The Free Online Aquaculture Dictionary



Suborder of fish (part of the order Perciformes) including fish such as wrasses (Labrus) and Parrotfish (Scarus)

Lactic Acid

An acid produced as a result of anaerobic respiration in muscles and red blood cells i.e. when glycogen is used as an energy source for respiration rather than oxygen. After production, it is converted back to glycogen in the liver. The build up of large amounts of lactic acid in the blood can lead to stress and toxic effects. High levels are usually a result of sustained, excessive exercise.


Area of the gills where exchange of gasses / waste products takes place. The lamellae consist of the Primary lamellae which are attached to the gill arch at one end, and are the main branches, with the smaller, Secondary lamellae radiating form the them. The secondary lamellae are loosely bound together by mucus.


The largest subclass of the Class Bivalvia. Contains the largest number of species (and almost all the edible forms including mussels, oysters etc.).


A pre-adult form. Generally free floating. Given to one or more of the stages of development prior to the adult form. Many species, particularly marine, have a larval stage, where the animals spend the start of their lives as part of the zooplankton community.


The time it takes for a toxin (or other harmful condition) to kill 50% of a fish population. The LD stands for "Lethal Dose" (LC50 - "lethal concentration" is sometimes used instead) Used as a guideline in the establishment of toxic concentrations etc. Other "LD" numbers are also sometimes used, the LD number referring to the percentage of the test population killed during the stated period. E.g. LD100 = 1 hour 


Low density polyethylene. Plastic, often used for items such as sheeting, bags 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 heat welded, but note that high density, medium density and low density polyethylene's will not weld to each other.


The water which permeates through, and out of any stored material. The main types of leachate which are of concern to aquaculturists are those which will directly impact on stock, if allowed to run, untreated, into the water course that supplies a farm. One of the most common leachate pollutants is that from silage or manure storage areas. These chemicals are typically extremely high in BOD and can cause fish kills by removing all the dissolved oxygen from a watercourse. Other forms of leachate which can cause problems include that from domestic and industrial waste tips and overflowing septic tanks. Many countries impose strict laws on the containment of leachates, however pollution incident still occur. These are most common in periods following heavy rain, where the leachate containment system may be overwhelmed by water and discharge direct to a watercourse.


Heavy metal which can be toxic to fish below levels of approx. 100 micrograms/litre. Usually only a problem in acid water areas. Not usually regarded as one of the main toxic heavy metals, as if it is present in such concentrations, other metals will usually have killed the fish first. May, like many other heavy metals, have the synergistic effect of increasing the uptake and toxicity of others esp. copper in the case of lead.


Causing death. E.g. a lethal concentration of a particular chemical will cause death in the animal.

Leuteinising hormone

A gonadotropin hormone, produced by the pituitary. In males it stimulates the production of testosterone. In females, it brings about ovulation. Often abbreviated LH

Leuteinising hormone releasing  hormone

See Lhrh


Abbreviation for Leuteinising hormone


Leutenising hormone releasing hormone.  Controls the release of gonadotropin from the pituitary gland (and hence the maturation process). Produced by the hypothalamus.  


"Leutenising hormone releasing  hormone analogue". Synthetic copy of Lhrh


The intensity and duration of light impacts on many aspects of aquaculture. See light intensity, daylength, smolting, photoperiodism

Light Intensity

Much debate surrounds the correct light intensity levels for different species. In general it is held that unless the water is very cloudy of coloured by drainage through peat (or similar) a normal, working level of light is sufficient for most species. Many texts quote "lux" levels. Lux meters can be purchased and give an indication of light intensity. The calculation of the rate of decrease of light intensity with depth is dependant on the turbidity and solids loadings of the water. In many intensive farms, the fish themselves prevent light penetration to any great depth. Although this might be thought to be a problem with sight feeder species such as salmonids it does not appear to be the case. Light coloured tanks can assist in the penetration of light into a tank by reflection from the sides and bottom of the tank. Although meters can be used to measure turbidity, the more common method is through use of a Secchi disc. See also Extinction coefficient.


Lightning can affect a fish farm in a number of ways. Directly, lightning strikes on farms will melt electrical circuits and cause machinery (such as pumps etc.) to break down. Direct hits onto water can also cause fish mortalities. Indirectly lightening strikes can interrupt power supplies, cutting them off altogether or causing surges which can damage sensitive equipment such as circuit boards, silicone chips etc. In areas where such equipment is used, an anti-surge device can be fitted to power cables to eliminate (or reduce the risk of this occurring.) See also Thunderstorms


Lime is a generic term used to encompass a variety of compounds all of which are alkaline. The compounds differ in their pH values and their buffering capacities, a summary is given in the table. Lime is used to increase the pH of water, disinfect ponds and pond bottoms and oxidise organic matter that has built up in the sediments. 


The process of addition of lime to a body of water or a pond.

Limiting nutrient

That which is most scarce in an environment, relative to the needs of a particular organism e.g. with phytoplankton blooms, the limiting factor is often phosphates, which is needed to continue the bloom. The phosphates in the environment are used up by the algae before any of the other substances that the phytoplankton need. The phosphate in this example is the Limiting nutrient.

Limiting Velocity

The maximum velocity of water possible without starting to erode a channel. The limiting velocity is dependant on the silt loading of the water. Waters carrying silts will increase abrasion and so increase erosion


Type of Spring see Spring


Pond liners are used for a number of reasons: to prevent water from leaking form the ponds, to prevent erosion of the pond and also to act as a barrier in areas of acid sulphate soils (or soils with other undesirable characteristics). The main factor in liner selection is lifespan. Many liner materials will decay with exposure to sunlight and so it is essential that they are not left exposed to sunlight for any length of time.


A large group of compounds which contains fats and oils. See saturated, polyunsaturated, EFA

Littoral zone

The area of bed and shore of a body of water where light penetration is sufficient for the establishment of rooted plants

Live Feed

The term live feed can be used to describe either naturally occurring animals (although this is usually referred to within the broad band of productivity) or animals which are produced (usually under artificial, controlled conditions) for feeding larval stages of farmed fish, crustaceans and bivalves. The types of live feed used include rotifers, artemia, algae and copepods. The use of live feed allows the correct nutrition to be given to farmed fish in their larval stages, where manufactured feeds are unable to guarantee this due to the microscopic size of the feed particle (see encapsulated feeds). many live feeds are enriched with special formulas (see EFA) to ensure they carry the correct nutrients to the fish that they are being fed to.

Logarithmic Scale

A logarithmic scale is one which increases in real terms by a factor of 10, for every increase in the scale of 1. For example a logarithmic scale of 1,2,3,4,5, in fact represents values of 1, 10, 100, 1000, 10000. Logarithmic scales are often used to simplify graphs and tables, where otherwise changes data at the lower end of the scale would be difficult to distinguish (e.g. a graph axis which would normally have values from 1 - 1,000,000 is shown by values of 1 - 7). An example of a logarithmic scale is the pH scale, where an increase of 1 represents a 10 fold rise in real terms, so a rise of 2.0 in pH (e.g. from 6.0 - 8.0) actually means a 100 fold increase.


Order of fish (part of the superorder Paracanthopterygii) includes fish such as Angler Fish (Lophius) with the first dorsal ray forming a lure.

Low Head Pump

Pumps which are specifically designed to deliver water to heads of between 0.5 and 8.0m. These differ from other pumps which are generally designed to supply smaller quantities of water at higher heads (eg centrifugal pumps, piston pumps or hydrams). The use of such pumps can often lead to higher pumping efficiencies and therefore lower electrical costs.

Luteinising hormone

See Leuteinising hormone

Luteinising hormone releasing  hormone

See Lhrh