Sea-to-sea survival of late-run adult steelhead (Oncorhynchus mykiss) from the Columbia and Snake rivers

Abstract

We used biotelemetry and genetic stock identification to assess sea-to-sea survival and run composition of 1212 late-migrating adult steelhead (anadromous Oncorhynchus mykiss) through the Columbia River and Snake River migratory corridors. The late run was predominated by steelhead from Idaho’s Clearwater and Salmon rivers that must pass eight large hydroelectric dams during both prespawn and postspawn migrations. In 2 years (2013 and 2014), prespawn survival to Snake River tributaries (>500 km) was 0.48–0.67 for the most abundant populations and was higher for females and 1-sea fish (i.e., fish that spend one winter at sea). Annual survival from Snake River tributary entry to postspawn kelt status was 0.14–0.17, with higher survival for females and those without hatchery fin clips. Kelt outmigration survival was 0.31–0.39 past four Snake River dams and 0.13–0.20 past all eight dams and was highest for smaller kelts. Full-cycle adult freshwater survival (sea-to-sea) including 16 dam passage events was 0.01–0.02. Younger steelhead and those without fin clips survived at the highest rates. This study uniquely partitioned mortality across prespawn, reproductive, and kelt life history stages and informs management strategies for this conservation-priority metapopulation.