Black bream CTmax across body mass and populations

Climate change is pushing temperatures towards intolerable limits for many fishes. Fish are ectothermic, or "cold-blooded", meaning that they function within a range of environmental temperatures, and exposure to extreme temperatures can become intolerable. The capacity of ectotherms to tolerate environmental warming has come into focus with ongoing climate change. Yet, we currently lack an understanding of how factors such as body size affects thermal limits, and whether different populations within a species differ in their capacity to cope with warming. To address this research gap, I used black bream (Acanthopagrus butcheri) to test two main hypotheses; first, that population-level differences exist in thermal tolerance, with northern populations being more tolerant of warming; and second; that thermal limits increase with increasing body size. Critical thermal maximum (CTmax) was used as a measure of heat tolerance to address the hypotheses. The first hypothesis was tested by measuring CTmax of fish from four distinct populations; a northern (Moore River), a mid-latitude (Serpentine River) and two southern (Blackwood River and Kalgan River) populations along a 5 degree latitudinal cline of the West Australian coastline. The influence of body mass on CTmax was tested in black bream across a ~500 g (mean = 52.4 g, range = 0.57 – 541 g) mass range. These data provide critical insight into the capacity of black bream to cope with environmental warming. These data highlight the importance of considering various factors when considering a species' vulnerability to climate change that can help prioritise conservation and management actions.