High genetic introgression in alpine brown trout (Salmo trutta L.) populations from Hardangervidda, Norway

We studied the effect of stocking brown trout in three alpine lakes (Skavatn, Midtre Grondalsvatn and Nordmannslagen) of the Hardangervidda mountain plateau by comparing microsatellite data from historical (1933 and 1967) and contemporary (2003) population samples. Historical and contemporary samples from a control lake (Krokavatn) and a contemporary sample from Gjuvsja (suspected donor lake) were included as well. A total of 331 brown trout were genotyped with 11 microsatellites. Very low level of genetic diversity is observed in the historical samples from Nordmannslagen and Midtre Grondalsvatn but not in the contemporary samples. In Skavatn historical and contemporary samples show similar genetic diversity as in the other contemporary samples. Admixture analyses with the structure program indicate high levels of introgression by nonnative brown trout in all three lakes. The stocking source for two lakes (i.e. from Gjuvsja) is partially verifiable via our analyses. The high introgression in these lakes is discussed and the management issues are briefly addressed.