No identified genetic effects of dam construction or releasing hatchery-reared brown trout (Salmo trutta) to a Norwegian wild brown trout population

In the River Gudbrandsdalslågen, a hydropower dam blocked upstream migration of brown trout in the early 1960s and reduced natural production along a 4.4-km minimum-flow reach. A fishway was constructed in 1966, and annual stocking of 15,000 juvenile trout began in 1973. We evaluated the genetic consequences using 91 neutral SNPs from historic and contemporary scale samples collected from mature trout over a century (1909-2015). Contemporary juveniles from multiple spawning areas in the main river, both upstream and downstream of the dam, and in tributaries were also analyzed to assess population structure and group migratory fish from the historic dataset to geographic origins. Contemporary samples formed three genetically distinct groups corresponding to the lower and upper main river and one tributary. Genetic differentiation increased with geographic distance but was not associated by the hydropower dam. Historic samples clustered with the contemporary lower main-river group. Despite significant isolation by time, differentiation among years was weak. This long-term genetic stability likely reflects a large effective population size, maintained habitat connectivity, and use of local broodstock for stocking.