Population‐Specific Migration Patterns of Wild Adult Summer‐Run Chinook Salmon Passing Wells Dam, Washington

Abstract

AbstractSummer‐run Chinook Salmon Oncorhynchus tshawytscha migrating over Wells Dam, Washington, enter a habitat characterized by blocked upstream access, high tributary water temperature regimes, and robust tribal and recreational fisheries. In 2011, we initiated a 2‐year radiotelemetry study to identify population‐specific run timing, movement, and mortality of naturally produced fish passing the dam. Five hundred seventeen salmon were radio‐tagged at Wells Dam over 2 years of study. The highest proportion (44%) of tagged fish escaped to the Okanogan River, but spawning populations from the Methow River (16%), Wenatchee River (6%), Entiat River (5%), and the Columbia River upstream (14%) and downstream of Wells Dam (14%) were also represented. In general, tributary‐spawning fish had significantly earlier run timing than did main‐stem–spawning fish. We observed very little movement among spawning tributaries, but a significant proportion of fish (~30%) were detected holding in the tailrace of Chief Joseph Dam on the Columbia River prior to spawning, including fish from populations many kilometers downstream of Wells Dam. Fallback was common in each year of the study, and we calculated that passage at Wells Dam was overestimated by 27.5% in 2011 and 32.0% in 2012 due to fallback and re‐ascension. Of the fish that remained upstream, 16% in 2011 and 22% in 2012 were estimated to have died prior to spawning, excluding fish that were known or suspected to have been harvested. In 2012, warm water temperatures (>20°C) in the Okanogan River resulted in a thermal barrier that delayed migration but did not affect spawning distribution within the river compared with 2011, when no significant thermal barrier was detected. Our results highlight some of the complex migration and distribution patterns of natural‐origin Chinook Salmon passing Wells Dam and should assist managers in upstream population and fishery modeling efforts.