Causes and Consequences of Changing Age at Maturity of Female Chinook Salmon in Western Alaska

Recent poor returns of Chinook salmon Oncorhynchus tshawytscha to rivers in western Alaska have been attributed to a number of potential causes, including climate change, overharvest, and by-catch in the Bering Sea. Subsistence fishermen along the Yukon and Kuskokwim rivers have expressed concern about decreasing size of returning adults, particularly the females. Although size-selective gill net fisheries are recognized as a potential cause of declining age and size of Chinook salmon, environmental variation is likely contributing to these declines through its influence on growth. Here, we use a ~30-year time series of female Chinook salmon age and growth data from scales collected at two escapement projects (Kogrukluk River in the Kuskokwim drainage and Andreafsky River in the Yukon drainage) to determine which periods of growth best explain the probability of female Chinook salmon maturing early (after three years at sea) or late (after five years at sea). We then examine correlations between growth achieved during these periods and environmental variation in the North Pacific Ocean. Finally, we discuss the potential demographic consequences of changing age/size of female Chinook salmon in western Alaska.