| Summary: | Supporting fish populations by restocking with artificially bred young fish is a common practice worldwide, which may sometimes have a negative effect on natural genetic diversity. In this study, the currently ongoing restocking program of brown trout (Salmo trutta Linnaeus, 1758) in the subarctic Pasvik river in Norway was investigated. Previous investigations of this population showed a loss of genetic diversity and both genetic and morphological differentiation between historical breeding stocks and wild trout populations. However, the genetic impact of the breeding program used in this river today has never been investigated. To accomplish this, I compared the parent fish, the offspring, and fish caught in the river using 16 short tandem repeat loci and estimated pairwise relatedness, inbreeding, genetic distances, and differentiation. The results showed, by estimating the effective population size, that the number of parents used in the breeding program was generally insufficient to maintain a genetically sustainable river population. Further, genetic diversity in stocked fish was typically not maximized due to lack of performing all possible crossings between males and females. In combination, this induced genetic drift in the offspring generations and thus genetic differentiation away from the river populations. In summary, the results from this study show that the current stocking practises used in the Pasvik river are not sustainable and that measures need to be taken to prevent continued loss of genetic diversity in this large-growing trout population.
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