Genetic structure in alpine brown trout Salmo trutta L. shows that indirect stocking affects native lake populations

Genetic variation of brown trout inhabiting six connected lakes (Krokavatn, Skavatn, Kollsvatn, Litlosvatn, Kvennsjøen and Øvre Krokavatn) of the Hardangervidda mountain plateau was analysed by 11 microsatellites covering a period of 24–35 years. Temporal changes were found and are deemed to be mostly related to stocking in the headwater lakes Krokavatn and Skavatn. A pronounced change was evident in one population (Kollsvatn) and was the result of downstream migration of stocked fish from these headwater lakes. This indirect stocking accounted for up to 50% in the STRUCTURE analysis of Kollsvatn fish and was less pronounced in the other downstream populations of Litlosvatn (c. 28%), Kvennsjøen (c. 17%) and Øvre Krokavatn (c. 13%). Since significant downstream migration of stocked fish affecting native populations occurs, nature management efforts must respond accordingly.