Temperature‐mediated en route migration mortality and travel rates of endangered Snake River sockeye salmon

Abstract

Abstract Conservation efforts for endangered Snake River sockeye salmon (Oncorhynchus nerka) have been hindered by high en route adult mortality during their ∼1450 km freshwater spawning migration. Identifying causal factors for this mortality has been difficult given very small (often <10 fish) annual returns in recent decades. However, several hundred hatchery‐bred fish returned in 2000 and we intercepted and radio‐tagged 31 in mid‐migration to monitor behaviours and survival. All fish initially migrated at similar rates, but later‐timed fish eventually slowed migration and were far more likely to be unsuccessful. Late‐season mortality was strongly associated with water temperatures near tolerance thresholds (21–24 °C). The data also suggest increased risk for fish in poor initial condition (i.e., with injuries or parasites) and probable recent selection against late‐timed salmon. Results parallel temperature‐ and condition‐related adult mortality in Columbia and Fraser River sockeye salmon populations and demonstrate the potential vulnerability of marginal southern populations to regional climate warming.