The Free Online Aquaculture Dictionary

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Rabbit Fish

Common name given to many species of the genus Siganus. A brackish / marine water fish. Naturally schooling, feeds on algae attached to substrates and also free floating algae. Usually found at depths 2 -6 meters. Some trials and few commercial farms growing Siganus in cages, esp. in Middle East


General term given to a straight sided artificial channel in which fish are held. Generally with a high water turnover rate of <1 hour. The advantages of raceways are that they can be easily built in series, with the water flowing from one to the other and that they are easy to empty of fish by using a simple crowding screen. To be self cleaning, raceways must be operated at high flow rates and/or high stocking densities, where the movement of the fish keep the faeces and uneaten feed from settling. Disadvantages of raceways are brought about primarily by poor mixing (due to the "slug flow" nature of the raceway) and include the gradual deterioration of the water quality along the length of the raceway (whereas round tanks tend to be more even) and difficulty in efficiently distributing additional oxygen throughout the raceway. Usually built with a width to depth ratio of between 2:1 and 4:1, with the length limited either by the amount of stock that can be held in a single holding unit for ease of management or the deterioration of water quality. The even nature of raceways means that they are somewhat flexible in that screens can be placed anywhere along the length, dividing a single unit into 2 or more smaller units. Modifications to the designs include rounded bottoms to concentrate solids for ease of cleaning by vacuuming (especially where small fish are involved) although this adds to the cost of construction, the inclusion of aeration along the length of the raceway (which serves to maintain more even oxygen concentrations along the length of the raceway, and also concentrates settled solids into specific areas to make cleaning easier. Other modifications include the addition of barriers in the raceway to create a swirling motion in the water where waste concentrates. Raceways are advantageous in that they can be constructed with basic building materials such as bricks, blocks or poured concrete and require little specialised labour. 


Term used to describe the way in which some species of marine larvae can become "trapped" in the surface tension of the water and clump together, resulting in mortalities. An air current provided across the water is usually sufficient to eliminate the problem.

Rapid Bioassessment Protocol

Another name for Biotic Index


Chemicals which are added to water samples to react with a specific chemical in the sample to produce a measurable effect (such as the formation of colour) which enable the determination of the concentration of a particular chemical(s) in the water- see also colourimeter


The process of taking water from a holding system which would otherwise be discarded from the system and reintroducing it to the same system. Prior to being reintroduced, the water is often treated to remove some of the wastes produced by the fish so that the water quality is maintained at a sufficient high level that it remains suitable for fish culture. The amount that is recirculated is often called the percent recirculation although this can be a misleading figure as it only takes account of the hydraulics of the system, rather than the biological processes which are occurring in it. A more accurate way of describing the amount that water is recirculated or reused, is to refer to the amount of water brought into a system per kg of feed given to the system. The recirculation of water increases the operating costs of a farm through the process of pumping the water round and also the additional costs associated with any water treatment (e.g. power for filters, cost of oxygen etc.). In most recirculation systems a compromise is reached between the operational costs, the capital costs and the amount of make up water required. Water is usually recirculated for one of the following reasons; limitations in the water quantity or quality available, restrictions in discharge of wastes into the environment, maintenance of stable water condition (such as year round raised temperatures). See also biological filtration, mechanical filtration, ozone. pH control, oxygenation.

Red Muscle

The Line of "dark" muscle that runs along the length of the fish. This muscle is used for the everyday functions of swimming and maintaining position in a current. The white muscle is brought into play only when additional bursts of energy are required. The red muscle has a rich blood supply (hence the colour) and is good site for intramuscular injections.

Red Tide

See Algal bloom


A word derived from a combination of the words Reduction and Oxidation. The REDOX measurement, in millivolts, is a measure of the potential of the water for oxidation or reduction processes. The higher the reading, the higher the availability of oxidising agents in the water. Optimum levels of oxidising and reduction agents occur at around 300 mV. And this is regarded to be the approximate REDOX level of very good quality water (e.g. for salmonids). REDOX level in excess of 500mV may prove toxic to life over prolonged periods, and REDOX levels of over 600mV are often maintained in systems where oxidising agents (such as ozone) are used to disinfect water. Low REDOX levels are a sign of poor water quality as the amount of oxidising compounds in the water is low, which limits the breakdown of organic matter.


The process where a molecule gain an electron. Reduction applies only to chemical reactions where a transfer of electrons occurs. The opposite of oxidation.


Device for measuring the salinity of water. More accurate and easy to use in the field than a hydrometer. Uses light refraction through a lens. The light refracts to a greater or lesser degree in different salinities. Relatively inexpensive.

Relative atomic mass

The ratio of the average mass of an atom of the naturally occurring form of an element to 1 /12 of the mass of a carbon-12 molecule. 

Relative Humidity

See humidity


A device for switching an electrical current on or off, i.e. a digital output, in response to changes in another current / signal. Relays either operate on a normally open (or normal) state, or a normally closed (or inverted) state. For example if the digital output is used for an alarm signal, the normally open state means that when there is no alarm condition, the circuit is open and no current is able to flow round it. When an alarm occurs the relay switch closes and forms a circuit. The normally closed state, is a closed circuit (i.e. there is a current flowing through it) when the system is normal, when there is an alarm condition, the relay opens breaking the circuit. When the circuit breaks the alarm panel sends out an alarm. This is more preferable to the "normally open" relay as if there is any break in the wiring of the circuit this will also make an alarm condition (making the system fail-safe). It should be checked that any relays that are used on the farm will revert to the required state should power fail. For example an alarm relay should revert to the alarm position (be it open or closed) when the power fails. However, a relay which is being used for oxygen may be required to stay in the state where it continues to supply oxygen if power fails (in which case a separate alarm relay should also be used to make the system more fail-safe). See also solenoid, analogue, controller

Remote Sensing

The gathering of information concerning the earths surface that do not involve contact with the surface or object under study. The techniques include aerial photography, multispectral and infra red imagery and radar. Usually carried out by satellite and aircraft, remote sensing can give information as to ground conditions over large areas at a relatively low cost. Also sometimes called G.I.S. (Global imaging systems). Remote sensing is often accompanied by a relatively small amount of "ground truthing" where an operative visits several areas to ensure that the information that is being received is being interpreted correctly. Used by some government agencies to focus on areas which are most suited to aquacultural development by looking at geology, communications, population and water distribution over large areas.


Collective term used to describe the species of crustacean which are walkers such as the Brachyura (Crabs) opposite Natantia

Residence Time

Term used to describe the average amount of time that waste remains in a vessel. Can be used to describe the turnover time of tanks, settlement chambers etc.

Resistance #1

Symbol R, measured in ohms. The ratio of the potential difference across an electrical component to the current passing through it. R = Voltage (V) / Current (A). It is therefore a measure of a components opposition to the flow of electric charge.

Resistance #2

Refers to the ability of pathogens to build up a tolerance to chemicals that should, under usual circumstances kill or inactivate them. Drug resistance is particularly a problem with bacteria which can build up a resistance to some (or all) of the antibiotics used to kill them. Once a strain of bacteria has developed with such a resistance, the farmer will find that a chemical which has always controlled a disease, is no longer effective. Resistant strains are most commonly formed when antibiotics and other chemicals are incorrectly administered, allowing some of the pathogens to survive. For example if a bacterial infection was only given three days dose of an antibiotic rather than the recommended full course of 10 days (varies according to the type of antibiotic), although the symptoms of the disease may disappear, as most of the bacteria have been killed, those remaining will be the ones with a small natural resistance to the antibiotic. As those that are susceptible to the antibiotic have now been killed off, these strains will multiply and become the dominant strain. When the farmer uses the drug the next time he might find that he has to use it of 5 days to see mortalities stop. If he still fails to use it for the full 10 days, he will have only killed the bacteria which have a mild resistance to it and those remaining will have a greater resistance and multiply. It is therefore of vital importance that the farmer sticks to the full treatment time to ensure that even bacteria with a slight resistance are killed off. Some farmers rotate the types of antibiotic used every two or three years, which helps the prevention of the build up of resistant strains. In some areas, where antibiotic use is poorly regulated, strains which are resistant to a number of antibiotics develop. These "super resistant" strains, can cause devastating effects on farms and farming areas. As many of the bacteria that affect humans are found in the wild, it is important that drugs that are used on humans, are not used on farms. The release of antibiotics into the environment through leaching from food and excretion by the fish, can cause low concentrations which assist in the development of resistance of any bacteria in that environment.


The process by which animals and plants metabolise organic substances, breaking them down into simpler components which produce energy. In most plants and animals the process of respiration requires oxygen, and carbon dioxide and heat production are the end products. The exchange of oxygen and carbon dioxide between the body and the environment is termed external respiration In most animals this takes place at special organs such as gills or lungs and is assisted by respiratory movements (e.g. breathing). Respiration at a cellular level is called Internal (or tissue) respiration. This occurs in two stages, 1. Glucose is broken down into pyruvate (called glycolysis) and 2. Pyruvate is broken down into carbon dioxide and water (the Krebs cycle) the former of these is anaerobic and does not require oxygen, whereas the latter is aerobic and requires oxygen, producing carbon dioxide as a waste product. The second stage (Krebs) is the main energy yielding process.


Water reuse is a process by which more production than would normally be possible is achieved from a given quantity of water. For example if the normal minimum requirements of a fish is for 1 litre per minute per kg of body weight, and a farmer adds oxygen to the water through aeration, so that he can hold 2kg for every litre per minute, he is reusing the water once. This term is often confused with Percent Recirculation, but differs in that no water needs to be pumped for the additional production to be achieved. The amount that water can be reused is governed by the limiting factors, the first of which is usually oxygen. As oxygen can be easily added to the water the next limiting factor is usually either carbon dioxide (at low pH) or ammonia (at high pH). Re-use

Reverse Osmosis

Method if obtaining freshwater from saltwater. The process uses a semi-permeable membrane through which pure water and not the salts will pass. The saltwater must be pressurised to approximately 25 bar, which makes it operationally expensive to produce large quantities of fresh water by this method. See Osmosis and osmoregulation


Type of Spring 


A quaternary ammonium compound used for the treatment of bacterial gill disease. Similar to Hyamine 3500 and Benzalkonium Chloride used at a rate of 5-1.0 mg/l


General term given to the eggs or ovaries of fish. Many roes have a commercial value, the highest being for caviar, the roe of the sturgeon. Other commercially produced roes are those from salmon, trout, lumpfish and sea urchins. Some roes, such as that of the Cod (Gadus morhua) is used as a wet diet in aquaculture. It has been particularly successful in use for elvers (Anguilla sp.) prior to weaning on to dry diets although fears for disease contamination limit it's use.


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


Microscopic ciliated animals. Commonly used in the culture of marine species where live diets are essential during early stages. The most commonly used species of rotifer is Brachionus plicatilis. Rotifers have the ability of retaining nutrients and are fed certain species of algae, so that when they are fed to the larval fish, they transfer the nutrients in the larvae in to the fish. they are also enriched with special compounds (such as HUFA mixes) which they retain and transfer to the fish in the same way.


A naturally occurring substance found in the roots and stems of several tropical plants. Jewel vine (Derris spp.), Lacepod (Lonchocarpus spp.), and hoary pea (Tephrosia spp.) are the more common plants from which rotenone is derived. Very potent biocide, used in some countries to kill off larvae and predatory fish prior to introducing parent fish for natural spawning or newly hatched fry. Also used in some areas for fishing, where it is added to the water course and the fish collected as they float to the surface. Commercially available in a 5% solution and administered at 14mg/l for pesticide control. 

Round Tanks

Type of holding unit. Round tanks have the advantage of a naturally self cleaning action. As the water swirls around the tank, solids are drawn towards the middle, where the outlet is situated. Due to this property, they are often used in hatcheries, where due to high feed rates, solids loadings (waste feed and faeces) can be very high and also in recirculation systems, to remove the solids as soon as possible, before they break down. Round tanks can be constructed of almost any material, the most common being fibreglass (for tanks 8m diameter and under), steel (lined or unlined) and concrete or concrete block. Other materials can be used as long as it is strong enough to hold the water without distortion and is non-corrosive, non-abrasive and non-toxic. Round tanks generally have a slope of about 1:50 (2%) on the bottom towards the centre outlet to increase solids removal efficiency. Other qualities of round tanks include a good mixing of the water, resulting in easy oxygenation and less contact of the fish with the tank sides / bottom, due to a higher ratio of tank volume : tank wall/bottom. Many species prefer the consistent current of a round tank to other more static systems. The disadvantages of round tanks include poor use of land area and difficulties in management (fish removal, screen cleaning), especially in larger diameter tanks (>5m). See also screening, d-ended tanks and raceways


Communications language used by computers to talk to devices such as mouse, printers, keyboards etc. Also used to transmit and receive commands over short distances (up to 10m) to monitoring and controlling devices. Limited to one device per cable. see also RS485.


Communications language used by computers to talk to multiple devices such as monitors and controllers over distances of up to 1km. Uses either two or four wire communication link. Advantages over RS232 include distance and simplicity of wiring. Must be converted to RS232 by protocol converter before communicating with PC. see also RS232


Generally refers to water, which has fallen as precipitation, which is channeled directly to streams, rivers etc.