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-The genetic structure of a fish population is usually thought to be stable over time. In the southern Baltic, Salmo trutta m. trutta (sea trout) populations have been characterized by low degree of genetic differentiation. All studied populations had been heavily stocked with mixed material for many years prior to the sampling period, including releases of Pomeranian sea trout to the Vistula River in Poland, Southern Baltic. However, the strategy of stocking became river based a few years before the sampling began. Juveniles from artificial reproduction are released only to their parental river, which reduces the mixing of the gene pool of fish from different populations. Changes in sea trout populations in the southern Baltic over time were studied using single nucleotide polymorphisms (SNPs). Genetic composition of populations of sea trout in the Vistula and Drwȩca river system were found to increasingly resemble the non-admixed hatchery population from Aquamar (Miastko, Poland), whereas the Pomeranian populations were stable. The implementation of a new stocking strategy for the restoration and protection of Vistula sea trout was noted as possible explanation. With the increase of natural breeding, artificial enhancement of sea trout populations should be reduced.

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