Linking aerobic scope to fitness in the wild reveals potential opportunities to help recover imperiled salmon populations

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 respirometry data, telemetry studies, long-term population monitoring, and in situ predator-prey experiments, we related AS to two Chinook salmon (Oncorhynchus tshawytscha) population bottlenecks in the wild, juvenile rearing and migration. We found that AS, which we quantified using the metabolic index (ɸ), was associated with 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 potentially high-impact temperature- and O₂-specific conservation and management actions using existing hydraulic engineering infrastructure could therefore exist when AS is between critical (ɸ_crit) and stable (ɸ_stable) values. Outside of this ecological threshold, changes in AS did not yield appreciable fitness benefits because successful rearing and migration were either exceptionally improbable (i.e., AS<ɸ_crit), or seemingly independent of AS (i.e., AS>ɸ_stable). 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.