Genetic monitoring of a long-term, large-scale experimental steelhead supplementation program

Declining salmonid populations often prompt the use of captive-reared fish to supplement wild stocks, but such programs risk negative genetic and ecological impacts. We evaluated six steelhead (Oncorhynchus mykiss) populations in the Hood Canal watershed, Puget Sound, Washington, including three supplemented and three unsupplemented control populations, over the span of 17 years to assess the effects of supplementation on several population genetics metrics. This program uniquely allowed natural spawning to occur before removing eyed eggs from redds for captive rearing, and later release as smolts or adults. Key genetic metrics-expected heterozygosity, allelic richness, and effective population size-remained stable from before to after supplementation in both the supplemented and non-supplemented populations. Parentage analyses confirmed successful reproduction by captively reared adults after they were released into the wild. These findings suggest that natural spawning prior to captive rearing, among other aspects of the program, lessened the genetic risks typically associated with artificial propagation such as loss of genetic diversity, or a reduction in effective population size. Our results highlight the potential for carefully designed supplementation programs to conserve genetic diversity and maintain effective population sizes in threatened steelhead populations.