Aerobic scope increases throughout an ecologically relevant temperature range in coho salmon

Aerobic scope (AS) has been proposed as a functional measurement that can be used to make predictions about the thermal niche of aquatic ectotherms and hence potential fitness outcomes under future warming scenarios. Some salmonid species and populations, for example, have been reported to exhibit different thermal profiles for their AS curves such that AS peaks around the modal river temperature encountered during the upriver spawning migration, suggesting species- and population-level adaptations to river temperature regimes. Interestingly, some other salmonid species and populations have been reported to exhibit AS curves that maintain an upwards trajectory throughout the ecologically-relevant temperature range rather than peaking at a modal temperature. To shed further light on this apparent dichotomy, we used adult coho salmon (Onchorhynchus kisutch) to test the prediction that peak AS coincides with population-specific, historically experienced river temperatures. We assessed AS at 10°C and 15°C, which represent a typical river migration temperature and the upper limit of the historically experienced temperature range, respectively. We also examined published data on AS in juvenile coho salmon in relation to new temperature data measured from their freshwater rearing environments. In both cases, AS was either maintained or increased modestly throughout the range of ecologically relevant temperatures. In light of existing evidence and the new data presented here, we suggest that when attempting to understand thermal optima for Pacific salmon and other species across life stages, AS is a useful metric of oxygen transport capacity but other thermally-sensitive physiological indices of performance and fitness should be considered in concert.