We used coded wire tag data to compare spawner age structure and seasonal patterns of age-specific size at date among fish harvested in the ocean from the four seasonal run timings (fall, late-fall, winter, and spring) of Chinook Salmon Oncorhynchus tshawytscha from the Central Valley, California, and we examined differences between the fall-run fish (the most abundant run) fromthe Sacramento and San Joaquin River basins. The runs varied in their ocean size at a common age and date, and within each run, monthly mean ocean sizes appeared to stop increasing when spawners began to return to freshwater. Despite support for multiple hypotheses, no single factor explained all of the variation among and within runs. Ocean size at a common date was well explained by a “juvenile head-start” hypothesis, predicting larger sizes for the spring and fall runs due to earlier ocean entry. Month of spawner return was well explained by a “premature adult migration” hypothesis, predicting earlier returns (within years, regardless of age) by winter- and spring-run fish spawning further upstream. However, neither release timing nor spawning elevation could fully explain observed patterns in spawner age structure, such as an unusually high occurrence of age-2 San Joaquin River fall-run spawners and the near absence of age-4 or older spawners in the winter run. Larger smolt size might explain earlier maturation by the San Joaquin versus SacramentoRiver fall run, but smolt size could not explain patterns in age structure across runs.Metabolic costs of holding upstream with large size might explain the lack of older spawners among the winter run but are inconsistent with the late-fall run having the highest frequency of age-4 and older spawners. Our results demonstrate multiple pathways by which differences both within and among the runs may contribute to differences in their fishery vulnerability and demographic decoupling, which could contribute to a stabilizing portfolio effect.
