Tolerance of an acute warming challenge declines with size in Nile tilapia: evidence of a link to capacity for oxygen uptake
It has been 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 inde-pendent of mass at an acclimation temperature of 26 °C. Tolerance of acute warming, however, declined significantly with mass when evaluated as the critical temperature for fatigue from aerobic swimming (CTSmax). The CTSmax 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. CTSmax elicited pronounced increases in oxygen uptake but maximum rates achieved prior to fatigue declined very significantly with mass. Mass-related variation in CTSmax and maximum oxygen uptake rates were positively correlated, which indicates a caus-al relationship. When fish populations are faced with acute thermal stress, larger indi-viduals may become constrained in their ability to perform aerobic activities at lower temperatures than smaller conspecifics. This could affect survival and fitness of larger fish in a world with more frequent and extreme heatwaves, with consequences for population productivity. Methods The data were collected on 18 tilapia acclimated to 26 °C, swimming respirometry was used to calculate standard and active metabolic rates, to calculate aerobic scope by difference. CTmax was measured by warming groups of animals in a tank until loss of equilibrium, CTSmax was measured by warming tilapia when they were swimming, until they fatigued. Data are calculated values, not raw oxygen data (for example). Also included are data for swimming performance at acclimation temperature, namely maximum aerobic speed, absolute maximum speed and the gait transition speed between these. Further, the measures of oxygen uptake during incremental warming in CTSmax are carried for each fish studied.