Fish know no borders—Implications of the genetic structure and mixed-stock composition to cross-border management of adfluvial brown trout

Effective management of migrating fish stocks requires an understanding of population structure and each stock's contribution to the fishery. Water bodies spanning across national borders pose a particular challenge. We investigated the genetic structure of 13 adfluvial brown trout populations from natal rivers and conducted a genetic mixed-stock analysis of brown trout from the cross-border Lake Pyaozero Basin in eastern Fennoscandia, using 16 microsatellite loci. Our analysis revealed a clear genetic structure of three distinct geographic groups, consisting of river-specific populations. The Western and Southern groups, which included 11 rivers, contributed over 90% to the mixed stock in the lacustrine rearing area. This mixed stock of several river-specific subpopulations renders poorly managed fishing in the area a significant threat to the entire native adfluvial brown trout population complex.