Brown trout in Oder estuary tributaries: genetic structure, stocking, and admixture.

Several rivers that are tributaries of the Oder estuary are inhabited by Salmo trutta L, the most important of which are Ina, Gowienica, and Wołczenica. Both forms of the species, sea trout and resident brown trout, are present. All rivers are traditionally stocked with either sea trout from the neighboring Pomeranian river Rega basin or resident brown trout from various locations. To examine populations in these rivers in terms of genetic structure, genetic diversity, and origin, they were analyzed using 13 microsatellite loci. Relatedness was also assessed for fish stocked in the same year. The obtained genotypes were compared with breeding stocks used for stocking in Poland. The analyses revealed a significant genetic distance between adult individuals from Ina and Rega Rivers and fish caught during electrofishing. Strong kinship relationships were identified in the sampled areas, with high proportions of fish originating from stocking and their dominance in numbers over wild juveniles, primarily in smaller tributaries. Additionally, clear separation in the origin of stocked individuals was observed. Adult trout from Ina and Rega are genetically closer to northern brown trout lineages, providing crucial information for the management and biodiversity conservation of Polish Salmo trutta populations.