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.

我们探究了持续有氧游泳引发的疲劳能否作为一种亚致死终点指标，用以界定鱼类对急性升温的耐受能力，以此作为临界热最大值（critical thermal maximum, CTmax）实验中失去平衡（loss of equilibrium, LOE）的替代方案。本研究选取两种鱼类作为实验对象：尼罗罗非鱼（Oreochromis niloticus）与巴拉圭巨脂鲤（Piaractus mesopotamicus）。每尾实验鱼均接受渐进式游泳测试，以确定其步态转换速度（gait transition speed, UGT）——此时鱼类首次进入疲劳前的非稳态厌氧游泳模式。经过充分恢复后，每尾鱼以自身UGT的85%强度进行游泳，并每30分钟升温1℃，以此确定其发生疲劳时的温度，记为CTswim。同时，每尾鱼还需接受标准临界热最大值（CTmax）实验：在静态条件下以与CTswim实验相同的升温速率升温，直至鱼类出现失去平衡（LOE）的现象。所有个体均在CTswim实验中发生疲劳，其疲劳时的平均温度较CTmax实验低约2℃。由此可见，若野生鱼类遭遇急性升温，其有氧代谢工作能力将在远低于直接威胁生存的温度下受到限制。CTswim实验中运动能力崩溃前的步态转换，与渐进式游泳测试中的步态转换在定性上无显著差异。这表明CTswim实验中的疲劳与鱼类无法满足运动与升温共同带来的组织氧需求密切相关。该结果与阐释鱼类升温耐受机制的氧容量限制热耐受（oxygen and capacity limited thermal tolerance, OCLTT）假说相符。综上，CTswim实验中的疲劳阈值是一个具有生态相关性的亚致死临界值，相较于CTmax实验中的LOE，该阈值对极端升温事件的响应更为敏感。