Abstract
Chinook salmon, Oncorhynchus tshawytscha (Walbaum), populations have declined rapidly along the western coast of North America since the year 2000, possibly because of factors such as habitat loss, altered hydrology and barriers to migration. However, few analyses have rigorously examined which of these factors actually explain historical patterns of extirpation. Data were compiled on flow regimes, habitat loss and migration barriers for 27 streams that historically supported autumn run salmon and 22 streams that supported spring runs. The probability of extirpation in streams supporting autumn run was predicted solely by migration barriers. All other factors were >10 5 times less likely to explain existing variation. By contrast, models for spring run salmon suggest that habitat loss and altered flow regimes were also predictors of extirpation. These results suggest that regional extirpation of Chinook salmon has been driven by multiple forms of environmental change, and restoration efforts must address a multitude of bottlenecks that now impact spring and autumn run populations.