Evaluating Surrogacy of Hatchery Releases for the Performance of Wild Yearling Chinook Salmon from the Snake River Basin

Abstract

The combined juvenile and adult detections of Snake River yearling Chinook salmon Oncorhynchus tshawytscha implanted with passive integrated transponder tags migrating through the hydroelectric facilities in the Federal Columbia River Power System were analyzed using the ROSTER statistical release–recapture model. This model was used to estimate the downriver survival of smolts, ocean survival, adult passage success, and smolt-to-adult ratios (SARs) for wild and hatchery-reared Chinook salmon released as yearlings in the Snake River basin from 1996 through 2004. Estimates from wild and hatchery release groups were compared to assess the extent to which hatchery stocks may be used as surrogates for endangered wild stocks. Wild release groups included both spring and summer Chinook salmon runs, while hatchery release groups were separated by run (spring versus summer). Overall, there was a significant difference between estimates from wild release groups and hatchery spring-run release groups for all survival measures except ocean survival but not between wild release groups and hatchery summer-run release groups. Estimates of adult passage success and SAR were significantly higher for wild groups than for hatchery spring groups. There was high correlation (r > 0.8) between estimates from wild fish and both spring and summer hatchery fish for all measures except adult passage success within a calendar year. It appears that hatchery summer Chinook salmon stocks from the Snake River basin may be used as surrogates for wild stocks for measuring survival on large temporal and spatial scales (e.g., annual basinwide measures). Because of differences between survival estimates from wild fish and hatchery spring fish, hatchery spring Chinook salmon may be used as an index for trends in the survival of wild stocks but not as direct surrogates.