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.

气候变化正将水温推向许多鱼类难以承受的极限。鱼类属于变温动物（ectothermic），即‘冷血动物’，这意味着它们的生理机能仅能在特定环境温度范围内正常运作，暴露于极端温度下会难以承受。随着气候变化的持续，变温动物耐受环境变暖的能力已成为研究焦点。然而，目前我们尚不明确体型等因素如何影响热极限，以及同一物种内不同种群应对变暖的能力是否存在差异。为填补这一研究空白，本研究以黑鲷（Acanthopagrus butcheri）为对象，验证两个核心假设：其一，热耐受性存在种群间差异，北部种群对变暖的耐受性更强；其二，热极限随体型增大而提高。本研究采用临界热最大值（Critical Thermal Maximum, CTmax）作为热耐受性的衡量指标，以验证上述假设。为验证第一个假设，研究测量了西澳大利亚海岸线上沿5度纬度梯度分布的四个不同种群黑鲷的CTmax：一个北部种群（摩尔河）、一个中纬度种群（蛇河）以及两个南部种群（布莱克伍德河和卡尔甘河）。为验证体型对CTmax的影响，研究选取了体重范围约500克（均值52.4克，范围0.57–541克）的黑鲷进行测试。这些数据为理解黑鲷应对环境变暖的能力提供了关键洞见，同时强调了在评估物种对气候变化的脆弱性时需考虑多种因素的重要性，这有助于优先制定保护与管理措施。