Upper thermal limits are ‘hard-wired’ across body mass but not populations of an estuarine fish

Climate warming is seeing temperatures breach exceptional thresholds as the frequency and intensity of heat waves increase. Efforts to forecast species vulnerability to climate warming often focus on upper thermal limits threatening survival, overlooking the role of intraspecific variation in determining vulnerability. Using an estuarine fish (black bream, Acanthopagrus butcheri) as a model, we explore how intraspecific variation in body mass and among populations affects upper thermal tolerance. Upper thermal limits were quantified using critical thermal maxima (CTmax) of wild fish. We used a ∼500 g (mean = 52.4 g, range = 0.57-541 g) mass range to test the relationship between body mass and thermal tolerance. Four distinct black bream populations were chosen along a 5° latitudinal cline to explore population differences in thermal limits. Contrary to expectations, there was no effect of body mass on upper thermal limits. However, significant population differences in thermal tolerance were observed that correlate with mean habitat temperatures. Specifically, the southern population had a significantly lower CTmax (35.57 ± 0.43 °C) compared to northern (36.32 ± 0.70 °C) and mid-latitude (36.36 ± 1.15 °C) populations. These data underscore the importance of observing intraspecific variation in thermal limits to reveal the capabilities of individuals within a species to cope with climate warming and improve the management of at-risk life stages and populations.