DISEASE AND PARASITE IMPLICATIONS OF THE COEXISTENCE OF WILD AND CULTURED ATLANTIC SALMON POPULATIONS

The practical difficulties in measuring the prevalence, incidence, and pathogenicity of diseases in wild Atlantic salmon populations cause s erious problems in determining the possible implications of disease. L imited research has been undertaken on wild salmon disease associated with environmental effects of fish farming, or with the disease implic ations of possible changes to the genetic make-up of wild salmonid sto cks as a consequence of farmed fish escaping. To date, no significant disease problems have been reported linked to these aspects. The great est disease risk to both farmed and wild stocks is through the introdu ction of exotic pathogens into areas where local stocks have no innate resistance. National and international legislative controls are in ex istence to reduce this risk, but these have not afforded total protect ion. Serious epizootics of furunculosis and Gyrodactylus salaris in st ocks of salmon indicate the severe consequences of new disease outbrea ks linked to movements of live fish for farming or restocking purposes . A wide range of infectious agents has been recorded from wild salmon and some of these (and from other species of wild fish) provide the p rimary source of infection leading to disease problems in fish farms. Although disease control has markedly improved in fish farms in recent years, problems still remain with some diseases, notably sea lice. It is likely that the lice population and other diseases in farms contri bute infection to local wild stocks, but the extent and consequences o f this have not been quantified. (C) British Crown copyright 1997.