Distribution and biological characteristics of escaped farmed salmon in a major subarctic wild salmon river: implications for monitoring

We report the occurrence, distribution, and biological characteristics of escaped farmed salmon in the River Teno in northernmost Europe, which supports one of the largest and most versatile wild Atlantic salmon (Salmo salar) populations in the world. Farmed salmon were caught during the fishing season (May-August) when their proportion in the catch varied between 0.0% and 0.7%. Occasional sampling after the fishing season revealed much higher proportions of escapees, up to 47%, indicating a potential for a more severe impact of farmed fish than the in-season monitoring is able to uncover. Peak migration of the wild salmon was in June or July, but that of escaped farmed fish was in August. Up to 88% of the escaped salmon caught in August showed gonad development, and scale analysis indicated that 4.5% of them were repeat spawners. Genetic analyses using microsatellite markers revealed highly significant genetic differentiation between wild salmon and escaped farmed fish (F-ST = 0.05) caught in the River Teno and a Norwegian farmed strain (F-ST = 0.10). The heterogeneity of escapees compared with the single farmed strain indicated that escapees apparently originate from multiple sources, which will complicate their genetic identification and the assessment of the level of hybridization with wild salmon.