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

Abstract

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 max) 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 (U GT), where it first engaged the unsteady anaerobic swimming mode that preceded fatigue. After suitable recovery, each fish was exercised at 85% of their own U GT and warmed 1°C every 30 min, to identify the temperature at which they fatigued, denoted as CT swim. Fish were also submitted to a standard CT max , warming at the same rate as CT swim , under static conditions until LOE. All individuals fatigued in CT swim , at a mean temperature approximately 2°C lower than their CT max. 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 swim was preceded by a gait transition qualitatively indistinguishable from that during the incremental swim test. This suggests that fatigue in CT swim 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 swim provides an ecologically relevant sub-lethal threshold that is more sensitive to extreme events than LOE at CT max .

Intraspecific variation in tolerance of warming in fishes

“…The protocol is simple and defines the temperature where survival is threatened because at LOE the fish cannot escape the conditions (Beitinger & Lutterschmidt, 2011). An alternative is the critical threshold temperature for fatigue from swimming (CT swim ), which may have greater ecological relevance because it defines the temperature where fish can no longer perform an ecologically essential activity (Figure 1), but this protocol has not yet been applied widely (Blasco et al ., 2020b). Lying inside a Fry‐TTP are more restricted zones (Figure 1) that are delimited by temperature‐dependent effects on the performance of activities that are essential for growth and reproduction (Brett, 1971; Cossins & Bowler, 1987; Currie & Schulte, 2014; Schulte et al ., 2011).…”

“…The critical threshold temperature for fatigue from swimming (CT swim ) is an alternative and potentially more ecologically relevant protocol. It involves imposing a fixed level of steady and sustained aerobic exercise upon a fish, in a swim flume, then warming (or cooling) the fish in steps until it fatigues (Blasco et al ., 2020b; Steinhausen et al ., 2008). Maximum CT swim occurs at a lower temperature than CT max (Blasco et al ., 2020b) so a Fry‐TTP derived with a CT swim protocol would lie inside of one derived by classic CT protocol.…”

Population variation in the thermal response to climate change reveals differing sensitivity in a benthic shark

“…Ultimately though, as conditions worsen, swimming activity eventually declines—regardless of motivational state. While the mechanisms behind thermal tolerance are still under active debate (Farrell, 2016; Jutfelt et al., 2018; Lefevre, 2016; Schulte, 2015), there is some evidence that under acute warming, declines in activity, specifically swimming, could be due to the inability to meet the tissue oxygen demand required for activity and warming (Blasco et al., 2020).…”

“…Furthermore, with increased energetic requirements imposed by rising temperature, both prey encounter rate and capture success can increase concomitantly with velocity (Nemeth, 1997;Papastamatiou et al, 2018;Shipley et al, 1996), which can potentially mitigate some energetic limitations imposed by temperature. (Farrell, 2016;Jutfelt et al, 2018;Lefevre, 2016;Schulte, 2015), there is some evidence that under acute warming, declines in activity, specifically swimming, could be due to the inability to meet the tissue oxygen demand required for activity and warming (Blasco et al, 2020).…”

Aerobic swimming in intensive finfish aquaculture: applications for production, mitigation and selection

“…The capacity of fish to meet the costs of aerobic swimming is of course finite. The species‐specific maximum capacity for aerobic swimming occurs at the maximum sustainable swimming speed ( U ms ), which is taken to mean a speed that can be sustained for hours or even days without fatigue (Beamish, 1978; Videler, 1993; Blasco et al, 2020). The critical swimming speed ( U crit ) has often been considered a relatively close approximation of U ms in fish (Hammer, 1995).…”