Long-term and seasonal genetic differentiation in wild and enhanced stocks of sea trout (Salmo trutta m. trutta L.) from the Vistula River, in the southern Baltic-Management implications

The Vistula River, rich with sea trout (Salmo trutta m. trutta L.) is the longest river in Poland, flowing into the southern Baltic Sea, with its population sustained mainly by enhancement programmes. The population source was declined and the proportion of Vistula winter and summer sea trout stocks existing in the river was drastically changed over a few decades. This paper is the first detailed genetic study of stocks of the species in the river. Five samples representing wild winter (1971), enhanced winter (2010/11) and summer (2003), and hatchery winter (2003) sea trout stocks were analysed. Twelve microsatellite loci were employed to investigate long-term and seasonal genetic differentiation affected by strong anthropogenic activities and thence to assist fish stock management in the Vistula River. The genetic structure of past and current stocks was observed. The substructure of the sea trout stocks, showing different spawning behaviour, was also exhibited. The drop of private alleles and bottleneck effect were noted, pointing the decrease of Vistula sea trout genetic sources in last 40 years. The pronounced differentiation (pairwise Fst values from 0.024 to 0.057) and high full-siblings share (68%) recorded for the hatchery stock, being the only source for supplementary stocking of winter sea trout stock, imply that this source should be used with caution and action to improve, and subsequently maintain, conditions for natural spawning should be prioritized.