GENETIC DIFFERENTIATION AMONG DANISH BROWN TROUT (SALMO-TRUTTA) POPULATIONS

Brown trout (Salmo trutta L.) from twelve locations in small rivers in Jutland, Denmark, were examined by allozyme electrophoresis. Seven of the locations are tributaries to the small (3.3 km2) Lake Hald. These and two other locations are assumed to have been mainly inaccessible to gene flow from outside for hundreds of years because of impassable dams. The levels of polymorphism indicated that little or no loss of genetic variation had occurred in these isolated populations compared to populations open to gene flow. In Lake Hald significant genetic differentiation among the tributary populations was detected. Intensive stocking with trout from a hatchery strain directly into the lake was shown to have had little or no effect on the genetic composition of the original populations. In contrast, transplantation of trout from one tributary to another within the lake system was successful. The geographical distribution of genetic variation indicated that the Lake Hald populations are genetically divergent from the other populations and for that reason special care concerning management practices in the lake is recommended.