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 g enetic variation had occurred in these isolated populations compared t o populations open to gene flow. In Lake Hald significant genetic diff erentiation among the tributary populations was detected. Intensive st ocking with trout from a hatchery strain directly into the lake was sh own to have had little or no effect on the genetic composition of the original populations. In contrast, transplantation of trout from one t ributary to another within the lake system was successful. The geograp hical distribution of genetic variation indicated that the Lake Hald p opulations are genetically divergent from the other populations and fo r that reason special care concerning management practices in the lake is recommended.