The Influence of Individual Fish Characteristics on Survival and Detection: Similarities across Two Salmonid Species

Abstract

Trait-selective mortality is of considerable management and conservation interest, especially when trends are similar across multiple species of conservation concern. In the Columbia River basin, thousands of juvenile Pacific salmonids Oncorhynchus spp. are collected each year and are tagged at juvenile bypass system (JBS) facilities located at hydroelectric dams, thus allowing the tracking of population-level performance metrics (e.g., juvenile survival and juvenile-to-adult survival). Several studies have suggested that juvenile salmonid survival is both size dependent and condition dependent, but little is known about trait-selective collection at JBS facilities. Traitselective collection (e.g., length-based or condition-based selectivity) is particularly important, as inferences to population-level performance metrics may be biased if both the survival and collection processes are influenced by similar characteristics. We used a capture-mark-recapture study to investigate length-and condition-selective survival and detection probabilities for two salmonid species in the Columbia River basin. In 2014, juvenile steelhead O. mykiss (n D 11,201) and yearling Chinook Salmon O. tshawytscha (n D 7,943) were PIT-tagged, measured (FL), examined for external condition characteristics (descaling, body injuries, fin damage, or disease symptoms), and released into the Lower Granite Dam JBS facility on the Snake River to continue seaward migration. Results indicated similar trends in both length-and condition-selective juvenile survival and detection probabilities. For both species, survival probability was higher for longer, nondegraded individuals (those without descaling, body injuries, or fin damage). Trends in detection probability were also consistent across species: shorter, degraded individuals were more likely to be detected at downstream JBS facilities than longer, healthier individuals. These results suggest that similar characteristics (FL and external condition) affect survival and detection processes for PIT-tagged steelhead and yearling Chinook Salmon and that JBS facilities may selectively

Associations among Fish Length, Dam Passage History, and Survival to Adulthood in Two At‐Risk Species of Pacific Salmon

“…Our results confirm those of Zabel et al (2005) and Hostetter et al (2015), who found that length was an important predictor of bypass probability for Snake River Chinook Salmon and steelhead at Snake River dams, with smaller fish being more likely to enter a bypass system. Brown et al (2013) also found that the bypass probability of yearling Chinook Salmon released from Lower Granite Dam decreased with increasing FL on average across multiple dams.…”

“…() and Hostetter et al. (), who found that length was an important predictor of bypass probability for Snake River Chinook Salmon and steelhead at Snake River dams, with smaller fish being more likely to enter a bypass system. Brown et al.…”

“…; Hostetter et al. ), and smaller fish and fish in poorer condition also have a lower return probability (Ward and Slaney ; Zabel and Williams ; Evans et al. ).…”

“…Another possible explanation for reduced return probabilities of bypassed fish is that smaller fish and those in poorer condition tend to enter bypass systems with a higher probability (Zabel et al 2005;Hostetter et al 2015), and smaller fish and fish in poorer condition also have a lower return probability (Ward and Slaney 1988;Zabel and Williams 2002;Evans et al 2014). This suggests that the apparent effects of bypass on return probability could be at least partly due to a correlation between bypass probability and fish size and condition and not due to bypass passage itself.…”

Combining Migration History, River Conditions, and Fish Condition to Examine Cross‐Life‐Stage Effects on Marine Survival in Chinook Salmon

“…Various mechanisms have been proposed to explain the observed patterns of D, including differences between B fish and R fish in terms of physiological readiness for seawater entry (Schreck et al 2006;Anderson et al 2012) and in traits such as fish size and physical injury, which may be subject to selectivity in the bypass system that is used to collect fish for barging Hostetter et al 2015). Evaluation of these and other hypotheses has been difficult because of the limited and mostly indirect information on the physical condition of B fish and R fish as they enter the marine environment.…”