The Free Online Aquaculture Dictionary

Packed column

A vessel, open to the atmosphere, containing a media, over which water falls by gravity. The media is typically "random pack" type. Packed columns are used for degassing, dechlorination and are the basis of design for trickle biological filters. See purpose of column for more details.



Devices for aeration which use a motor to drive a partially submerged set of blades through the water to create splashing. Transfer of oxygen occurs through the interface between the water droplet and the air. As in all aeration devices, the lower the saturation level of the water the easier (and quicker), that oxygen is transferred into the water. Paddlewheel aerators have the added advantage over many other aeration devices in that is creation of flow is limited to the upper layers of water (to 1m approx. deep) and so is largely ineffective in destratification. Typical oxygen transfer rate is in the region of 1.2 - 2.4 kg oxygen per kW hour at standard oxygenation test conditions (20oC, 0mg/l oxygen) and 0.25 - 0.5kg oxygen per hour at 20oC, 6mg/l oxygen.


The attractiveness, in terms of taste, of a feed substance.


Device that uses high pressure to transfer substances (typically dyes for tagging or sometimes drugs) into the external tissues of an animal. Process does not use needles or involve any penetration of the skin, other than that of the substance entering it, this reduces the risk of infection. Dye marks fade over time and their life is dependant on the location and depth of penetration. 

Pantothenic acid

One of the constituents of the vitamin B complex. Can cause nutritional gill disease if deficient in diet.


Superorder of fish containing orders Percopsiformes, Gadiformes, Batrachoidiformes and Lophiformes. Mostly marine fish with stout bodies inhabiting deep waters or gullies and caves in shallow waters.


Precipitate chemical formed in formaldehyde. Very toxic to marine life.

Paralytic Shellfish Poisoning

Build up of toxins in filter feeding animals such as blue mussel (mytilus edulis) as a result of feeding on certain species of algae and bacteria (esp. cyanobacteria) which can lead to the poisoning of humans on consumption.


An animal that lives part of all of it's life at the expense of the health of another animal (the host)


A young salmon or trout living and feeding in freshwater, before it's migration to sea.

Parr marks

Thumbprint shaped markings along the flanks of some species, particularly salmonids, where they occur in the juvenile (parr) stage.

Parts per million

Measure of concentration. Equivalent to mg/l. Abbreviated as ppm.


A disease causing agent


Able to cause a disease condition.


Abbreviation for Polyethylene (plastic), usually abbreviated as HDPE (High Density Polyethylene), MDPE (Medium Density Polyethylene), and LDPE (Low Density Polyethylene)


Fibrous plant debris produced by the partial disintegration of plant material in wet areas. Impacts on aquaculture through the addition of humic acids to the water as it percolates through the peat. The addition of the acid to the water causes a drop in the pH level. Water from peat areas may also take on a brown colouration, which when extreme, may result in the inability of fish to feed or react to photoperiod stimulation.


The third and final free swimming stage of a mollusc, prior to settlement or attachment to a substrate. see also veliger, spat


Term used to describe a crustacean which is about to moult. Peeler crabs are valued as a fishing bait in some countries. This is due to pheromones which are released at the time of peeling, which fish detect and home in on. The peel crab or soft shell crab is easy prey for predators. See also Soft Shell


Group of marine / brackish water decapod crustaceans, commonly termed shrimps, includes the most important species of crustaceans to aquaculture Penaeus. monodon, P. japonicus, P. vannamei and P. stylirostris.

Percent Recirculation

An often confusing and misleading term which is calculated in a variety of methods. Many recirculation systems are sold claiming to be a particular percent recirculation. The main two ways in which the calculations are made are shown below, along with an example of how they can give very different results:

 Many people use % recirculation to describe the amount of new water, in relation to the tank or system volume that is brought in each day. E.g. If a system holds 100,000 litres of water, and each day 10,000 litres of water was brought in, the system would be said to be 90% recirculated Others think of % recirculation as the percentage of system flow that is recirculated. E.g. If a system had 1000 litres per minute going round it, and new water was added at 100 litres per minute, it would be said to be 90% recirculated. 

For example : A farm has a holding capacity of 200m3 and on a flow through basis discharges 7200m3 per day. The farm decides it would like a recirculation system. The first expression of percent recirculation indicates the percentage of farm volume that is replaced each day. A 90% recirculation system will require only 10% of the system volume per day as make up water = 20m3 rather than the flow through discharge of 7200m3 per day The second expression of percent recirculation expresses the percent of through flow water that is replaced by recirculated water. Hence a 90% recirculation system will require 720m3 of make up water per day, with the other 6480m3 being made up of recirculated water. The difference between using the two expressions here is considerable! There are many other ways which people could use to refer to the percent recirculation. As the percent recirculation figure takes little account of the amount of stock being held in the system for the water available, it is a bit of a "red herring" all round. It is important when asking about the percentage recirculation to enquire how it is calculated and also whether this includes the water which may need to be discharged from a system, when a tank is emptied. This can increase the water use considerably if there is not storage capacity within the system. It can also increase operational costs as new water brought into the system may need to be treated and heated/cooled.


Order of fish (part of the superorder Acanthopterygii). The largest and most diversified of all orders, includes 6880 species in 18 sub orders including Percoidei, Mugiloidei, Labroidei, Blenniodei, Gobiodei, Acanthuroidei, Scombroidei, Anabantoidei and Channoidei. Two dorsal fins, pelvic fins with one spine, ctenoid scales.


Suborder of fish (part of the order Perciformes) including over 4000 species of fish, such as Grouper (Epinephelus), Nile Perch (Lates), Yellowtail (Seriola) Sea Bream (Sparus), Tilapia (Oreochromis), Damselfish (Pomacantrus).


Order of fish (part of the superorder Paracanthopterygii) includes fish such as the Trout Perches of North America


Trade name for a zeolite, used for water softening processes.


Clear plastic material, trade name for polymethylmethacrylate. Used in some applications where a cheap, clear vessel or window is required. Easily scratched, cannot be welded. See also PET-G


Chemicals which are used to kill off unwanted animals. Many pesticides are used in aquaculture for the treatment of parasitic infections. Pesticides can be problematic when used in the water catchment areas of farm water supplies (including in some cases groundwater) The table gives the maximum permissible concentrations which are tolerated by fish, however these may vary with length of exposure, other stress present at the time and species.


Plastic material, clear. Has the advantage over Perspex that it can be welded with standard heat gun welding equipment


Pinpoint haemorrhaging. Shows as a series of small red dots across the tissue surface. Typical of systemic bacteria diseases such as "enteric red mouth".

Petri Dish

Shallow glass or plastic flat bottomed dish with a lid. Used primarily in laboratories for the culture of bacteria and other microorganisms on specially prepared media. Invented by J.R.Petri (1852 - 1921)


A logarithmic scale for expressing the acidity of a solution. pH is an abbreviation of "potential of hydrogen" and is in effect a measure of the amount of hydrogen ions, expressed on moles per cubic decimeter (10m x 10m x 10m, or 1000m3) The pH scale goes from 0 - 14. A value of 7 is neutral (i.e. neither acid or alkaline). Values below 7 indicate an acid solution, whilst values greater than 7 indicate an alkaline solution. For every increase in pH of 1.0, the alkalinity increases ten fold (and for every decrease of 1.0, the acidity increases ten fold. Therefore a solution at pH8.5 has ten times less acidity of a solution at pH7.5. Rapid changes in the pH of the water can stress fish (and also other organisms such as bacteria in biological filters).

pH Control

The control of pH in aquaculture is essential in a number of different type of systems. The most common use of pH control is to counteract the acidity produced by the process of nitrification in recirculation systems. Without pH control in such systems, the pH would rapidly fall to a level where it became toxic to the fish and also the bacteria in the biological filter. pH control is also used for some systems where the incoming water supply is such that if unchanged, the pH would harm the fish. Such systems are rarely continuous, but are a protection against extreme pH levels experienced during short periods (e.g. the fall in pH which can occur in areas of peat following heavy or prolonged rainfall.). pH control systems are usually to increase the pH, as the amount of raw materials required to achieve this (e.g. a carbonate of bicarbonate source) are relatively small compared to adding acid to the water. Due to the increasing, logarithmic nature of the pH scale, the addition of only a small amount of alkalinity will see a large rise in pH when the water is acidic to begin with. Conversely to reduce the pH of a very alkaline water, requires large amounts of acid, as very alkaline water has a very high alkalinity which must be countered by the acid. As a result it is not usually feasible to reduce the pH unless it is for very small quantities of water. One of the most effective and cost effective ways of overcoming a high pH is to recirculate the water through biological filters, which as part of the process of nitrification reduce the alkalinity and therefore the pH of the water. It is important in such systems however to ensure that there is little or no denitrification taking place (this will only occur in anaerobic areas or those of <2.5mg/l dissolved oxygen) as the process increases the alkalinity of the water. pH control systems can be either manually or automatically controlled. For flow through systems, where the pH may be subject to speedy changes, an automated control system consisting of a pH probe and controller. In such a system the controller may control the speed of a dosing pump or similar device. For recirculation systems, which are much more consistent in their pH, a manual control is often used and adjusted daily. Many recirculation systems will still have a pH monitor to give an alarm as the increase in pH in such systems, is indicative of a reduction in the amount of nitrification taking place in the biological filters.


A cell that is able to engulf and break down foreign particles, cell debris and disease producing micro-organisms in the body. Form an important part of the natural defence mechanism in most animals.

Phaffia rhodozyma

A basidiomycetous yeast, containing more than 0.1% (of it's dry weight) astaxanthin. The yeast is cultured as the one of the main sources of astaxanthin for fish feeds. At the end of fermentation, the yeast is is inactivated and conserved  by pH reduction. It is then dried, which results in the rupture of the yeast wall which has the effect of increasing the bioavailability of the astaxanthin.

Pharyngeal Teeth

Bony plates at the back of the mouth used for grinding. Found in many species, particularly herbivores and also some omnivores, but especially those of the sub-order Cyprinoidei. Pharyngeal

Phase Contrast

Type of microscope that gives a greater contrast, especially when viewing largely transparent samples.

Phases of moon

see moon phases


The physical appearance and/or characteristics of an organism. Determined by it's genes see also genotype.


Amino Acid


A chemical substance emitted by an organism as a specific signal to another organism, usually of the same species. these substances play an important part in the spawning of many species, used to attract mates and signal to others that they are spawning. The substances are usually volatile organic acids or alcohols and are effective at minute concentrations.

Phospholipid cell membranes

A layer of unsaturated lipid which surrounds body cells in fish. Enables the transfer of nutrients across the cell wall. In coldwater species, more unsaturated fatty acids must be fed to ensure the lipid in the layer remains fluid. See EFA


Non metallic element. An essential element for living organisms, found in the tissues (especially bones and teeth) and cells. Present in fish feeds as a result of the inclusion of fish and other animal meals. Excreted by fish as a result of more phosphorous being available in the feed than the fish requires. In general 5-15g of phosphate are excreted per kg of dry diets fed to fish. Phosphorous is often the limiting nutrient in relation to algal blooms and plant growth, an excessive amount released into the environment from a fish farm can therefore increase the plant growth in a water course. It is because of this that the release of phosphorous (or phosphates which are salts of phosphorous) from farms are usually one of the items regulated by the environment agencies.


The response of an organism to changes in the daylength. The manipulation of the daylength can lead to changes in spawning times, migration times and the extension of the daily feeding cycle, leading to faster growth rates. 


The chemical process by which green plants make organic compounds from a combination of carbon dioxide and water in the presence of sunlight. The process of absorption of carbon dioxide by the plants during daylight hours (which results in the release of oxygen) is reversed during the hours of darkness, when the plants absorb oxygen and release carbon dioxide. This can lead to a reduction in the dissolved oxygen concentration of waters containing plant matter (algae or higher plant types) which culminates in a minimum concentration, which is usually experienced just before dawn. Even in comparatively clean waters, the effect of algae can reduce the dissolved oxygen concentrations to less than 80% saturated at dawn. It is important for all farmers to check, at varying times of the year, and during varying weather conditions, the extent of the fall of dissolved oxygen concentrations at dawn. This will then assist in predicting low oxygen levels and being able to take preventative action such as feeding early in the day so that the maximum oxygen demand by the fish occurs in the afternoon and evening, when there is plenty of oxygen available. An addition precaution is to install aeration or oxygenation devices on a simple timer so that they start when the oxygen is likely to be at it's lowest. The use of oxygen controllers is preferable, but the cost of such items often prohibits their use. In such circumstances, timers are an effective solution.


Taxonomic category, often restricted to the animal kingdom. Corresponds to the category "division" in botany


Small highly diverse organisms relying on currents to keep them afloat. Primarily algae such as bacillariophyta and dinoflagellates. Reliant on sunlight, a carbon source and a supply of nutrients to photosynthesise. Due to the reliance on sunlight, phytoplankton are only found in the surface waters where sufficient light penetrates.


Symbol P, denotes a constant value of 3.141592…. Used in the calculation of radius, circumference and other calculations relating to circles and spheres.

Piggy Eyes

Term used to describe a deformity where the eyes are very small and underdeveloped. Has been attributed to low dissolved oxygen levels and/or high concentrations of toxic waste compounds (such as ammonia) in egg silos, where channeling flow results in areas of the silo with poor water exchange.


A compound that gives colour to body tissues. Naturally found in tissues it is of particular importance in providing colour to the fish for camouflage. Used in some fish diets to enhance the colour of the flesh to make a product that is more readily accepted by markets. Chemicals such as astaxanthin and canthaxanthin (collectively known as carotenoids) are used, for example, to give farmed salmon and trout a pink colouration to their flesh, which would other wise be white. The pink colouration mimics the colour found in wild fish, which is obtained by the consumption shrimps, which naturally contain the pigment. Chlorophyll, which gives algae it's green colour is also classed as a pigment. The addition of pigments to salmonid feeds  composes only 0.005 - 0.01 of the feed volume, but can comprise as much as 15% of the price of the feed. Pigments are also very important in the development of eggs and larvae of many species.


Chemical used to inhibit the effects of dopamine and assist in the induction of spawning.


Pipes vary in their material of construction and also their ability to withstand pressure, which is usually governed by the wall thickness, the diameter of the pipe and the material it is made from. The pressure that a pipe of a certain wall thickness is able to withstand increases with a decrease in diameter of the pipe. For example a 20mm diameter pipe with a wall thickness of 3mm will withstand approximately twice the pressure of a 50mm pipe with the same wall thickness. It should be remembered that a pipework system is only as secure as the weakest part and so the pressure ratings of pipes used in different parts of systems often varies. The external diameter of a pipe is constant no matter what the pressure rating. This enables pipes and fittings to be universal to that size e.g. a Class B pipe will fit into a Class E socket as the external dimension of the pipe is the same, all that has to be checked are that the pipes are of the same material, or if they are not that the method of joining (e.g. solvent welding, heat welding etc. ) is suitable for both pipes. Almost the only different pipe materials that are compatible for solvent welding together are UPVC and ABS. Pipes generally fall into three categories, drainage (i.e. those that are perforated), sewage i.e. operating at low pressures, often with fittings with rubber seals etc. and pressure pipe. There are other more specialist types such as double walled, but these rarely have applications in aquaculture. Sewage pipe is often overlooked as a cheaper option to pressure pipe in many applications, and if the security of solvent welded joints are required, pressure pipe solvent joints can be used for sewage pipe. The ratings for pressure pipe generally start at a pressure of about 5 bar, the equivalent of 50m head of water. Such pressures are rarely encountered in aquaculture and so pipes of higher pressure rating and higher cost are usually a waste of money, see also the entry of particular materials.

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Pipe pressure ratings

Pipes are usually supplied with a pressure rating. This is the maximum normal operating pressure that the pipe can safely withstand. 

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An all encompassing term often used to group pipes and all associated fittings

PVC and ABS Pressure Pipe


Chemical that kills fish. many piscicides are also toxic to other vertebrate and invertebrate life .so care should be taken. See also insecticide, saponin, tea seed cake

Piston pump

See dosing pump

PIT tag

Device containing a microchip that is injected beneath the skin, containing information about the individual fish. The chip can be read externally by a special electronic reader ( much like a bar-code reader), which means that the information can be  gained from a fish at several stages through it's life, as it does not need to be killed. "PIT" stands for Passive Integrated Transponder

Pituitary Gland

A small gland attached to the hypothalamus by a thin stalk. Divided into two sections, the gland is collectively responsible for the control of many hormones (such as gonadotropin). It is often used, in ground form, as an injection to stimulate the spawning process in some species, especially where fish are spawned out of their normal season. Pituitary glands are not necessarily species specific, for example those from salmonids have been used for the artificial spawning of cyprinids. Dried pituitary extract is commercially available and is usually dissolved in solutions such as acetone, before injection onto the fish. The quality of dried pituitary may vary between suppliers and also the time of year it is harvested. The amount of gonadotropin hormone (GTH) that is contained may vary from 300 - 1400µg (1µg = 0.001mg) per gland. A table showing the best times to collect is given. It is for this reason that CPE (Carp pituitary extract), which has a known concentration is more successful than dried whole pituitary gland.


see Proliferative Kidney Disease


The collective term for all organisms (plant, bacteria and animal) which drift passively, or are unable to move against prevailing currents. The plankton includes four groups primary producers (e.g. the phytoplankton), primary consumers (i.e. those which consume the primary producers), secondary consumers (i.e. those which consume the primary consumers and sometimes also the primary producers) and the decomposers (those which use dead tissue of other organisms for their nutrients). See also zooplankton, nekton.


See phytoplankton, algae, emergent plants, floating leaved plants, macrophytes

Plastic Welding

The fusion of two pieces (usually of the same plastic type) with a directed hot air jet and plastic welding rod. Used increasingly for the manufacture of tanks, floats and other items. Useful for repair of pipework (either temporary or permanent). Welding temperatures for different plastics are given below. It is easier to obtain a neat finish with some plastics rather than others. If polyethylene and polycarbonate are over heated they become like melted wax, and then reform in almost their original colour when they cool. However, if UPVC or ABS are overheated they burn, leaving a permanent scorch mark. 


Material that can be shaped by the application of heat or pressure. Most are based on synthetic resins although some are the product of natural substances (such as cellulose derivatives). Plastics fall into one of two categories, thermoplastic or thermosetting. Most of the plastics used in the aquaculture industry are of the thermoplastic type. Plastics are in wide use in the aquaculture industry, largely as a result if the relatively low production costs associated with them. The diversity of plastics offers a range of tolerances to different chemicals, and their use is often more suitable than metals. The table gives a method for recognising different types of plastic when a small shaving of the plastic is held with tweezers and lit with a match.


Precious metal…..if your water has a high concentration of this in it, concentrate on getting it out of the water and selling it…it will be a lot more profitable than growing fish!


Order of fish (part of the superorder Acanthopterygii) includes flatfishes such as Plaice (Pleuronectes), Sole (Solea), Turbot (Scophthalmus) and Halibut (Hippoglossus)


Cold blooded - Temperature of the animal mirrors that of the environment, with the animals temperature remaining approximately 0.2-0.5oC above that of the environment. Even though fish are cold blooded, they still produce heat. This heat is lost into the environment around them, see heat gain for more information.


An undesirable change in the physical, chemical or biological characteristics of the environment.


Plastic. No smoke, drips like a candle and smells of wax. Plastic with high chemical resistance. Joined by butt welding, fusion welding or plastic welding. High resistance to chemicals but relatively low resistance to abrasion. Often used for potable water supplies. Sheet polyethylene is used extensively in the production of tanks and other structures. Flexibility is higher than many other plastics. Welding is easy compared to materials such as UPVC where overheated plastic burns and leaves an untidy finish. Polyethylene returns to it's original colour if overheated, although the weld may not be as strong. Main use of polyethylene is in the moulding of tanks etc. The low cost of the material and the speed of tank production, in comparison to fibreglass, makes the tanks much cheaper. There are however limitations of such processes and the production of tanks of >3m diameter is very difficult. Rotationally moulded polyethylene is usually carried out with medium density polyethylene, whereas sheets purchased from plastic stock holders are usually of high density. The different densities of polyethylene are often incompatible for welding..


Plastic. No smoke when lit, pulls to form a thread, smells of burnt horn.


Plastic. Yellowish sooty smoke when lit, sweetish smell
ABS Blackish smoke, soot flakes, sweetish smell. Used very infrequently in aquaculture.


This involves the treatment of fertilised eggs, very early in development, which causes the genetic make up of the fish to be altered. It's main application is in the production of triploid fish (where a temperature or pressure shock is often used as the treatment) the term "Polyploidy" means more than the haploid number of sets of  chromosomes. The haploid number is represented as 'n', so Polyploidy refers to any fish with more than one set eg: Diploid (2 sets and the normal natural number), Triploid (3 sets), Tetraploid (4 sets)  


Plastic. No smoke when exposed to a flame, drips like a candle and smells of burnt oil. Light weight with high impact strength. Joined by butt welding, fusion welding or plastic welding. Suitable for working temperatures between 0 and 90oC, suitable for handling foodstuffs. High purity polypropylene is available and is sometimes used in marine hatcheries (particularly for shellfish) where water purity can be critical to correct larval development. Thermal expansion is higher than most other plastic materials. The requirement for heat fusion welding usually prevents polyethylene pipe form being routinely used, as solvent weld plastic such as UPVC and ABS are easier and quicker to install. Polyethylene sheeting is sometimes used for the construction of cheap tanks and vessels such as biofilters.


Structures made from a steel frame, covered with a single or multiple layer of plastic sheeting. Used extensively for cheap housing of tanks and ponds, especially where heated or chilled water is used, or the control of light (photoperiod) is required. Insulation layers (such as rock wool, bubble wrap or air blown between two sheets) may be used. Typical life span of polytunnel is 3-5 years before resheeting is required. The wind loadings that polytunnel require vary with the design, but 90 -100 mph wind speed is usually the maximum quoted. Polytunnels can become poor working environments due to spilled food very quickly becoming rancid due to high humidity levels. Good ventilation systems to control humidity are often installed to alleviate this.


A fatty acid with more than one double bond (see EFA). As fish have a high percentage of polyunsaturated fats, fish meal is generally used a source for diets. They are more expensive than saturated fats, and are usually used to make up a small but essential percentage of the fishes total fat diet. Used for phospholipid cell membranes

Polyvinyl chloride (PVC)

Plastic which may be in either a plasticised (PVC) or unplasticised (UPVC or PVCU), the latter being rigid and the more widely used in aquaculture. Plasticised PVC is used in the production of some tarpaulins etc. where flexibility is an essential part of the function of the end product. Some PVC compounds may leach chemicals into the water. The quality of PVC and UPVC should always be checked with the supplier before use in recirculation systems, where the build up of leached chemicals may become a problem with some species. Blackish smoke and acrid smell when lit. Has a tendency to burn and leave scorch marks when overheated during heat welding. Very good for solvent welding which makes it the plastic of choice for most piping systems. UPVC can be directly solvent welded to ABS. See also Chlorinate Polyvinylchlorid

 PVC pipe and fittings

Polyvinyliden Fluoride

Plastic for ultra high purity systems, e.g. used to carry deionised water in the electronics industry. Rarely used in aquaculture except possibly in some laboratory conditions due to high cost.

Positive Displacement Pumps

Pumps that deliver a specific volume of water for each cycle, irrespective of the head that they are pumping against. Such pumps include piston, plunger and screw pumps. The most common use of these pumps in aquaculture is as dosing pumps, where a fixed amount of chemical is required to be added to a system, regardless of the inlet or outlet head. If positive displacement pumps are allowed to pump against a head greater than that which they can deliver or if valves are shut off whilst in operation, damage can be done to the pump.

Potable water

Water that is suitable for drinking by humans

Potassium Permanganate

Compound forming purple crystals which when dissolved in water give an intense purple colour to the water. Sometimes used for the detection of leaks through ponds etc. where the purple colour can be an indication of where water seepage is coming from. Also sometimes used as a disinfectant at a rate of 24mg/l or treatment of external bacterial infections such as columnaris in pond fish (at a rate of 2 - 4 ppm).

Power Station Effluents

See heated effluents


See polypropylene


Parts per million. Used to refer to the concentration of a substance in the water. Equal to mg/l (milligrams per litre). For example 100ppm means that there are 100 parts of a substance for every 1,000,000 parts of water.


A decapod crustacean living in freshwater. Some prawn species such as the Blue claw prawn Macrobrachium rosenbergii are commercially significant for aquaculture. The grouping of prawns as freshwater decapods and shrimps as marine decapods is open to debate. Although this is the general division, some countries such as the UK refer to shrimps as decapods <50mm and prawns >50mm.This can lead to marketing confusion between the terms prawn and shrimp, with prawns being regarded as large decapods and shrimps as small decapods.


A suspension of small solid particles, produced in a liquid by a chemical reaction.


Advanced development, the onset of maturity before it is normally due. e.g. "precocious parr" refers to salmonids (usually male) which remain on the  spawning grounds where they were hatched and reach sexual maturity at a size of only approx. 100mm after a year, rather than the true adult fish which are returning to the area after one or more years absence at a size of several kilograms.



Force acting on an area. Expressed in a variety of ways for which a conversion table is given. Pressure is important in aquaculture from the design of systems (to enable sizing of pumps and prediction of flows) to applications such as the induction of triploidy through subjecting eggs to high pressure treatments. Many diffusers have optimum pressure ratings, where they are most efficient at delivering the required gas into the water. see also friction loss water, friction loss air

Pressure Swing Adsorption

The process of separating oxygen and nitrogen gas. The process uses carbon molecular sieves, which are substances with precisely sized passages. These sieves allow the smaller nitrogen molecules to pass through, whilst adsorbing the oxygen which has molecules to large to pass through. When the sieve becomes saturated  with oxygen molecules, they are dislodged by a sudden jump in pressure (hence the name of the process) and the process can start again. Systems usually use two or more beds to smooth out the gas production. the process is widely used for small commercial oxygen generation systems. Purity of oxygen from the process varies between 75 and 95% according to manufacturer and operating level (typically the more oxygen a system produces, the lower the purity)  See also cryogenic fractionation

Primary consumer

Animal which feeds on the primary producers (the phytoplankton)

Primary Lamellae

See Lamellae

Primary producer

Organism which uses nutrients and sunlight to exist and does not prey on other organisms as a food source. Typically phytoplankton.

Primary production

The amount of primary producers per given volume or area of water.


The use of systems with large numbers of bacteria and other organisms, which prevents the ability for any one organism to become too dominant. Used in some systems to restrict the multiplication rates of pathogens. Largely unproven. See also Green Water Systems


In aquaculture - The capacity of a pond to produce cultivated fish. Measured in weight per area per annum. In general terms, it refers to the total weight of all organisms rather than just the cultivated fish.

Profundal Zone

Area of the bed of a body of water, where there is insufficient light penetration for rooted plants to become established.


One of a group of naturally occurring or synthetically produced hormones that ensure that the normal course of pregnancy / gonad development is followed.

Proliferative Kidney Disease

Disease of fish (especially trout) caused by small protozoan parasites. Of commercial importance for trout farms in many parts of Europe and North America. Affected fish are generally anaemic, with exophthalmia, ascites, haemorrhaged liver and greyish areas in the kidney. Trout will get an apparent immunity after the first infection.

Propeller Pump

Devices that use a propeller as the driving force to pump water, rather than an impeller. Particularly suited to applications where a high flow is required at a low head (typically < 8m). At very low heads, i.e. < 3m, propeller flow pumps can be considerably more energy efficient than other types of pump. Used primarily for irrigation, propeller pumps are now finding favour with recirculation systems, where large volumes of water are pumped at low heads. The pumps may be single or multistage (i.e. with one or more than one impeller).


Superorder of fish including the orders Salmoniformes and Myctophiformes. Predominantly slender, predatory fishes. Most have adipose fin without rays. Many are anadromous


A single celled animal


Extendable. Often used to refer to the ability to extend mouth to forage for, or grasp food


See Paralytic Shellfish Poisoning


Polyunsaturated fatty acid see EFA

Pump Ashore

The term given to land based, marine farms, where the seawater is pumped onto the land, and then discharged back top the ocean after use. The main drawbacks associated with pump ashore systems are the cost of pumping the water, the high capital cost when compared to cage farming techniques and the fouling of pipes by marine organisms. For some species, fish reared in pump ashore systems can demand a higher price, this is due to the ability to guarantee harvest in rough conditions, when cages are too dangerous to work on, and also, if circular tanks are used, the growing of fish in a current can improve the flesh quality by making it firmer. 


see Propeller pumps, centrifugal pumps, positive displacement pump


See polyvinylchloride

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Pvc-C (CPVC)

See Chlorinate Polyvinyl Chloride


See polyvinyl chloride


see Polyvinyliden Fluoride