The Free Online Aquaculture Dictionary


Term that refers to leaved plants.


Granules of Magnesium Hydroxide. Usually available in either coarse, medium or fine. The fine are the grade of choice for aquaculture as they dissolve quicker. Used in fluidised beds or sand filters to buffer recirculation systems. Problems have been found in marine recirculation due to calcium carbonate coming out of the seawater and coating the granules. Will not dissolve at pH of > 8.3. See also lime

Magnesium Carbonate

A compound, usually available in powder form, used as a buffer in recirculation systems and hatcheries to prevent low pH levels. See also lime

Magnesium Hydroxide

See Magnaspheres

Malachite Green

Chemical used primarily in hatcheries to kill fungal infections, especially of eggs and broodstock. Now banned in many counties due to carcinogenic links. If used for livestock, it is important that the "zinc-free" chemical is used. Also sometimes used in conjunction with formalin at for stubborn external parasites such as Ichthyopthirius. Malachite green has also been used for applications such as spraying fish ponds to eliminate fungus (Saprolegnia) and White Spot (Ichthyopthirius) at 0.15 mg/l.

Malachite Green #2  

Used as a herbicide to control filamentous algae (except Chara sp.). Applied at 0.23g / m3. See also malachite green for health and safety issues. 


Largest subclass of Crustacea, including prawns, shrimps, crabs, lobsters, woodlice etc. See also Decapoda


Method of direct sex reversal by the treatment of animals with doses of male hormones in the feed during the early stages of development (typically at first feeding). Although the development of male fish is undesirable for species such as salmonids, eels and flatfish due to early maturity, it is of benefit to tilapia and catfish farmers where the males are the more desirable gender. The use of hormones in fish for consumption is problematical in some countries, where laws prevent the direct use of hormones in this way. Most countries however permit the use of hormones in potential brood fish that are not to be used for human consumption. Treated fish, which would otherwise have been female, cannot be stripped of sperm, and these fish are dissected and the milt used to fertilise eggs which results in the production of all female off spring. This is desirable in some species where the females mature later than the males, or grow larger, faster. The most common hormone used for masculinisation is 17a-methyl testosterone, which is used at a rate of between 1-60mg/kg feed for a period of between 25 and 100days following first feeding. The administration of very high levels will have the effect of sterilising the fish. The hormone is fully dissolved with ethyl alcohol prior to mixing with the feed. A more even mix can be achieved if the feed is spread out and the hormone sprayed onto the feed. After mixing the food is dried to allow the ethyl alcohol to evaporate off before feeding it to the fish. The feed can be stored in a freezer for later use.


An appendage originally used for walking which has been adapted to also be able to be used for feeding. I.e. The front legs of many crustaceans, which have developed claws.


Medium density polyethylene. Plastic, often used for items such as tanks, floats etc. Has a high degree of resistance to chemicals and is easy to keep clean. Very difficult to adhere to with glues etc. silicone is one of the only adhesives that will give some bonding. Easily welded, but note that high density, medium density and low density polyethylene's will not weld to each other.

Mechanical Filtration

Term used to describe a physical process (i.e. one not reliant on chemicals or biological organisms) to remove solid particles from the water.  mechanical filters fall into three categories - settlement, screen filtration and bed filtration. 

Settlement involves passing water through a tank which has a residence time and slow enough current sufficient to allow the particles to sink to the bottom of the tank, from where they can be removed. Modifications to improve settlement tank design include the use of directed water flow, either through a series of baffles, which ensure that the water stays in the tank for the maximum amount of time, or by using a swirl of water to draw the particles to the centre of a round tank, from where they can be removed. (This works in the same way as when you put some sand in a glass and stir it with a spoon. The sand will ed up in a pile in the centre of the glass.) Settlement s a good method to remove large particles from the water column, but can be an expensive method of removing fine solids, due to the large tank size required.

Screen filtration involves passing the water through mesh or bars. Such a process inevitably results in the screen becoming blocked with removed particles. For large solids such as leaves and weed, the screens can be manually cleaned, but for smaller particles, including fish waste, self-cleaning systems are necessary. Careful design of manually cleaned screens can reduce management time, examples are given in the diagrams (buttons left). Automatically cleaned filters usually either use a rake / brush system or water jets to clean the screen. In general, rake/brush systems are used for large solids and water jets for smaller solids. Water jet cleaned filters follow either a drum, disc or belt method, where the screen material is constantly moving and cleaned as it rotates. Such filters are capable of removing solids as small as 6 microns. These filters have the advantage that they have a very low head loss (typically 10-70mm) which reduces the pumping costs in recirculation. they can also, compared to bed filters handle very large flows at comparatively low capital cost. See also drumfilters, discfilter, conveyor filters

Bed filters use a volume of sand or other media through which the water percolates. The filters then reverse their flow and backwash. The backwash cycle can be either automatic or manually started. These types of filters are ideal for small hatcheries, but the water pressure required to force the water through the bed results in high operational costs and limits their use in larger systems. Sand filters in recirculation systems pose two challenges. Firstly they are energy expensive due to the pumping power required, and secondly the sand acts a a suitable media for biofiltration which results in the filters blocking more often due to the bacterial growth. There are sand medias available which have been specially treated to reduce the bacterial growth.  See sand filters


Term used to describe any substrate, which is used to either support living organisms (e.g. biofiltration / shellfish cultch), filter material from the water (see mechanical filtration) or perform a task such as degassing.


Gynogenotes produced through meiotic gynogenesis, e.g. through retention of the 2nd polar body. Meiogynotes are characterized with certain rates of heterozygousity due to recombination and crossing over during meiosis


A type of cell division which gives rise to four reproductive cells (gametes) each with half the chromosome number (i.e. two single as opposed to two pairs)  of the parent cell. see also haploid

Membrane diffusers

Diffusers manufactured out of a flexible plastic membrane. Often tubular in shape, although some designs are flat plates sealed onto a rigid body. In general membrane diffusers give a medium bubble size although some achieve a fine bubble. The advantage of membrane diffusers is that the pores close when no air is flowing through. This prevents clogging of the pores, as is found with some ceramic and other rigid diffusers. Membrane diffusers typically have a head loss of less than 0.1bar ((0.147psi) which is much less than rigid diffusers, which reduces operational costs.


Metal. Extremely toxic to fish. Maximum continuous safe level = 0.00005ppm, 0.002 maximum for short periods. 

Mesh Size

Mesh size is one of the terms used to describe and specify nets. Mesh size is Usually measured across the longest length of the mesh when it is stretched (i.e. from corner to corner). Selecting the correct mesh size is important for eliminating escapes, reducing damage to the stock, prevention of wild fish from entering the cage and minimisation of fouling.


Term, used to describe a body of water that is between Oligotrophic and Eutrophic. Typical of low lying areas where there is extensive or little agriculture.


The chemical processes that occur with every living organism. The chemicals at the end of the chemical process are referred to as metabolites.


A product of an animals metabolism. In general terms this is used to collectively describe the wastes (including organic matter, nitrogenous compounds such as ammonia and urea and carbon dioxide) produced by the fish.


See heavy metals, or the entry for the specific metal you are  interested in.


An organism with more than one cell. Many of the larger parasites are metazoans, such as Gyrodactylus. Opposite Protozoa which are single celled animals


Colourless, odourless gas (CH4). The simplest hydrocarbon and the main constituent of natural gas. Produced during the process of breakdown of organic matter. Can be converted to methanol by catalytic oxidation


(Methyl alcohol). A colourless liquid (CH3OH). Used in aquaculture for providing a carbon source for the denitrification process. Was once known as "wood alcohol" due to it's former manufacture through the dry distillation of wood.


Milligram, equal to 0.001 gram


Milligrams per litre. Used to refer to the concentration of a substance in the water. Equal to ppm (parts per million). For example 100mg/l means that there are 100 milligrams of a substance in every litre of water.


A unit of measurement equal to 0.001 millimetres ( 1/1000 mm)


The opening in an egg that allows the entrance of a single sperm to fertilise the egg.


Term used by some chemists term to describe concentration - equates to; ion charge x mmol. The main place it is used in relation to aquaculture is to describe alkalinity where 1 meq is equivalent to 50mg/l as CaCO3.

milligrams per litre

see mg/l


Common term used to describe the sperm


Millilitres per litre. Term used to describe a concentration of one liquid in another. Equivalent to parts per thousand or grams per litre.

Modulating (Valve)

An electrically (or hydraulically) operated valve that can vary it's position (eg closed, 10% open, 20% open etc.) in response to an analogue control signal. Modulating valves are useful for controlling processes such as oxygenation, dosing etc. but are considerably more expensive systems than solenoid valves which can only be open or closed in response to a digital signal. Modulating valves should be installed with thought as to how they will operate on failure of the control signal. Three options are usually that the valve will either remain in it's last position, open fully or close fully.


A syrup which is a by product of sugar refining. Used in some feed formulations as a binder, but mainly to keep the dust levels down to a minimum.


The relative molecular mass expressed in grams. For example carbon has a molecular mass of 12. So a 1 mol concentration of carbon would comprise 12g in 1kg or litre.


The smallest part of a chemical compound that can take part in a chemical reaction.


Phylum. Including the class Bivalvia, see molluscs


Common term for animals of the phylum Mollusca.  Includes groups such as the bivalves (mussels, oysters etc.), cephalopods (squid, octopus etc.) and gastropods (abalone, snails). Over 80,000 species in total with fossils back to the Cambrian period.

Moon phases

The different phases of the moon (i.e. from a new moon when it is completely in shadow, to a full moon, when it is completely visible) control the height of tides through gravitational pull, with the higher tides (spring tides) occurring during periods of new and full moon. The moon phases also impact on some species. Most notable is the impact on the migration of some eel species such as the European Eel (anguilla anguilla) which show a much stronger urge to migrate to sea when there is a new moon. Than when there is a full moon. Such urges are apparent even on heavily clouded nights, when a partial or full moon contribute a minimal amount of light. It is unknown, whether it is the darkness which affects the eels or whether it is related to gravitational pull.


Order of fish (part of the superorder Osteoglossifirmes) includes fish with electric organs


Term used to describe the activity of sperm. More active sperm are generally more likely to succeed in fertilising an egg.


One of the most commonly used fish anaesthetics. Chemical name tricaine methanesulphonate. Administered as a bath.

Mucous membrane

General term for a moist layer of cells and it's immediate underlying connective tissues lining cavities and tubes. Excreted by goblet cells.


Slimy excretion produced by specialised goblet cells. See also hyperplasia


Suborder of fish (part of the order Perciformes) including fish such as Mullet (Mugil)

Multi Stage

Term used to describe pumps which use two or more impellers to drive the water. Typically such pumps are used where higher pressures are required than can be achieved with a single impeller. Multistage pumps may be submersible or dry types and are used in aquaculture in applications such as supply from boreholes and wash pumps for mechanical screen filters.


Edible bivalve. Most commonly farmed variety is Mytilus edulis, the Blue Mussel. Easily farmed on ropes or other cultch. Seed stock (spat) generally from natural settlement. See link for developmental stages

Mycobacteriosis / Nocardiosis

Systemic Bacterial disease which affects a wide range of freshwater and marine species. In particular is a problem with aquarium fish. Some of these bacteria are capable of infecting humans. the bacteria can enter the skin through cuts and abrasions and can produce granulomas around some of the joints. Infected humans must be treated with antibiotics for a considerable time as the disease is very persistent. Fish in organically rich systems (such as activated sludge systems) have been linked with the disease and there have been cases of processors of such fish, contracting the disease. 


Order of fish (part of the superorder Protacanthopterygii) includes fish such as lantern fish (Myctophum) and Bombay duck (Harpadon)

Myxobacterial gill disease

See Bacterial Gill Disease

Myxosomas cerbralis

The causative agent (pathogen) of Whirling Disease