Sibship within samples of brown trout ( Salmo trutta ) and implications for supportive breeding

We analysed family relationships among brown trout from two small tributary populations that have been suggested as a source of individuals for supportive breeding, using variation at eight microsatellite loci. As a control, we analysed a sample of supposedly unrelated individuals representing a large anadromous population, and we simulated unrelated individuals based on the allelic distributions in all three samples. Two different approaches were used: (1) pairwise estimates of relatedness between individuals and (2) a method for partitioning individuals into half-sib and full-sib families. The anadromous population did not show evidence of a significant number of closely related individuals. In both tributary populations, however, the distributions of pairwise relatedness estimates suggested the presence of several related individuals, and sibship reconstruction suggested fewer families consisting of more individuals than were observed for the simulated individuals. The expected increase of inbreeding coefficient in the two samples due to family structure was 0.026 and 0.030 respectively. Moreover, tests for recent bottlenecks yielded significant outcomes in both populations suggesting a history of low effective population sizes. Depending on the effective population size of captive spawners and past effective population sizes in the populations it could be beneficial to conduct sib-avoidance matings, though this cannot eliminate inbreeding but only delay it. Alternatively, individuals from different populations could be crossed. Sibship reconstruction provided the clearest evidence for family structure, but pairwise relatedness is the best measure for designing mating schemes, as it allows for mating as unrelated individuals as possible rather than just avoiding mating between sibs.