Genetic impact of escaped farmed Atlantic salmon (Salmo salar L.) on native populations: Use of DNA profiling to assess freshwater performance of wild, farmed, and hybrid progeny in a natural river environment

Since Atlantic salmon (Salmo salar L.) used for farming are usually genetically different from local wild populations, breeding of escaped farmed salmon potentially results in genetic changes in wild populations. To determine the likelihood and impact of such genetic change, an experiment was undertaken, in a natural spawning tributary of the Burrishoole system in western Ireland, to compare the performance of wild, farmed, and hybrid Atlantic salmon progeny. Juveniles were assigned to family and group parentage by DNA profiling based on composite genotypes at seven minisatellite loci. Survival of the progeny of farmed salmon to the smolt stage was significantly lower than that of wild salmon, with increased mortality being greatest in the period from the eyed egg to the first summer. However, progeny of farmed salmon grew fastest and competitively displaced the smaller native fish downstream. The offspring of farmed salmon showed a reduced incidence of male parr maturity compared with native fish. The latter also showed a greater tendency to migrate as autumn pre-smolts. Growth and performance of hybrids were generally either intermediate or not significantly different from the wild fish. The demonstration that farmed and hybrid progeny can survive in the wild to the smolt stage, taken together with unpublished data that show that these smolts can survive at sea and home to their river of origin, indicates that escaped farmed salmon can produce long-term genetic changes in natural populations. These changes affect both single-locus and high-heritability quantitative traits, e.g. growth, sea age of maturity. While some of these changes may be advantageous from an angling management perspective, they are likely, in specific circumstances, to reduce population fitness and productivity. Full assessment of these changes will require details of marine survival, homing and reproductive performance of the adults together with information on the F₂ generation.