INTERACTIONS BETWEEN WILD AND CULTURED ATLANTIC SALMON - A REVIEW OF THE NORWEGIAN EXPERIENCE

Most Norwegian salmon populations are characterized by small numbers of fish. The proportion of cultured salmon in these populations has increased together with the rapid growth of the Norwegian salmon farming industry. In several spawning populations, fish of cultured origin now exceed the number of wild fish. The cultured salmon occurring in Norwegian streams are largely dominated by escapees from fish farms, although some are released for stock enhancement and ocean ranching purposes. Life history characters in cultured salmon-for instance, age and size at spawning, time of spawning and migratory behavior-often differ from those of the local stock of wild salmon. Possible ecological effects of interactions in the freshwater stages are discussed. The most serious effects so far have been the introductions to wild populations of lethal parasites and diseases with cultured fish. In recent years, more than 30 populations of salmon have been completely wiped out by the monogean parasite Gyrodactylus salaris. High mortality of adult salmon has also been observed as a result of furunculosis in some streams. In biochemically detectable loci, small but statistically significant differences in allele frequencies exist between populations within and between rivers. Possible long-term effects of genetic interbreeding and erosion of local adaptations are discussed. Increased numbers of cultured salmon may also increase the fishing intensity on wild salmon. Suggestions to reduce negative effects of interactions from cultured fish are proposed.