The impact of stocking on the genetic structure of European grayling Thymallus thymallus, Salmonidae) in two alpine rivers

The European grayling (Thymallus thymallus) is considered to be threatened in several European regions. In recent decades fishery managers have increasingly turned to stocking programs as one way to combat the negative effect that human influence has had on population densities. The present study surveyed the genetic structure of two Danube drainage populations at the Inn and Drau rivers, in Austria, on the basis of mtDNA sequences. Data were placed in the context of the phylogeographic structure of European Thymallus and thus could reveal unexpected geographical mixing due to stocking with allochthonous individuals. Our analyses revealed that regular stocking of fish not originating from their natal Rivers has left genetic traces in both systems surveyed. These traces may be classified as marginal for the Inn river and its tributaries in which 97% of the graylings investigated carried haplotypes belonging to the northern alpine lineage, corresponding to the region through which the Inn flows. In contrast, the genetic composition of the Drau population, situated in the southern Alps, has been seriously altered through the stocking of fish belonging to the northern alpine mtDNA lineage as only 62% of the fishes sampled carried haplotypes representing the native southern alpine lineage.