| Summary: | Many stocks of fish consist of mixtures of individuals originating from different populations. This is particularly true for many salmon and trout stocks, where fish of different genetic background are being found in the same rivers and/or lakes due to stocking activities or straying caused by increased aquaculture activities. The interpretation of results from studies of survival and behaviour of fish from such “mixed stocks” require information of the genetic background of individual fish. We used genetic analysis combined with radiotelemetry to study upstream migration of Atlantic salmon (Salmo salar) in a Danish lowland river. The river has a small population of native salmon, but salmon juveniles from Irish, Scottish and Swedish populations have been stocked and return as adults. A total of 39 salmon were caught by electrofishing and tagged by surgical implantation. A tissue sample (fin clip) from each tagged salmon was analysed using microsatellite DNA analysis of 6 loci. Assignment tests were used to infer the population of origin. The results showed that the salmon run was composed of approximately 1/3 “native fish”, 1/3 foreign stocked fish and 1/3 escaped farmed salmon. The results indicate that stocked, foreign salmon had a slightly higher mortality and moved more up and down in the river than the native salmon did, but all salmon had problems passing the physical obstructions in the river. The DNA analyses enabled us to compare the behaviour of fish of different genetic origin, but the interpretation of the results was hampered by a high mortality of tagged fish. This study demonstrates that the combination of recent genetic methods and telemetry provides a potent tool for better management of mixed stock fisheries
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