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

We investigated whether fatigue from sustained aerobic swimming provides a sub-lethal endpoint to define tolerance of acute warming in fishes, as an alternative to loss of equilibrium (LOE) during a critical thermal maximum (CT) protocol. Two species were studied, Nile tilapia () and pacu (). Each fish underwent an incremental swim test to determine gait transition speed (), where it first engaged the unsteady anaerobic swimming mode that preceded fatigue. After suitable recovery, each fish was exercised at 85% of their own and warmed 1°C every 30 min, to identify the temperature at which they fatigued, denoted as CT Fish were also submitted to a standard CT, warming at the same rate as CT, under static conditions until LOE. All individuals fatigued in CT, at a mean temperature approximately 2°C lower than their CT 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 CT was preceded by a gait transition qualitatively indistinguishable from that during the incremental swim test. This suggests that fatigue in CT 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 CT provides an ecologically relevant sub-lethal threshold that is more sensitive to extreme events than LOE at CT.