Range-wide genetic assignment over half a century reveals long-distance migration in Atlantic salmon

Atlantic Salmon (Salmo salar) populations throughout the North Atlantic have declined in recent decades largely resulting from reduced survival during the marine life history phase. Nonetheless, our understanding of the marine phase, distribution patterns and migratory routes remains limited. Here, we assigned archived individual samples (n=3511) collected throughout the North Atlantic over a half century (1968-2018) to region of origin using range-wide genetic assignment. In the Northwest Atlantic, the distribution of assignments reinforced the importance of the Labrador Sea with 73% of all regional groups detected there including 5 European groups and European individuals detected from southern Newfoundland to Greenland. Similarly, in the Northeast Atlantic in addition to European groups present, several North American populations (n=6) were detected in waters around the Faroe Islands. Based on the distribution of assigned fish, estimates of trans-Atlantic migration distance averaged 4000 km and 2900 km for North American and European salmon respectively. Overall our analysis highlights the widespread importance of the Labrador Sea and Faroe Islands to the species marine distribution patterns, the prevalence of long distance trans-Atlantic migration (>4000 km) and suggests that environmental conditions experienced by many Atlantic salmon populations span much of the North Atlantic Ocean.