ASSESSMENT OF THE STOCKED OR WILD ORIGIN OF ANADROMOUS BROWN TROUT (SALMO-TRUTTA L) IN A DANISH RIVER SYSTEM, USING MITOCHONDRIAL-DNA RFLP ANALYSIS

Declines in the number of anadromous brown trout in the Karup River in Denmark, due to environmental degradation, led to the stocking of lar ge numbers of hatchery trout during the 1980s. This practice was gradu ally replaced by stocking with the offspring of electrofished local tr out. The genetic contribution of the hatchery fish to the current popu lation of anadromous trout in the river was estimated by restriction f ragment length polymorphism analysis of mitochondrial DNA, using seven restriction endonucleases. Fish from the hatchery strain as well as f rom five locations in the river system, and from a further unstocked r iver were screened. Eight haplotypes were observed. The distribution a nd frequencies of the observed haplotypes revealed little genetic diff erentiation among stocked populations. The hatchery strain differed si gnificantly from the stocked populations. One haplotype which was foun d at a high frequency in the hatchery strain was almost absent from th e stocked populations. This suggests that the genetic contribution of the hatchery trout to the current population is much less than would b e expected from the number of stocked fish. The possible reasons for t he failure of the hatchery trout to contribute to the gene pool, and a lso the implications for conservation biology, are discussed.