Genetic effects of supportive stockings on native pikeperch populations in boreal lakes--cases, three different outcomes.

The genetic consequences and gene flow of pikeperch (Sander lucioperca) stocking were assessed in three boreal lakes based on admixture model analysis and comparison of the pre- and post-release patterns of genetic variability at 9 DNA microsatellite loci in the recipient populations. In two out of the three cases, the releases of fish from foreign populations caused significant changes in the genetic structure of the recipient population. The largest changes were observed in Lake Oulujärvi, where the post-release sample was almost identical to the released Lake Vanajanselkä population, and about 90% of the catch was composed of the released population. The genetic composition of Lake Lohjanjärvi pikeperch also shifted markedly towards that of the released Lake Vanajanselkä population, and about half of the later catch was of released Vanajanselkä origin. In Lake Vanajanselkä, in contrast, releases of pikeperch from lakes Painio and Averia had only a small impact on the genetic structure of the pikeperch population. These results indicate that the current stocking practices create an effective artificial gene flow that may strongly shape and reduce the genetic differentiation among the remaining native pikeperch populations. A common feature of all three cases was the lack of prior appraisal of the potential genetic and ecological risks in relation to the expected benefits of the release programmes.