Influence of stream temperature and human disturbance on prespawn mortality of Chinook Salmon in the Willamette River basin.

This article focuses on identifying factors contributing to high prespawn mortality (PSM) of adult female Chinook Salmon (Oncorhynchus tshawytscha) outplanted above dams in Fall Creek, a tributary of the Willamette River basin, Oregon, and developing a decision support model to guide adaptive management. Using data from 2010–2017 and 2020–2021, the study estimated PSM rates averaging 66%, with rates often exceeding 50%, and explored three main hypotheses for nontransport stressors influencing PSM: thermal exposure during migration, human disturbance (swimming and fishing), and elevated water temperatures below the dam. Simulation results indicated that excluding human activities in July and August could reduce PSM by nearly half, and managing stream temperatures below the dam in June also showed potential benefits. Sensitivity and adaptive management analyses highlighted the importance of targeted management actions and ongoing monitoring to reduce uncertainty and improve salmon recovery efforts in regulated river systems.