Tracing the genetic origin of brown trout (Salmo trutta) re-colonizing the Ecker reservoir in the Hartz National Park, Germany

The Ecker reservoir and its main tributary had been free of brown trout (Salmo trutta) for several decades due to cumulative effects of natural and anthropogenic acidification. However, after the decline of emissions in the 1990s and the resulting rise of water pH to suitable for brown trout values, the species began to recolonize its original habitats. In the main tributary first brown trout individuals were caught in 2008 and in the reservoir in later years as well. Stocking could be excluded in both areas. Therefore, the present study was aimed to trace the genetic origin of these brown trout by genotyping eight microsatellite loci in samples collected in the reservoir, its main tributary, potential refugia and - for comparison - from two areas downstream of the dam being physically isolated for about 70 years. Genetic variability within populations (average number of alleles per locus), genetic differentiation between populations (FST values and genetic distances), occurrence of certain alleles and results of assignment tests indicated that the Ecker reservoir was re-colonized from two sources: the Große Peseke, a small direct inflow into the reservoir, and the Fuhler Lohnsbach, a parallel flowing brook connected to the reservoir by a pipe. Genetic data also supported recolonization of the main tributary from the reservoir but not in the opposite direction. Moreover, bottleneck effects were evident in brown trout populations upstream of the dam compared to the two populations downstream of the dam.