The Free Online Aquaculture Dictionary

W

Water leaving a processing plant or production facility.

See flow rate

The process by which an egg absorbs water and stores it into the perivitteline space. Occurs soon after fertilisation. Eggs are often left in a bucket of water to harden and absorb water before handling. Once eggs are water hardened, they can be moved and handled safely for a short period.

General terms for all the physical, chemical and biological; factors which influence the use of the water.

Level below which the ground is saturated with water. May vary with rainfall and pumping of boreholes. The pumping of a borehole will draw down the water table around it which may affect other boreholes in the same area. Many authorities require a test pumping period, during which other boreholes are monitored, prior to issuing a permanent pumping license.

The area of land draining into a river or water body

Usually measured from peak to peak or trough to trough

The process of changing from one diet to another. Typically applied to fish which have been fed a live diet (such as artemia) and are then "weaned" onto a commercial dry diet. The weaning process is often one of the most critical in the development of a farm fish. Carried out incorrectly, poor weaning can lead to starved fish or fouled water (caused by too much uneaten food). The key aspects of successful weaning are feed distribution and constant availability, together with a set program of gradual change from one diet to another over a period of about 10-20 days.

Weather can have an important bearing on aquaculture - Sunny weather can lead to algal blooms which can cause oxygen depletion in the early morning, a large pH shift throughout the day caused by the carbon dioxide produced as a product of respiration (including photosynthesis), and also the occurrence of some potentially toxic chemicals to be released into the water (see cyanobacteria). Heavy rain can cause flooding of tanks/ponds leading to the escape of fish, can increase suspended solid loadings to a point where the gills can be damaged and become hyperplasic, and can also lead to the addition of toxic chemicals/organic matter into a water course through leaching or run off. High winds van lead to damage of farm systems including telephone wires that carry alarm signals etc., winds can also cause problems through leaves and other matter being blown into the water course and blocking screens etc., Extreme cold can cause the phenomenon known as anchor ice and snow melt from hills can block screens and pipes. Cold winters can impact on the length of the growing season, and high temperatures reduce the amount of dissolved oxygen available in the water for the fish. 

A dam type of structure installed to provide additional head from a river supply to a farm, or for measurement of the flow of the river. Measurement weirs are either flat crested or V shaped. V notch weirs are often the option chosen as they give good accuracy at both low and high flows, whereas flat topped weirs can be inaccurate at low flows. 

see Plastic welding

See nutritional gill disease

A feedstuff that is (or is mostly) in it's raw state. Such diets include trash fish.

The sum of the length of the sides and bottom of a channel, which are covered by water. The value is used in the calculation of friction loss and flow calculations for channels.

A parasitic disease of trout caused by the protozoan Myxosoma cerebralis. The parasite enters spine of the fish at a stage before the cartilage has turned to bone. Causes bent spines which force the fish to swim in the characteristic "whirling" motion. The spores of the parasites can remain in the muds of ponds etc. for a long time. Prevention is by maintaining the young fish on ground water or un contaminated supplies in concrete or plastic tanks until ossification has occurred. The addition of lime (e.g. Calcium oxide) to the muds of ponds when dried at a rate of 400g/m2 will kill the spores through increasing the pH of the mud.

The muscle that forms most of the "flesh" of the fish. Used only occasionally during the seizing of prey, the evasion of predators and threats and during activities such as leaping waterfalls (e.g. in the case of some migratory fish). The use of the white muscle involves the metabolism of large amounts of energy, which is supplied by glycogen. The metabolism of glycogen causes a build up of lactic acid in the bloodstream, and if the fish has to use it's white muscle too much (such as when it is being caught on the end of a line and struggling for a long time), the lactic acid can build up to toxic levels, and may eventually kill the fish.

Disease caused by the parasite Ichthyopthirius multifilus. The mature parasite on the fish is typically 0.5mm in size and encysts on the skin and gills. This results in the appearance of white spots, each of which is an individual parasite.  The disease is commonly treated with a mixture of 3.68g malachite green per litre of formalin, administered at a rate of 25 ppm. A single treatment will rarely cure the problem however, as the chemicals only encourage the parasite to leave the fish and does not kill those multiplying on the bottom of the tanks / pond. If possible water flow rates should be increased and the bottom regularly brushed to rid the parasites. If increasing the flow rate is not possible, lowering the tank can increase the tank turn over time and make it more self cleaning. Often two or three treatments over a period of 10-14 days are necessary to rid the disease. Usually occurs in Spring as temperatures warm up. The parasites cause irritation to the fish and they can often be seen "flashing" whilst rubbing themselves on the tank sides and bottom.

A once common method used to determine the dissolved oxygen concentration by titration. Now rarely used due to the accuracy and low price of oxygen meters.

The death of animals in a water body as a result of oxygen depletion brought about by heavy ice cover preventing oxygen transfer across the water surface, and photosynthesis. The breakdown of organic matter on the bed of the pond and in the water column also uses up the oxygen. Fish may either die as a result of no oxygen or from chemicals such as hydrogen sulphide (which cannot escape the water due to the ice), brought about by the anaerobic decomposition of wastes.

Electrical wiring on fish farms should always be waterproof. Although low voltage wires for some feeders and alarm systems will operate when damp or wet, with little or no danger to operatives, corrosion can soon make the wiring unreliable. 

The statutory time that fish must wait before harvest after being fed or administered chemicals, particularly antibiotics (but can also refer to herbicides and other chemicals used) Often expressed in degree days