The purpose of this webpage and associated downloadable document is to examine the parasites which affect the raising of fish for human consumption in commercial aquaculture production systems, how they are monitored, and the current treatments available, as well as other management methods in order to encourage responsible use of parasiticides.
Fish have an anomalous status as far as the Veterinary Surgeons Act 1966 and the Medicines Regulations are concerned. Fish are not covered by the Veterinary Surgeons Act 1966 which means that non-veterinarians can legally diagnose medical conditions in fish, which is not the case in any other livestock. However, the Medicines Regulations stipulate that Veterinary Prescription Only Medicines may only be prescribed by veterinary surgeons and may only be supplied either directly by them or by pharmacists or Suitably Qualified Persons to a prescription issued by them.
The main species of food fish farmed in the UK are Atlantic salmon (Salmo salar) and Rainbow trout (Oncorhynchus mykiss), although other trout species such as brown trout (Salmo trutta) are produced along with some more exotic fish species such as tilapia, Arctic charr and sea bass and Halibut.The methods and systems employed to keep and farm fish will vary not only according to the species, but also even to the age or stage of development of the fish being reared.Atlantic salmon are hatched in freshwater, in purpose-built hatcheries, and the parr are reared in freshwater tanks on land or pens in lochs, until they undergo a major metabolic change which adapts them for life in seawater. This is called smoltification, and is genetically pre-programmed in fish in the wild to take account of the time for the parr to swim from the point of hatch, usually far up a small stream or burn, to the estuary and then to sea for the next stage of their life cycle. In farmed salmon, the time taken from hatch to smoltification is usually about 15 months, and the fish grow to about 80 grams in weight in this time. At this point they are transferred to sea pens, where their most rapid phase of growth takes place, reaching around 3 to 5 kg in weight at harvest after approximately 15 months at sea. The sea pens are large anchored suspended nets, which although providing the fish with a natural environment, at the same time carry the risk of contact with wild fish and aquatic pathogens in the surrounding environment – a challenge fairly unique to a number of methods of fish farming.
Rainbow trout are kept in freshwater (and seawater) in tanks and net pens, ponds or concrete raceways, with the water supply from various sources such as an adjacent river, spring or borehole. Although mechanical filtration is employed to clean the water on entry to the farm, again the challenge here can be the potential proximity of wild fish as well as other hosts such as wild birds.
Recirculation units are becoming more widespread – these may be used for a variety of situations including salmon hatcheries and the rearing of more exotic species such as halibut and even cod. Although they represent a significant investment to start-up, they have the advantage of keeping the fish in a protected environment especially as far as pathogen challenges including parasites are concerned.
Farmed fish can be affected by both ectoparasites and endoparasites.
The most important ectoparasites affecting farmed salmon and some farmed rainbow trout are from the parasitic crustacean group which includes sea lice which are divided into two main species, Lepeophtheirus salmonis and Caligus elongatus, as well as freshwater copepods Argulus foliaceous and Ergasilus sieboldi. Common endoparasites include Eubothrium sp. in farmed salmon and Proteocephalus sp. in farmed rainbow trout.
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