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.

俄勒冈州上威拉米特河（UWR）的冬季冬季黑头黑头鲷Oncorhynchus mykiss最初于1999年被确认为濒临灭绝，到2017年下降至创纪录的低位543条成年鱼类。这种无鳍种已严重受不可逾越的水坝造成的栖息地丧失的影响，夹杂物的捕食和水污染。非本地孵化场硬头鱼造成的遗传和生态风险也可能限制其恢复。在这里，我们使用了15个微卫星标记来研究天然产生的O. mykiss幼虫的遗传组成。是从最下游的UWR位置和整个上盆地的其他地点收集的。我们基于1012位个体的结果确定了正在进行的孵化场计划释放的夏季硬头孵化场的一些自然产量，但已终止的非本地冬季冬季硬头孵化场计划的影响更大。孵化场计划的遗传影响几乎不在大坝上方的栖息地中，而是集中在大坝以下的栖息地和装有孵化场钢头的河流中。尽管提供的信息丰富，但我们的微卫星数据缺乏足够的能力来将单个鱼类分配给杂交类别，因此在解释少数基因座的相似结果时，我们应谨慎行事。综上所述，我们的研究结果表明，鱼类分拣设施可以帮助保护重新引入大坝上方的硬头种群，而且即使停止放养，孵化场计划也可以对自然产生的种群产生持久的遗传影响。为了保护原生UWR硬头鱼的遗传完整性，我们建议管理人员考虑用于再引入的硬头鱼的血统，继续实施限制现有孵化场计划遗传风险的政策，并探索减轻已终止计划的持续风险的方法。