Perils of brown trout ( Salmo spp.) mitigation-driven translocations: a case study from the Vlasina Plateau, Southeast Serbia

In brown trout, instances of genetic homogenization after introducing hatchery-reared allochthonous Atlantic strains are well documented. Therefore, mitigation-driven translocation with autochthonous lineages is gaining support. However, when the origin of local donor populations is not clear they should be genetically characterized prior to translocation. Here we present a case study from the Vlasina Plateau in Southeast Serbia, where two drainages, Danube and Aegean, are adjoining. The status of Danubian and Aegean brown trout populations was investigated with respect to inter-relationship and genetic admixture using mtDNA and microsatellite loci. Results revealed a complex genetic structure and demonstrated different levels of introgressive hybridization from the Aegean populations (which belong to the Adriatic evolutionary lineage) into the Danubian populations as a direct consequence of human mediated translocations. While most introgressed Danubian locations showed low to intermediate proportions of allochtonous genes this was not the case with the upper Jerma location, where all individuals were characterized as pure Adriatic lineage of brown trout. Contrarily, Aegean locations from the Plateau are inhabited by pure Adriatic brown trout. Finally, our analysis clearly demonstrates that the biological invasion followed a stepping-stone scenario via the upper Jerma, which served as a local donor population for supplementing other Danubian populations, and excludes the possibility of brown trout invading through physical connections between the two drainages that were established because of Vlasina hydroelectric power plants. Furthermore, evidence of increased stocking in the wider region is additionally supported by the detection of low level introgression with the domesticated Atlantic brown trout.