Evidence that tolerance of acute warming declines with increasing body mass within fish species

It has been proposed that larger individuals within fish species may be more sensitive to warm temperatures because of limitations in their capacity to provide oxygen for aerobic metabolic activities. We investigated this in four populations: captive European seabass (Dicentrarchus labrax) and Nile tilapia (Oreochromis niloticus), and wild-caught European seabass and gilthead seabream (Sparus aurata), spanning an ∼10-fold body mass range (20-800 g). Tolerance of acute warming declined significantly with mass in all populations, when evaluated as the temperature of fatigue from sustained aerobic exercise, using the critical thermal maximum for swimming (CTSmax) protocol. Mass-specific maximum rates of oxygen uptake achieved prior to fatigue (ṀO2,max-heat) declined very significantly with mass, and were positively correlated with variation in CTSmax in all populations, which could indicate a causal relationship. In some individuals, however, CTSmax did not require their full cardiorespiratory capacity because ṀO2,max-heat was lower than active metabolic rate (AMR) at acclimation temperature. Mass-independent ṀO2,max-heat and AMR scaled in a similar manner with body mass, indicating that the CTSmax protocol did not reveal a greater size-related cardiorespiratory limitation than was observed at benign environmental temperatures. Therefore, further work is required to understand the mechanisms underlying the mass-related decline in tolerance of warming. Nonetheless, larger individuals within both farmed and wild fish populations appear to be more sensitive to acute heat stress than smaller conspecifics. This could have implications for size distributions of fish populations in a world with more frequent and extreme heatwaves.