One-generation effects of farmed fish immigration on the genetic differentiation of wild Atlantic salmon in Norway

Abstract

One-generation effects of various amounts of farmed fish immigration on the genetic differentiation among wild salmon stocks in Norway were examined. Known allel frequencies at three electrophoretic loci and population sizes of five Norwegian salmon stocks and one farmed fish strain were input in formulae for gene flow, and the results measured as reduction in G-st at electrophoretic loci.In general, a raised immigration rate from a common source will reduce the existing genetic differentiation between stocks. In absolute terms, the reduction in wild stock G-st will be largest in the first generation, because the genetic differences between immigrant and recipient are largest then. Both the magnitude and pattern of immigration effects are substantilly influenced by whether immigrant number is equal in all rivers (model I) or proportional to local population size (model II). Assuming equal fitness of immigrants and recipients, the currently reported farmed fish proportion among spawners of about 30% (the mean value in 50 Norwegian reivers as reported for 1989) was estimated to reduce existing genetic differentiation (measured as G-st) between the wild stocks involved by almost 70% in one generation under model I, and by more than 50% under model II. Each of the wild stocks will take on the genetic characteristics of farmed fish at corresponding rates. First-generation effects on G-st of burst immigrations in connection with rearing pen wrecks may be used to estimate farmed fish fitness in nature.