Abstract
It is proposed that larger individuals within fish species may be more sensitive to global warming, due to limitations in their capacity to provide oxygen for aerobic metabolic activities. This could affect size distributions of populations in a warmer world but evidence is lacking. In Nile tilapia Oreochromis niloticus (n=18, mass range 21-313g), capacity to provide oxygen for aerobic activities (aerobic scope) was independent of mass at acclimation temperature (26°C). Tolerance of acute warming, however, declined significantly with mass when evaluated as critical temperature for fatigue from aerobic swimming (CT swim ). The CT swim protocol challenges a fish to meet the oxygen demands of constant intense aerobic exercise while their demands for basal metabolism are accelerated by incremental warming, culminating in fatigue. CT swim elicited pronounced increases in oxygen uptake but maximum rates achieved prior to fatigue declined very significantly with mass. Mass-related variation in CT swim and maximum oxygen uptake rates were positively correlated, which may indicate a causal relationship. When faced with acute thermal stress, larger fishes within populations may become constrained in their ability to swim at lower temperatures than smaller con-specifics. This could affect survival and fitness of larger fish in a world with more frequent and extreme heatwaves, with consequences for population productivity.