Temperature dependence of fish performance in the wild: links with species biogeography and physiological thermal tolerance

Summary Temperature strongly regulates the distribution and fitness of ectotherms, and many studies have measured the temperature dependence of physiological performance in controlled laboratory settings. In contrast, little is known about how temperature influences ectotherm performance in the wild, so the ecological significance of physiological performance as measured in the laboratory is unclear. Our aim was to measure the temperature dependence of performance in the wild for several species of fishes and to explore how temperatures that maximize performance in the wild ( T optE ) are related to species biogeographies. We gathered body activity and growth data from the wild for nine tropical and temperate fish species, and by fitting thermal performance curves to these data, compared T optE to species‐specific warm range boundary temperatures (the average temperature of the warmest month at equatorward range limits). To explore the degree to which trends in the wild reflect trends in physiological performance measured in the laboratory, we also compiled published data on the temperature dependence of aerobic metabolic scope in fishes and compared these to our wild fish data. We found T optE in the wild was strongly correlated with warm range boundary temperatures, and that the difference between these two temperatures (the ‘environmental heating tolerance’) was smaller for more‐tropical species. Comparison with laboratory data revealed that T optE approaches warm boundary temperatures in the wild at the same rate that the optimal temperature for aerobic scope ( T opt AS ) approaches upper critical temperatures (upper T crit ) for aerobic scope in the laboratory, meaning that environmental heating tolerances in wild fishes closely mirror physiological heating tolerance (i.e. upper T crit − T opt AS ) in captive fishes. Our comparison of field‐ and laboratory‐derived data highlights the ecological significance of aerobic metabolic scope in fishes and suggests wild fish species tend to perform best near the highest temperatures encountered in their range while maintaining a safety margin from the deleterious effects of upper critical temperatures.