The Free Online Aquaculture Dictionary

V

Vaccination

The process of administering vaccines. The effectiveness of vaccination is dependant on a number of factors the first of which is the method of vaccination, injection is the most effective method which gives the best results both in terms of protection level and speed of protection; immersion or spraying the fish gives a poorer protection level and can take up to two months for protection to be realized; oral administration is largely ineffective and can take a long time for protection to develop, if at all. Other factors influencing the efficiency of the process are the size of the fish (fish of below 5 grams are unlikely to develop a high rate of protection due to a poorly developed immune system), the temperature at the time of vaccination and the temperature in the following weeks (at low temperatures the metabolic rate of the fish is reduced and so developing protection can take longer) and the presence of any other diseases or stresses which would divert the energies of the fish away from producing the antibodies relevant to the vaccine. Vaccination will in general last up to a year. Automatic machines and semi-automatic machines are available for injection which speed the process up.

Vaccines

A preparation of non-virulent disease organisms (which may be alive or dead) that induces the defence system of the animal to produce antibodies against them. Once produced these antibodies can be called on at any time to deal with virulent disease organisms should they challenge the animal. Some vaccines are very strain specific, and may work in one area for a specific disease but not be as effective in another area. 

Vector

An organism that is capable of carrying and transmitting pathogens from one host to another. Examples include ticks, which may affect more than one animal and in the process of feeding on the first host, take in viruses or bacteria. When feeding on the next host, these pathogens can transfer into the bloodstream.

Veliger

The second free swimming stage of molluscs prior to settlement or attachment to a substrate. See also pediveliger

Velocity

A function of distance traveled per a given amount of time (e.g.: metres per second, miles per hour, kilometers per hour). The water velocity of tanks has a bearing on the swimming speed of the fish and also the self cleaning ability of the tank. It also has implication in the distribution of feed throughout a tank, especially if there is a single, automated feeding point. The ideal velocity for water in a tank could be said to be that which achieves a high degree of self cleaning, whilst not unduly stressing the fish by causing them to burn up energy (which could otherwise be used for growth) as they try to maintain themselves in a fixed position in the water column. It has been shown that species which have a natural habitat of flowing rivers and streams, use less energy sitting in a steady flow of water, whilst waiting for the food to come to them, than they do in still water, where they must exert energy to keep on the move and find food.

Ventral

Relating to the lower abdomen area. The underside of the body

Ventral Fin

The single (rather than paired like the pelvic fins) fin located on the underside of the fish, behind the anus. Sometimes called the anal fin.

Venturi

  

A device which draws one fluid media (e.g. gas or liquid) into another, using the velocity of the second fluid media (caused by a pressure difference) as the energy source. See diagram. As the water passes through a restriction in a pipe, it forms a vacuum at the end of the restriction. A hole bored into the pipe at the point where this vacuum occurs will cause air (or any other gas or liquid) to be drawn into the main flow. The amount of gas/liquid drawn in is a direct function of the difference in pressure between the venturi intake and the outlet, and the bore of the restriction between them. Venturis are efficient methods of mixing chemicals and gasses into water, and in aquaculture are sometimes used for aeration, oxygenation and ozoneation. The operational costs of venturis is high due to the cost of the increased pumping pressure required for the unit to operate. Venturis have their applications in some systems where there is more pressure available, than is required by the rest of the system components.

Vertebrate

 Animals of the subphylum "vertebrata" (of the phylum "chordata"). The vertebrates differ from non-vertebrate (invertebrate) chordates in that they possess skulls and backbones which may be of cartilage or bone.

Entry edited by Henrik Kreiberg

Vertical transmission

The transfer of a disease agent from a parent fish to one of it's progeny through reproduction see also Horizontal transmission.

Vibrio

A family of bacteria, many of which par pathogenic e.g. Vibrio anguillarum  (the causative agent of the systemic disease Vibriosis) or V.Viscosus (the causative agent of Winter Ulcer disease). Occurs mainly in marine fish, but infections have been established in some freshwater sites, especially where fish are fed a diet including trash fish (see also cod roe). Most marine species are susceptible including anadromous species such as Atlantic Salmon and European eels. Infections of Vibriosis are more likely to occur in Spring, especially following the transfer of Atlantic Salmon to sea or other stressful periods.

Virulent

An organism capable of causing a disease condition

Virus

An infective agent many times smaller than bacteria that are capable of infecting and multiplying in cells. Viruses are the most feared type of disease as they are often untreatable. Very few vaccines are available for viral diseases of fish. For many farms, the only remedy for some viral infections is complete slaughter of stock and disinfection of the farm. This is often enforced by government agencies, to protect other farms in the locality.

Entry edited by Henrik Kreiberg

Viscera

The internal organs. Often used to describe processing wastes.

Viviparous

Fish which gives birth to live young which have already reached an advanced state of development e.g. guppy (Poecilia reticulata)

Von Bayer Trough

A V-shaped trough 30cm long, which is used to count and size eggs. 

Vortex

A whirling mass of water, where solids particles are drawn into the centre.

Vulcanising

Process of bonding rubber compounds through heat. See also heat welding

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