Assessing Freshwater and Marine Environmental Influences on Life‐Stage‐Specific Survival Rates of Snake River Spring–Summer Chinook Salmon and Steelhead

Abstract

AbstractPacific salmon Oncorhynchus spp. from the Snake River basin experience a wide range of environmental conditions during their freshwater, estuarine, and marine residence, which in turn influence their survival rates at each life stage. In addition, researchers have found that juvenile out‐migration conditions can influence subsequent survival during estuarine and marine residence, a concept known as the hydrosystem‐related, delayed‐mortality hypothesis. In this analysis, we calculated seasonal, life‐stage‐specific survival rate estimates for Snake River spring–summer Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss and conducted multiple‐regression analyses to identify the freshwater and marine environmental factors associated with survival at each life stage. We also conducted correlation analyses to test the hydrosystem‐related, delayed‐mortality hypothesis. We found that the freshwater variables we examined (the percentage of river flow spilled over out‐migration dams and water transit time) were important for characterizing the variation in survival rates not only during freshwater out‐migration but also during estuarine and marine residence. Of the marine factors examined, we found that the Pacific Decadal Oscillation index was the most important variable for characterizing the variation in the marine and cumulative smolt‐to‐adult survival rates of both species. In support of the hydrosystem‐related, delayed‐mortality hypothesis, we found that freshwater and marine survival rates were correlated, indicating that a portion of the mortality expressed after leaving the hydrosystem is related to processes affected by downstream migration conditions. Our results indicate that improvements in life‐stage‐specific and smolt‐to‐adult survival may be achievable across a range of marine conditions through increasing spill percentages and reducing water transit times during juvenile salmon out‐migration.