Aquatic ectotherms are hypothesized to be vulnerable to warming and deoxygenation associated with environmental change because temperature and oxygen (O2) supply can restrict aerobic scope (AS) in captivity. However, evidence of a direct association between AS and fitness in the wild is lacking, inspiring debate about the circumstances under which AS is the primary driver of population fluctuations. Using the metabolic index (ɸ), we related AS to two Chinook salmon (Oncorhynchus tshawytscha) population bottlenecks in the wild, juvenile rearing and migration. We found that AS governed success probability for these bottlenecks only under a relatively narrow window of viable environmental conditions, depending on intraspecific metabolic trait diversity and hydrologic conditions. Opportunities for high-impact temperature- and O2-specific conservation and management actions using existing hydraulic engineering infrastructure therefore exist when AS is between critical (ɸcrit) and stable (ɸstable) values. Outside of this ecological threshold, increases in AS did not yield appreciable fitness benefits because successful rearing and migration were either exceptionally improbable (i.e., ASɸstable). In addition, AS impairments likely increased susceptibility to predation, and this may have been involved in the putative association between AS and fitness in the wild. We propose that ɸstable may serve as a more conservative benchmark than ɸcrit to prevent extirpations and recover imperiled populations under a changing climate.

Funding provided by: United States Bureau of ReclamationROR ID: https://ror.org/00ezrrm21Award Number: R18AP00136 Funding provided by: United States Bureau of ReclamationROR ID: https://ror.org/00ezrrm21Award Number: R21AC10455