Environmental correlates of male life history variation among coho salmon populations from two Oregon coastal basins

Documenting the spatial scale and possible causes of life history variation among populations of coho salmon Oncorhynchus kisutch can improve ecologic al understanding and guide efforts to conserve potentially adaptive intrasp ecific diversity. In this paper I use 14 years of spawning ground survey da ta for 10 populations from the Coos and Coquille basins on the southern Ore gon coast to examine interpopulation differences in the proportion, of male s that spawn as 2-year-old jacks instead of as 3-year-old adults. There wer e significant between-population differences in the proportions of males sp awning as jacks, and mean "jack proportion" was significantly and positivel y correlated with mean spawner density (fish/km). Two principal components (PCI and PC2) summarizing 98% of the variation in variables approximating s tream level environmental conditions explained 60% of the variation in mean jack proportion and 32% of the variation in mean spawner density. Mean jac k proportion was negatively and significantly related to PCI, which loaded positively for stream gradient, stream elevation, and smelt and adult migra tion distance, and to PC2, which loaded positively for distance and negativ ely for gradient. Mean spawner density was similarly, though not significan tly, related to the two principal components. The coefficient of variation for jack proportion was negatively and significantly correlated with mean j ack proportion and mean spawner density. Higher-density populations consist ently had a higher proportion of spawning males returning as jacks. Because jacks provide the only gene flow between otherwise isolated brood years an d may buffer populations from severe brood year loss due to environmental s tochasticity, the frequency of this life history strategy critically affect s the genetic structure and ecological function in regional coho salmon pop ulations. Explicitly considering the genetic and ecological role of jacks s hould help improve conservation efforts and future decisions regarding the status of regional coho salmon populations.