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Recent poor returns of Chinook salmon to the Yukon-Kuskokwim delta area of western Alaska have coincided with a trend toward decreasing size and age of adults. However, the degree to which changing size is caused by size-selective harvest or variation in growing conditions is unknown. In this study, we investigated changing growth and maturity in the females of two populations of western Alaskan Chinook salmon, the Andreafsky River (Yukon River drainage) and the Kogrukluk River (Kuskokwim River drainage) using scales collected over 31 brood years (1975–2005). The second year of marine growth (SW2) best predicted maturity in female Chinook salmon, with individuals showing greater SW2 growth more likely to mature earlier. Annual variation in average SW2 growth was associated with the Western Pacifi c Index, consistent with the western distribution of these Chinook salmon stocks during their second year of marine life; however, over half of the variation in mean SW2 growth was left unexplained by our environmental models. Temporal trends were the strongest for freshwater growth. We hypothesize that this may be partly explained by declining size of adult female salmon through negative eff ects on egg size and size of emergent fry.

Changing Growth and Maturity in Western Alaskan Chinook Salmon, Oncorhynchus tshawytscha, Brood Years 1975–2005