The Influence of Individual Fish Characteristics on Survival and Detection: Consistent Trends Across Two Salmonid Species

Trait-selective mortality is of considerable management and conservation interest, especially when trends are consistent across multiple species of conservation concern. We used a capture-mark-recapture study to investigate length- and condition-selective survival and detection probabilities across two salmonid Oncorhynchus species during out-migration through a section of the Federal Columbia River Power System. Juvenile steelhead (n = 11,201) and Chinook (n = 7,943) were tagged (passive integrated transponder [PIT] tag), measured (fork length), condition-scored (descaling, body injuries, fin damage, disease symptoms), and released into the Lower Granite Dam juvenile bypass system facility on the Snake River in 2014. Results indicated a consistent trend of both length- and condition-selective out-migration survival and detection probabilities. For both steelhead and yearling Chinook, survival probabilities were higher for longer, non-degraded individuals. Trends in detection probability were also consistent across species, where shorter, degraded individuals were more likely to be detected at downstream juvenile bypass system facilities compared to longer, healthier individuals. These results indicate that similar characteristics (length and condition) affect both the survival and detection processes for PIT-tagged steelhead and Chinook in the Columbia River basin and suggest that juvenile bypass system facilities may selectively collect smaller, degraded individuals that have lower survival probabilities.