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Because winter-run steelhead populations in the Willamette River are listed as Threatened under the Endangered Species Act, there is concern about potential genetic and ecological effects of non-native summer-run steelhead on threatened winter-run populations. However, at most life stages, and in most Willamette River habitats, it is difficult to identify naturally-produced summer-run fish compared to winter-run fish (all hatchery-origin summer steelhead smolts are marked with an adipose clip). As a consequence, it is generally unknown to what extent summer steelhead successfully spawn, how many juveniles are produced, and when and where they reside in the system. In this study, NWFSC's Don VanDoornik uses genetic stock identification (GSI) techniques to identify the stock of origin of steelhead in the Willamette River. The resulting information will be used by the Oregon Department of Fish and Wildlife to guide management of their steelhead hatchery programs in the Willamette River. Steelhead genetic data (microsatellite DNA genotypes)