Reproductive success of hatchery-produced and wild-born brown trout in an experimental stream

1. Although releases of hatchery-produced salmonids to support conspecific wild populations have increased dramatically during recent decades, little information is available about the performance in the wild of hatchery fish and their offspring. Important factors determining the success and genetic outcomes of supportive breeding programmes include (i) the relative reproductive success of released hatchery fish in the wild, and (ii) the extent to which the propagation affects the variance in reproductive success in the population as a whole. 2. We performed two field experiments on brown trout Salmo trutta from the River Dalalven in Sweden, where we examined reproductive success in an experimental stream. In experiment 1 we compared reproductive success between trout from a seventh-generation hatchery stock of native origin and wild-born trout from the river. In experiment 2, we compared reproductive success between seventh-generation hatchery trout and hatchery-reared trout derived from wild-born parents. Individual reproductive success, based on the number of offspring assigned using microsatellite markers, was assessed on three occasions after reproduction: immediately after hatching and after the first and second growth seasons. 3. In experiment 1 there were no significant differences in reproductive success between seventh-generation hatchery trout and wild-born trout. In experiment 2, males from wild-born parents were more successful than males from the seventh-generation hatchery stock, but this difference was not observed among females. 4. There was some evidence for a positive association between body size and reproductive success among females but not males. For males, the number of mates was significantly associated with reproductive success, but this relationship was not evident among females. 5. The variance in reproductive success was pronounced in both experiments, yielding estimates of the ratio between the genetically effective size and the census size of our experimental populations ranging from 0.12 to 0.59. 6. Synthesis and applications. Our results suggest that the reproductive success in the wild of hatchery-produced and wild-born trout with a common genetic background may be rather similar. These findings, in combination with the pronounced variance in reproductive success observed among breeders, indicate that supportive breeding can be managed to increase not only the census but also the genetically effective size of small, endangered salmonid populations. However, to minimize negative effects of hatchery selection, it is important to give priority to the restoration of natural habitats and thereby increase the reproductive output from individuals in the wild.