The study was undertaken on three adjacent rivers in NW Ireland, on one of which an
Atlantic salmon Salmo salar freshwater juvenile rearing unit is situated. Two markers which distinguished farm and wild populations 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 populations. 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 the two rivers adjacent to the one with the juvenile rearing unit. The farm strain showed a significant reduction in mean heterozygosity (0·281&0·057), over three minisatellite loci examined, compared to wild samples (0·532&0·063). The occurrence of farm genotypes and the independent occurrence of mtDNA and minisatellite markers in several parr samples from the river indicated that escaped juvenile salmon completed 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 males in the lower part of the river was also indicated but this could have been due to mature male 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 haplotype in adults netted in the estuary.