GENETIC CHANGES IN AN ATLANTIC SALMON POPULATION RESULTING FROM ESCAPED JUVENILE FARM SALMON

The study was undertaken on three adjacent rivers in NW Ireland, on on e of which an Atlantic salmon Salmo salar freshwater juvenile rearing unit is situated. Two markers which distinguished farm and wild popula tions were used. An Ava II-B RFLP in the ND1 region of mtDNA was at a frequency of 0.58 in the farm strain but absent in the wild population s. Allele E at minisatellite locus Ssa-A45/2/1 was at a frequency of 0 .91 in farm samples, but at a maximum of 0.41 in the populations in th e two rivers adjacent to the one with the juvenile rearing unit. The f arm strain showed a significant reduction in mean heterozygosity (0.28 1 +/- 0.057), over three minisatellite loci examined, compared to wild samples (0.532 +/- 0.063). The occurrence of farm genotypes and the i ndependent occurrence of mtDNA and minisatellite markers in several pa rr samples from the river indicated that escaped juvenile salmon compl eted their life cycle, bred and interbred with native fish, upon their return to the river. Escaped fish homed accurately, as adults, to the site of escape, i.e. the area adjacent to the hatchery outflow in the upstream part of the river. Breeding of makes in the lower part of th e river was also indicated but this could have been due to mature make parr which had moved downstream. The return of adults of farm origin to the river to breed was indicated by the presence of the Ava II-B ha plotype in adults netted in the estuary. (C) 1998 The Fisheries Societ y of the British Isles.