Genetic stock identification of sea trout (Salmo trutta L.) along the British North Sea Coast shows prevalent long-distance migration

Knowledge of migration behaviour and spatio-temporal habitat use is important for management and conservation, not least for diadromous species where population dynamics are affected by processes occurring in both freshwater and marine habitats. Sampling from a commercial capture fishery for the ecologically and economically important salmonid Salmo trutta L. we genotyped 189 single nucleotide polymorphisms and carried out genetic assignment of origin for 916 migratory sea trout sampled along ~500 km of Britain’s North Sea coast. Population contributions to stocks showed strong geographical differences. Trout from rivers in the northern part of this coast (Tweed, Aln, Coquet, and Tyne) contributed the majority of migrants, irrespective of location, but fish from local rivers contributed to catches in most locations. Almost all (99%) trout assigned to native British populations and a large proportion (34%) of the sampled catches originated in rivers more than 100 km from capture sites, suggesting long-distance migration to be a common trait, confirming and enlarging historical trends from tagging data. Repeated sampling across years indicated short-term stability in migration patterns. Findings illustrate the scope for genetic stock identification to identify marine habitat use and monitor population-specific migration behaviours in species where stock assessment has not traditionally incorporated marine phase dynamics.