Abstract Documenting the spatial scale and possible causes of life history variation among populations of coho salmon Oncorhynchus kisutch can improve ecological understanding and guide efforts to conserve potentially adaptive intraspecific diversity. In this paper I use 14 years of spawning ground survey data for 10 populations from the Coos and Coquille basins on the southern Oregon coast to examine interpopulation differences in the proportion of males that spawn as 2-year-old jacks instead of as 3-year-old adults. There were significant between-population differences in the proportions of males spawning as jacks, and mean “jack proportion” was significantly and positively correlated with mean spawner density (fish/km). Two principal components (PC1 and PC2) summarizing 98% of the variation in variables approximating stream level environmental conditions explained 60% of the variation in mean jack proportion and 32% of the variation in mean spawner density. Mean jack proportion was negatively and signi...