Daily maximum temperatures affect recovery of Brook char ( Salvelinus fontinalis ) metabolic rate from an acute thermal challenge

The intensity and frequency of warm events are projected to increase with climate warming, imposing significant challenges for ectothermic species. Daily maximum temperature, or Tmax, in salmonid habitats is exceeding the purported upper temperature limits for these species. Recovery from thermal challenges has been thought to be facilitated by cool daily thermal minima (Tmin), with Tmin above 20°C currently triggering the closure of some recreational fisheries. Here, we sought to investigate the relatively poorly understood effect of Tmax on salmonid metabolic rate and recovery. To this end, we acclimated Brook char (Salvelinus fontinalis) to an ecologically relevant diel thermal cycle with Tmin fixed at 19°C and Tmax set to 24°C for a minimum of one month. We then monitored oxygen consumption rate over 4 days throughout the diel thermal cycle. On the 5th day, Tmax was either 24, 26 or 27 °C, and we continued to monitor oxygen consumption during an 18-h recovery period. After exposure to a Tmax of 26 or 27°C and recovery at 19°C, oxygen consumption rate did not recover to resting levels. Tmax appears to have the potential to adversely affect aerobic metabolism and thus is worth considering in the management of salmonid recreational fisheries. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.