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

Most Norwegian salmon populations are characterized by small numbers o f fish. The proportion of cultured salmon in these populations has inc reased 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 Norw egian streams are largely dominated by escapees from fish farms, altho ugh some are released for stock enhancement and ocean ranching purpose s. Life history characters in cultured salmon-for instance, age and si ze at spawning, time of spawning and migratory behavior-often differ f rom those of the local stock of wild salmon. Possible ecological effec ts of interactions in the freshwater stages are discussed. The most se rious effects so far have been the introductions to wild populations o f lethal parasites and diseases with cultured fish. In recent years, m ore than 30 populations of salmon have been completely wiped out by th e monogean parasite Gyrodactylus salaris. High mortality of adult salm on 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 an d between rivers. Possible long-term effects of genetic interbreeding and erosion of local adaptations are discussed. Increased numbers of c ultured salmon may also increase the fishing intensity on wild salmon. Suggestions to reduce negative effects of interactions from cultured fish are proposed.