The sea trout, Salmo trutta L. population in the Vistula River was the largest in the Baltic Sea. Its primary spawning grounds were located in the Carpathian tributaries in the upper river basin. The fish ascended to spawn in two runs: in winter when the fish were immature and spent nearly a whole year in the river, and in summer when mature fish ascended the river and spawned within a few months. This work presents the fisheries and stocking history and scientific studies of this population from the late nineteenth century. The consequences of the most important changes in the sea trout habitat are tracked from construction in the upper river basin in the 1940s and the damming of the river in its middle reaches in Włocławek in 1969. Despite intense stocking that has been conducted for over one hundred years, catches have declined from over 100 tons to nearly zero in recent years. The current state of the population and the possibilities of restoring it are discussed in light of genetic studies.

BackgroundNative populations of Atlantic salmon in Poland, from the southern Baltic region, became extinct in the 1980s. Attempts to restitute salmon populations in Poland have been based on a Latvian salmon population from the Daugava river. Releases of hatchery reared smolts started in 1986, but to date, only one population with confirmed natural reproduction has been observed in the Slupia river. Our aim was to investigate the genetic differentiation of salmon populations in the southern Baltic using a 7K SNP (single nucleotide polymorphism) array in order to assess the impact of salmon restitution in Poland.MethodsOne hundred and forty salmon samples were collected from: the Polish Slupia river including wild salmon and individuals from two hatcheries, the Swedish Morrum river and the Lithuanian Neman river. All samples were genotyped using an Atlantic salmon 7K SNP array. A set of 3218 diagnostic SNPs was used for genetic analyses.ResultsGenetic structure analyses indicated that the individuals from the investigated populations were clustered into three groups i.e. one clade that included individuals from both hatcheries and the wild population from the Polish Slupia river, which was clearly separated from the other clades. An assignment test showed that there were no stray fish from the Morrum or Neman rivers in the sample analyzed from the Slupia river. Global FST over polymorphic loci was high (0.177). A strong genetic differentiation was observed between the Lithuanian and Swedish populations (FST = 0.28).ConclusionsWild juvenile salmon specimens that were sampled from the Slupia river were the progeny of fish released from hatcheries and, most likely, were not progeny of stray fish from Sweden or Lithuania. Strong genetic differences were observed between the salmon populations from the three studied locations. Our recommendation is that future stocking activities that aim at restituting salmon populations in Poland include stocking material from the Lithuanian Neman river because of its closer geographic proximity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12711-015-0121-9) contains supplementary material, which is available to authorized users.

Sea trout (Salmo trutta m. trutta) is a migratory form of brown trout common in the Baltic Sea. Nine populations from the southeast Baltic (Poland; Lithuania; Denmark, Bornholm; Estonia and Russia) were genotyped using iPLEX Gold technology (Sequenom) with 62 informative SNPs. A diagnostic panel of 23 SNPs was applied to estimate genetic differentiation and assess the population structure of Baltic sea trout. The highest level of pairwise FST differences was observed between the Russian (East Gulf of Finland) and Polish (Baltic main basin) populations. The lowest differences were between the two Polish and the Polish and Lithuanian populations. A genetic similarity was noted between the Estonian Riguldi River and Danish Bornholm populations, and this finding was supported by a Bayesian and factorial correspondence analysis. Diversity within populations was highest for populations from Estonia and lowest for the Lithuanian population. Genetic structure analysis indicated that individuals from the nine populations were clustered into four groups.

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Sea trout, Salmo trutta L., populations in the Slupia River basin have been affected by mass stocking with smolts and fry. This work is focused on a small-scale differentiation in sea trout populations from one basin with a strong emphasis on the relationship between wild and hatchery origin fish. A total of 216 sea trout were genotyped at 10 microsatellite loci. Global F ST obtained by AMOVA was low at 0.0165. Pairwise F ST were significant for all tests except wild and stocked adults. The highest pairwise difference was found between the hatchery sample and Kwacza (F ST = 0.038). Analysis of the genetic structure revealed micro-geographical differentiation with four subpopulations. The quality of the artificial spawning was found not to be adequate with a high risk of adverse effects to the whole population. All future stocking actions in the basin should consider the existing population structures.