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 serious problems in determining the possible implications of disease. Limited research has been undertaken on wild salmon disease associated with environmental effects of fish farming, or with the disease implications of possible changes to the genetic make-up of wild salmonid stocks as a consequence of farmed fish escaping. To date, no significant disease problems have been reported linked to these aspects. The greatest disease risk to both farmed and wild stocks is through the introduction of exotic pathogens into areas where local stocks have no innate resistance. National and international legislative controls are in existence to reduce this risk, but these have not afforded total protection. Serious epizootics of furunculosis and Gyrodactylus salaris in stocks of salmon indicate the severe consequences of new disease outbreaks 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 primary 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 contribute infection to local wild stocks, but the extent and consequences of this have not been quantified.