Yukon River Chinook salmon (Oncorhynchus tshawytscha) populations are declining for unknown reasons, creating
hardship for thousands of stakeholders in subsistence and commercial fisheries. An informed response to this crisis requires
understanding the major sources of variation in Chinook salmon productivity. However, simple stock–recruitment models leave
much of the variation in this system’s productivity unexplained. We tested adding environmental predictors to stock–recruitment
models for two Yukon drainage spawning streams in interior Alaska — the Chena and Salcha rivers. Low productivity was
strongly associated with high stream discharge during the summer of freshwater residency for young-of-the-year Chinook
salmon. This association was more consistent with the hypothesis that sustained high discharge negatively affects foraging
conditions than with acute mortality during floods. Productivity may have also been reduced in years when incubating eggs
experienced major floods or cold summers and falls. These freshwater effects — especially density dependence and high
discharge — helped explain population declines in both rivers. They are plausible as contributors to the decline of Chinook
salmon throughout the Yukon River drainage.
