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TROUT

194. In the hatchery moribund or dead embryos and infertile eggs are quickly infected with fungus which can spread to healthy embryos if left unchecked. We understand that present control is by malachite green wash and removal of dead eggs. The latter is time consuming, possible only with a monolayer system, and can lead to the death of adjacent eggs by mechanical disturbance.

195. On leaving the hatchery, often with its bore hole water environment, for river water the fry become exposed to a wide range of pathogens and parasites. Fry anaemia is caused by the bacterium Cytophaga psychrophila which is not easy to control other than by strict attention to hygiene. Antibiotic treatments are occasionally effective. Protozoan infestations are frequently controlled at present by formaldehyde bath or formalin with malachite green (but see paragraph 217). Whirling disease which is caused by a sporozoan has no treatment but can be avoided by management measures. Non-specific gill irritation which is sometimes induced by high silt levels can be cleared by hyamine bath. Infectious pancreatic necrosis can be avoided by refraining from introducing fry in sites where it is present.

196. Adult fish are subject to a wide range of problems, some more serious than others. Endemic bacterial problems include enteric redmouth for which vaccines are available and bacterial kidney disease (which is notifiable) for which there is no effective treatment or vaccine. Cold-water diseases caused by C. psychrophila can also occur. We learned from trout farmers that parasite-induced proliferative kidney disease is a major economic problem on farms in the south of England and is currently controlled by use of malachite green, with or without stress reduction, and that external fungal infections can be controlled with malachite green. Eyefluke can be a problem and control by mollusc removal is the only remedy. Viral diseases include infectious pancreatic necrosis which causes few problems in larger fish, and two serious notifiable diseases which are important in other European countries: viral haemorrhagic septicaemia and infectious haemopoietic necrosis. Apart from these diseases, major losses still occur through accidental oxygen deprivation, hence the emphasis in this report on the prevention of such occurrences, and non-specific mortality of unknown aetiology.