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

Felipe R Blasco,Shaun S Killen,David J Mckenzie,Francisco Tadeu Rantin,Edwin W Taylor,Andrew J Esbaugh

doi:10.1242/jeb.218602

Abstract

ABSTRACTWe 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 (CTmax) protocol. 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 exercised 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.

Highlights

Mean standard metabolic rate (SMR) was lower in pacu than in tilapia, but there were no differences in Active metabolic rate (AMR) or aerobic scope (AS) (Table 1)
Tolerance of acute warming: CTmax for swimming (CTswim) versus critical thermal maximum (CTmax) At their acclimation temperature, all fishes swam using a steady aerobic gait for at least 9 h at 85% of their UGT without any sign whatsoever of fatigue
During the CTswim protocol, all individual fishes swam using the steady aerobic gait until a certain warm temperature, at which they started to engage unsteady burst and coast anaerobic swimming, which was followed by fatigue with the fish falling back against the rear screen or refusing to swim

Summary

INTRODUCTION

Aquatic habitats are undergoing increasingly rapid human-induced environmental change, especially in response to climate warming. If limitations to tissue oxygen supply do underlie a collapse in performance during warming, this could be tested by imposing a level of aerobic metabolic work upon a fish and measuring their capacity to sustain it while their overall tissue oxygen demands are being accelerated by progressive increases in temperature. We expected to gain qualitative evidence that the collapse in performance at CTswim was linked to an inability to meet tissue oxygen demand, in particular that fatigue was preceded by a gait transition from steady aerobic to unsteady anaerobic swimming. This would indicate that the CTswim protocol could be useful in testing the expectations of the OCLTT paradigm. We measured rates of oxygen uptake during the initial swim test and during the CTswim, to gain insight into how acute warming influenced overall capacity for oxygen supply, relative to capacity at the initial acclimation temperature

MATERIALS AND METHODS

RESULTS

DISCUSSION