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A study was made of rivers in Northwest Ireland where escapes of adult Atlantic salmon (Salmo salar) are known to have occurred from adjacent sea cages. Two markers that showed substantial frequency differences between these farm and wild populations were used: an Ava II-B mtDNA haplotype and allele E at minisatellite locus Ssa-A45/2/1. Farmed populations also showed a significant reduction in mean heterozygosity over the three minisatellite loci examined. Independent occurrence of mtDNA and minisatellite DNA markers in several juvenile samples indicated interbreeding of escaped farm salmon with wild salmon. The proportion of juveniles of maternal farm parentage in two rivers ranged from 18% in 1993 to 2% in 1995 with an average of 7% in both rivers (1993-1995) and a maximum frequency of 70% in an individual sample. Only a small proportion of 29 000 adult farm salmon that escaped in spring …