for example some species lay their eggs in a nest rather then letting the drift freely in the water column.


Gadus Morhua, demersal marine fish, provides one of the main crops for Northern Hemisphere fishermen. Falling stocks have lead to increased prices and at the time of writing (1999) the culture of cod is under serious investigation, with some farms already producing small amounts. One of the main drawbacks to the culture of Cod is that they have a poor percentage yield of fillet. This is due to the large head and heavy structure of the skeleton and fins.

Cod Roe

Ovaries of the fish Gadus Morhua. Cod roes are sometimes used as a stimulant to encourage fish to feed e.g. they are sometimes mixed in with commercial diets to wean elvers (small eels) on to the commercial diets. Other roes can be used, but the availability and small size of the cod roe makes it a suitable candidate. Some farmers try to steer clear of using roes (or any wet fish) as it can be a source of bacterial and viral infections.

Coefficient of roughness

A value given to different surface types to assist in the calculation of friction losses in pipes and channels


Sticking together e.g. of particles

Cold water disease

See fin rot


A particle made up of many smaller particles. One that is capable of being relatively easily broken up into smaller particles.


Colouration of the water by either chemicals dissolved in the water or solid particles has a particular impact for species which require light as part of their lifecycle. For example species which are sight feeders (which includes most carnivorous and many of the omnivorous species) may find it difficult to locate feed pellets. This may then lead to poor growth and wastage of feed. Some areas can have water coloured so much, that normal physiological processes which are governed by light (such as spawning in some species), are not carried out, as the fish are not registering the small amounts of light that are reaching them. Removal of colouration from the water (especially if it is caused by very fine solids or dissolved chemicals) can be very expensive. Colouration of the water in high rate recirculation systems occurs as a result of long chain organic molecules building up, which are too complex for the bacteria in the biofilters to break down. These can be broken down by the addition of an oxidising agent such as ozone, into smaller, simpler molecules, which the bacteria can then break down. Alternatively in seawater systems, fractionation can remove a high percentage of this colouration. Colouration of the water can also affect the ability of the farmer to see his fish, and may sometimes delay the detection of a disease condition, leading to higher mortalities. During periods of high colouration, more attention should be paid to health checks because of this.


A device for use with water quality testing. Reagents are added to a sample, which react with specific chemicals in the sample to produce a colour, the depth of which is indicative of the concentration of the chemical. The colourimeter is used to compare the original sample with the depth of colour that has developed in the treated sample. Devices range from a simple manual comparison to electrical devices which pass a beam of light through the sample and are very accurate. The list of parameters that can be checked is long, with most of the usual required fish farming tests included (with the exception of B.O.D. an C.O.D.) Before using any test kit which involves the se of reagents in a brackish or seawater sample, it is worthwhile checking that they measure correctly in seawater, as many do not.

Columnaris disease

A bacterial infection caused by the bacteria Flexibacter columnaris. Gives rise to ulcers and damage to the skin and gills. A disease which occurs in freshwaters worldwide, especially at higher temperatures.


An organism that lives in union with another species ( a host ) but is not injurious to it. An example is the Clown Fish which lives in anemones.

Common Salt

Refers to NaCl (Sodium chloride) see Salt

Communication languages

The main communication languages used in equipment designed for aquacultural use are RS232 and RS485. There are other languages used, however these are the most common. RS232 is the same language that a computer uses to talk to, and receive information form a keyboard, mouse, printer etc. It's limitations are that an RS232 communication can only talk to one piece of equipment, through any cable at a time. The maximum distance that RS232 can travel, without losing performance is also a limitation as it is limited to about 25m. RS485 on the other hand can communicate with many devices on the same cable and can communicate (as long as the correct cable is used) up to distances of over 1000m. The requirement for RS485 is paramount due to the distances that the communications must travel on fish farms. It is also possible to transmit RS485 from remote locations via radio signals, although the success of this is dependant on the terrain and the local laws regarding the use of transmitters.

Compensation Point

The point at which there is insufficient light penetration for photosynthesis to be properly achieved see extinction coefficient


The aerobic breakdown of organic material by bacteria and fungus (occasionally higher animals will also be involved such as insects etc. but these rarely, significantly contribute to the composting process). Composting has been used mainly on an experimental basis in aquaculture, where either solid wastes from a farm (i.e. faeces and uneaten feed) or viscera and other fish processing wastes.


A substance formed by a combination of elements, in a fixed proportion. Compounds cannot be separated by physical means.

Compressed (fish)

When applied to the shape of a fish, this means a narrow, flat, deep body i.e. one with a small distance from one side to the other, but a large distance from the top to bottom.


A device for providing high pressure air (or other gas). The main distinguishing feature of compressors, is that a true compressor cannot be restricted in it's output (for example closing the outlet valve off so that the pressure rises) without causing damage to the compressor. Most compressors provide a pulsed flow of air rather than a smooth consistent pressure. As a result, compressors are usually linked to air receiving tanks, which serve to smooth the pulses out. The compressor starts and stops to maintain the correct pressure in the tank.


The amount of a given substance as a proportion of another substance (usually water in the case of aquaculture). Concentrations are usually referred to in terms of part per million (ppm), milligrams per litre (mg/l) or parts per thousand (ppt). The table enables you to convert one concentration to express it in terms of another.

Conditioned response

The "automatic" action taken by an animal in response to a stimulus. A behavior that is the result of training or experience. For example fish on a farm will often swim towards a person standing by the tank as they know that when someone stands by the tank, they get food


Ability to conduct electricity. A measure of the ionic concentration of waters. In general water with a high conductivity has a good buffering capacity. Not used as extensively in aquaculture as many other water quality measurements, which give readings (such as salinity, alkalinity etc.) more directly relevant. It can be used in some circumstances to give a general indication of changing water quality (e.g. pollution, if the likely pollutants are known).

Congenital disease

A disease that is passed on from the parent fish to their young and is present at birth.


The specimen(s) under experimental conditions that remain "untreated". e.g. in tests to assess the resistance of a fish to a particular chemical, several tanks will be administered the chemical, but the control will not. This is a safeguard against another variable affecting the fish. If all the fish died, this may be seen as an effect of the chemical, but if the control fish also die, it can be seen to be the effect of something else.


A device which regulates a process. The types of controllers typically used in aquaculture are for either turning machinery on or off, regulating the speed of the machinery or operation of automatic valves. Controller typically receive information form probes etc. (such as temperature probes). They then interpret the signal from the probe and convert it to an output signal depending on how they have been programmed. Although controllers can be purchased for self programming, most are purchased already pre programmed for certain programs and functions. Controllers have either a digital or analogue output, although some controllers have more than one output channel and therefore can have a combination of digital and analogue outputs. This might be required for example if an analogue signal was required to carry out an operation such as dosing of lime, but a digital alarm was required if the pH was too high or too low. Although such combined systems save money as only one controller and probe are required, they can provide a weak link in a system as if the unit fails, as both the alarm and control functions are lost.

Conversion Software

Follow the link to the left for DOS program which automatically converts area, volume, flow rate etc.

Conveyor Filter

A mechanical filter which uses a moving belt across the water channel to filter the water. The belt is washed by pressure wash jets as it rotates. Also called belt filters. Usually used for mesh sizes of >200 microns, due to problems with the strength of finer mesh materials. Some efforts have been made to construct finer mesh conveyor filters, however the thickness of the belt to achieve a combination of correct filtration and strength means that there is often a build up of solids and oils in the mesh resulting in the need for increased maintenance time. Conveyor filters have a slight benefit over rotating filters as they are more gentle on the faeces and less likely to break up the particles. Their inability to filter effectively to much below 100 microns however precludes their use in a lot of systems. See also disc filter, drumfilter


see Chilling



Crustaceans found in marine and freshwater environments. Typically 0.5 - 2.0mm long. Important members of the zooplankton. Beginning to be used for feeding in the larval stages of some marine fish, such as Halibut (Hippoglossus hippoglossus).


Metal, chemical symbol Cu. Toxic to fish a low concentrations when dissolved in water. Dissolution into water to toxic concentrations only usually occurs when pH<6.0. Safe concentrations of  copper in the water are in the region of 25-50 micrograms per litre. LC50 occurs commonly at 300 - 1000 micrograms per litre.

Copper Oxychloride

Herbicide used to control Filamentous algae (except Chara sp.) Applied at 1kg/ha

Copper Sulphate

Chemical symbol CuSO4. Compound used for the treatment of some diseases. Main uses are for the treatment of bacterial gill disease, but is also used for some protozoa and fungal infections. Used at a rate of 500ppm as a dip for 1 minute. Should be used in well buffered water only as toxicity can occur in poorly buffered waters. Some operators use as a bath for up to 30 minutes, dose rates are 0.5ppm for poorly buffered waters to 2ppm for well buffered water. In particularly bad outbreaks of external bacterial and protozoal diseases, copper sulphate is sometimes used for longer periods (several hours) at 0.2ppm. This is only recommended in well buffered waters.

Also used in some areas for the eradication of snails from a water course, to break the life cycle in some flukes (such as eye fluke - Diplostomum spathacaeum). Usually requires long exposure (over 1 hour), as snails will retract into their shells and appear to be poisoned, only to "come alive" again once the copper sulphate has passed them by.

Also used as a herbicide to control algae. Applied at 0.5 - 1g / m3 in pH < 7.0, to 1.5g / m3 in pH > 7.0, Toxic to fish, therefore only - 1/3 pond should be treated at any one time. 

Copper toxicity

see copper


The outer covering of the eye.


Chemical or electrochemical attack on the surface of a metal e.g. rusting. Corrosion is particularly a problem for marine farms, where annual painting of non galvanised steel or iron equipment is essential, to prevent equipment failure. Sacrificial anodes are often used on pumps and other devices to limit corrosion.


A general term for hormones produced by the adrenal glands. Includes cortisol and cortisone which regulate the use of nutrients in the body. 


A hormone produced by the adrenal gland that promotes the synthesis and storage of glucose. It is used in response to stress, to suppresses inflammation and regulates the deposition of fat in the body. When a fish is stressed, the cortisol levels increase causing more glucose to be released in to the bloodstream. This can result in the build up of lactic acid in the body, as once it has been used for energy, the glucose breaks down into lactic acid. Cortisol concentration in the blood is often used as an indicator to measure stress levels.

Corynebacterial disease

See bacterial kidney disease


Gram positive (see Gram's staining) bacteria, which is the causative agent of Bacterial Kidney disease.


The condition caused by infection with the protozoa, Costia necatrix

Costia necatrix

A protozoan parasite of fish . Affects the skin and gills. If present in sufficient numbers, can cause rapid mortalities, similar to viral infections. In larger fish, a skin infection often results in the production of excess mucus, which gives a blueish tinge to the fish. Treatment is usually by chemicals such as formalin at a concentration of 200 - 250ppm for 1 hour

Crashing - (1of 2)

Term used to describe the failure of a biological filter. Commonly regarded to be caused by the death of the bacteria in the filter due to changes in their environment (either due to stress caused by changes in the water quality in excess of their normal, acclimated range, or through a lack of available nutrients). Often however, especially with regard water quality, the bacteria are not killed off but cease to perform due to there normal levels whilst they try to acclimate to the new conditions. Changes in water quality which are likely to actually kill the bacteria will probably kill the fish first. Once stressed, the bacteria cease to perform as efficiently which leads to a build up of metabolites in recirculated water systems. This in turn exacerbates the problem as the stress to the bacteria is compounded by increased levels of ammonia and BOD. This can then result in a downward spiral, where the bacteria become more and more stressed by the ever worsening water quality, sometimes to a point where they barely function at all. All the farmer sees is the water quality worsening and his fish suffering.

Crashing - (2 of 2)

Term used to describe the death of an algal bloom, usually due to the exhaustion of nutrients or the lack of light penetration, usually caused by the bloom itself. The death of a bloom can lead to fish kills, either through the release of toxins into the water (from some species of algae) or more commonly, through the reduction in dissolved oxygen concentrations as, as the dead algae decomposes. In green water systems, this makes the management of a stable algae bloom, as important as the management of the fish.

Crayfish Handlers Disease 

See Seal finger


The mating of different, unrelated strains of a particular species. Used as a tool to prevent inbreeding. Many farms keep different parent groups separate, to avoid breeding siblings together. It is generally recommended that the minimum number of fish in each group should be 50 males and 50 females with at least three groups. Pit tags are now often used as a method of identifying each group, so that valuable tank space is not used up keeping each strain separately. Another option is to "trade" broodstock, eggs or sperm with other farms, however this brings  with it the problems of disease risk and also the introduction of unproven stock.

Crowding Screen

A device for concentrating the fish in a particular area of a tank or raceway, to enable them to be easily removed. Crowding screens are usually constructed of a rigid mesh, with soft sides (brushes or rubber flaps) which serve to follow the contours of the tank and therefore reduce the ability of the fish to escape round the screen. The soft edges also reduce damage to the tank during use.


The general term given to hard shelled animals. Includes crabs, lobsters, shrimps, water fleas (Daphnia) and Copepods. Segmented body, typically free swimming nauplius larvae which go through a series of moults and/or metamorphosis before becoming an adult

Cryogenic fractionation

The separation of gasses by cooling them until they enter their liquid state. large scale gas production companies use this method to produce liquid oxygen, liquid nitrogen etc. As gasses have different boiling points (the temperature at which they change form from liquid to gas). Oxygen has a boiling point of -183oC, and nitrogen a boiling point of -195.8oC.  therefore by cooling the gas mixture to -183oC, the oxygen can be collected as liquid and the nitrogen remains in gas form. See also oxygen generators


The freezing of eggs and milt to very low temperatures (typically -196oC, the temperature of liquid Nitrogen)  in order to store them for later use. Once correctly frozen , and assuming correct thawing procedures, theoretical storage time can range from 200 - 32,000 years!. Techniques for storage of sperm are well documented. The process is easier due to the smaller and less complex size of the sperm in relation to the eggs. Cryopreservation has a number of applications; to store milt so that checks can be made for diseases etc. prior to use and for the preservation of certain strains. The process of freezing is a very delicate one, which if carried out incorrectly will result in the formation of ice crystals, which can damage the integrity of cells. Often chemicals such as cryoprotectorants are added to reduce the risk of this damage occurring. 


A chemical which is used to restrict the amount of damage caused by small ice crystals when gametes are frozen. A large range of chemicals are used, but the more common include ethylene glycol, methanol, DMSO. Insufficient concentrations, these are toxic to the gametes and so must be used very carefully.


One of the two main teleost scale types (see also cycloid). Scale with serrated edges which are often quite sharp. Typical of basses and perches.


A substrate that is introduced to encourage the settlement of mollusc larvae (spat). Examples of items used include ropes, tiles, rocks, pipes and wooden posts.


The controlled rearing of plants or animals. Also used to refer to a stock of plants /animals, especially phytoplankton and zooplankton.


(Copper sulphate with triethanolamine) - Herbicide used to control algae. Applied in liquid form - 1.85 ml / m3. Granular form - 66 kg / ha. Administered 2 -3 times a year. Dilute the liquid form to at least 10 : 1 for good coverage of the water. 


Commonly called blue-green algae. These organisms behave like algae but are actually zooplankton. Responsible for the release of toxins into the water which can cause fish kills and flesh taints. When filter feeders such as blue mussel (Mytilus edulis) consume cyanobacteria, the toxins can build up in the body, causing poisoning to humans on consumption (called PSP - paralytic shellfish poisoning)


One of the two main teleost scale types (see also ctenoid). Scale with smooth edges, generally circular in shape. Typical of carps and salmonids


Common term used to describe fish of the suborder Cyprinoidei


Order of fish (part of the superorder of Ostariophysi) includes two suborders, Characoidei and Cyprinoidei


Suborder of fish (part of the order Cypriniformes), includes fish such as Carp (Cyprinus), Labeo, barbus, and Roach (Rutilus). Adipose fin absent, protractile toothless mouth, pharyngeal teeth.


Sac containing liquid or pus. Not necessarily associated with infection.


The material inside a cell, not including the nucleus