Genetic effects of hatchery rearing in Atlantic salmon

Six polymorphic enzyme loci were examined electrophoretically in a sample of wild Atalntic salmon smolts from the Burrishoole river in western Ireland and in samples of artificially-reared fry hatched in 1981 and parr hatched in 1979. These hatchery reared fish were the progeny of five generations of artificially reared sea ranched salmon which had originally come from the Burrishoole river. Selection for growth and disease resistance was practised and between ten and 30 females and similar numbers of males were used as parents in each generation. Gene frequencies differed significantly at a number of loci between the wild and the artificially reared samples. Erosion of genetic variability, as measured by mean heterozygosity and mean number of alleles over the six loci, was evident in both hatchery reared samples. It is argued that the observed genetic changes are caused by founder effects and genetic drift rather than selection by some aspects of the artificial rearing regime. The importance of using adequate numbers of parents in hatchery rearing is stressed, since it is shown that differences between wild and reared populations are as great as between natural populations from Irish rivers.