High rates of prespawn mortality, when adult salmon die after completing migration but prior to spawning, can lead to population declines and can impede recovery of threatened stocks. In this study, annual prespawn mortality of female Chinook Salmon Oncorhynchus tshawytscha ranged from 1% to 100% over 14 years in seven study reaches located throughout the upper Willamette River basin, Oregon. Prespawn mortality rates were positively correlated with the annual maximum 7‐d average maximum stream temperature and the percentage of spawning fish of hatchery origin. Observed prespawn mortality rates varied considerably, but annual female prespawn rates were consistently >80% where maximum temperatures exceeded 20°C and the composition of spawning fish was >80% hatchery origin. In several spawning tributaries, prespawn mortality rates generally decreased at higher elevations. The proximate cause of prespawn death was not evaluated here, and observed patterns likely reflected additional factors that influence mortality either directly or indirectly, such as handling, dam passage, fishing pressure, instream habitat, energetic budget, fish density, and pathogen loads.

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