Using aerobic exercise to evaluate sub-lethal tolerance of acute warming in fishes

We investigated whether fatigue from sustained aerobic swimming provides a sublethal endpoint to define tolerance of acute warming in fishes, as an alternative to loss of equilibrium (LOE) during a critical thermal maximum protocol (CTmax). Two species were studied, Nile tilapia Oreochromis niloticus and pacu Piaractus mesopotamicus. Each fish underwent an incremental swim test to determine gait transition speed (UGT), where it first engaged the unsteady anaerobic swimming mode that preceded fatigue. After suitable recovery each fish was swum at 85% of their own UGT and warmed 1°C every 30 min, to identify the temperature at which they fatigued, denoted as CTswim. Fish were also submitted to a standard CTmax, warming at the same rate as CTswim, under static conditions until LOE. All individuals fatigued in CTswim, at a mean temperature approximately 2°C lower than their CTmax. Therefore, if exposed to acute warming in the wild, the ability to perform aerobic metabolic work would be constrained at temperatures significantly below those that directly threatened survival. The collapse in performance at CTswim was preceded by a gait transition qualitatively indistinguishable from that during the incremental swim test. This suggests that fatigue in CTswim was linked to an inability to meet the tissue oxygen demands of exercise plus warming. This is consistent with the oxygen and capacity limited thermal tolerance (OCLTT) hypothesis, regarding the mechanism underlying tolerance of warming in fishes. Overall, fatigue at CTswim provides an ecologically relevant sub-lethal threshold that is more sensitive to extreme events than LOE at CTmax.本研究旨在探究持续有氧游泳产生的疲劳能否作为鱼类急性热耐受的亚致死终点，以替代临界热极限（CTmax）实验中的平衡丧失（LOE）指标。本研究选取两种受试鱼类：尼罗罗非鱼（Oreochromis niloticus）与锯腹脂鲤（Piaractus mesopotamicus）。每尾受试鱼均接受渐进式游泳测试以确定其步态转换速度（UGT），该速度为鱼类首次切换至疲劳前的非稳态厌氧游泳模式的临界速度。经过充分恢复后，每尾鱼以自身UGT的85%强度进行游泳，并以每30分钟升温1℃的速率进行热暴露，直至其出现疲劳，此时对应的温度记为CTswim。同时，每尾鱼还接受标准临界热极限（CTmax）实验：在静态条件下以与CTswim实验相同的升温速率进行热暴露，直至其出现平衡丧失（LOE）。所有受试鱼均在CTswim实验中出现疲劳，其疲劳时的平均体温较CTmax实验时低约2℃。因此，若野生鱼类遭遇急性热暴露，其有氧代谢做功能力将在远低于直接威胁生存的温度下就受到限制。CTswim实验中游泳性能崩溃前出现的步态转换，与渐进式游泳测试中的步态转换在定性层面无显著差异。这表明CTswim实验中的疲劳与机体无法满足运动与热暴露共同带来的组织氧需求相关，该结果与阐释鱼类耐热性机制的氧容量限制性热耐受（OCLTT）假说相符。综上，CTswim实验中的疲劳可作为一个具有生态学相关性的亚致死阈值，相较于CTmax实验中的LOE，该阈值对极端热事件更为敏感。