Lives of the Snake River Steelhead Trout Populations

Bimodal run timing of summer steelhead was observed in the Columbia basin historically. Snake River steelhead primarily returned after August 25. Fish in the early mode (A-run) tended to be younger and smaller (<77.5 cm) than later fish (B-run). B-run fish supposedly spawn only in certain Snake River tributaries, whereas A-run fish spawn throughout the Columbia basin. But it is still unclear how life history, body size, and spawning distribution correlate to the bimodal migration. We analyzed traits in 16 locations across the Snake River basin. All locations produced smaller adults and later returns. Median lengths of B-run populations were close to the defining criterion. In contrast, A-run populations produced few >77.5 cm. Average 2-ocean proportion was 52.1% for A-run and 82.0% for B-run populations. Freshwater and saltwater ages were positively correlated; average age at spawning is greater in populations with older smolts. Sex ratio was female biased but older populations have more females. We conclude 1) the A/B dichotomy is a gradient with characteristic endpoints, 2) life histories are becoming more homogenous as A-run timing becomes later and B-run size becomes smaller, 3) the A/B dichotomy masks important diversity in terms of age structure, sex ratio, and spawn timing.