Reservoir entrapment and dam passage mortality of juvenile Chinook salmon in the Middle Fork Willamette River

Abstract – High‐head dams in Oregon’s Willamette River basin inhibit seaward migration and present significant mortality risks to ESA‐listed juvenile Chinook salmon (Oncorhynchus tshawytscha). Over 7 years, we passively collected 13,365 downstream‐migrating juvenile salmon in rivers above and below Willamette dams. Most salmon emigrated from upstream sites in February–June, but passed dams in November–February when reservoirs were drawn down near annual lows, and access to deep‐water passage routes improved. Samples collected above reservoirs were dominated by subyearlings, whereas below‐dam samples were a phenotypically diverse mix of subyearling, yearling and older salmon. The life history data indicated that Willamette reservoirs seasonally entrap many salmon and some sea‐ready smolts probably residualise. Annual dam‐passage mortality estimates were 8–59% (mean = 26%). Individual salmon mortality risk increased significantly with body length and varied with reservoir elevation and discharge. Operational changes that allow timely volitional emigration and development of less hazardous passage routes would benefit this threatened population.