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We explored the nature of declines in size and age at maturity in 2 populations of Chinook salmon Oncorhynchus tshawytscha in western Alaska, USA, using multidimensional probabilistic maturation reaction norms (PMRNs) accounting for growth history. Individual growth histories informed by retrospective scale analysis were used to construct PMRNs and to describe the relative influence of different life-history periods on age at maturity. Similar results were found in both populations. Models accounting for growth history uniformly outperformed size-at-age models, suggesting the importance of growth history for the determination of matu ration. The second year of marine growth was found to have a disproportionate influence on the age at maturity in both sexes. Males tended to grow more than females in length during the second year at sea, possibly as a consequence of females storing more energy in preparation for the high cost of female gonad development. Finally, we found that growth thresholds for maturation have shown a long-term decline in both sexes. This suggests that declines in the average age at maturity of western Alaskan Chinook salmon may have been caused in part by adaptation to environmental or fisheries-induced selection.

Changing maturation reaction norms and the effects of growth history in Alaskan Chinook salmon