Genetic interactions among native and introduced stocks of Oncorhynchus mykiss in the upper Willamette River, Oregon

摘要

First recognized as threatened with extinction in 1999, native winter steelhead Oncorhynchus mykiss from Oregon's Upper Willamette River (UWR) declined to a record-low 543 adult fish in 2017. This anadromous species has been seriously affected by habitat loss caused by impassable dams, intense predation from pinnipeds, and water pollution. Genetic and ecological risks posed by non-native hatchery steelhead may also limit recovery. Here we used 15 microsatellite markers to investigate the genetic composition of naturally-produced juvenile O. mykiss, collected from the most downstream UWR location and other sites throughout the upper basin. Our results, based on 1012 individuals, identified some natural production by summer steelhead released by ongoing hatchery programs, but even greater influence from discontinued non-native winter steelhead hatchery programs. Genetic influence from hatchery programs was nearly absent in above-dam habitats, but concentrated in below-dam habitats and rivers stocked with hatchery steelhead. Although informative, our microsatellite data lacked sufficient power to assign individual fish to hybrid classes, and we urge caution when interpreting similar results from few loci. Taken together, our findings suggest that fish sorting facilities can assist with the conservation of steelhead populations reintroduced above dams, and that hatchery programs can have a lasting genetic influence over naturally-spawning populations, even after stocking has ceased. To conserve the genetic integrity of native UWR steelhead, we recommend that managers consider the lineage of steelhead used for reintroductions, continue to implement policies that limit genetic risks from existing hatchery programs and explore ways to alleviate persistent risks from discontinued programs.