Determinants of life history variability in the chinook salmon (Oncorhynchus tshawytscha) of Western Alaska

Abstract

Chinook Salmon from western Alaska have experienced recent declines in abundance, size, and age at maturity. Declines have led to hardships for the region's dependent subsistence and commercial users. Thus there is a managerial need to better understand factors effecting life-history expression in these populations. I used retrospective scale analysis and run reconstructions to investigate the causes of declines in age at maturity and the effect of the marine environment on growth, maturation, and survival in two western Alaskan Chinook Salmon populations subject to long-term monitoring: the East Fork Andreafsky River and the Kogrukluk River (tributaries of the Yukon River and Kuskokwim River respectively). The expression of age at maturation exhibited sex-specific responses to variability in growth. Additionally, thresholds for maturation, as described by a newly presented measure of maturation reaction norms that accounts for growth history, were found to have declined in both sexes. This can be interpreted as indirect evidence that observed declines in age at maturity represent an evolutionary response. I also found that sea surface temperatures in the Bering Sea exert strong control on the expression of life history variability. Warmer sea surface temperatures appear to lead to a younger age at maturity, largely through the vector of augmented growth. However, warmer sea surface temperatures additionally appeared to decrease the average age of male recruits by lowering growth thresholds for early male maturation. Despite the demonstrated relationship between Bering Sea surface temperatures and age at maturation, a lack of a temporal trend in sea surface temperatures during the period of analysis (1977-2013) suggests that temperature alone cannot explain documented declines in average age. However, this result suggests that the average age at maturation of western Alaskan Chinook Salmon will continue to decline with future predicted warming of the Bering Sea as a consequence of climate change.