Acute stress-induced mortality in big Atlantic salmon at high temperatures is associated with insufficient oxygen uptake capacity

Thermal studies on fish can help us to understand their robustness to warming climates. Most experiments are performed on smaller individuals and may not represent larger life-stages owing to physiological scaling effects, particularly with regards to thermal tolerance and respiratory capacities. In this study, respirometry experiments were performed on big Atlantic salmon (Salmo salar) (≈4 kg) following 3-weeks acclimation to seawater of 9 °C or 19 °C. Additionally, gill and heart morphology traits were assessed. At 9 °C metabolic rates resembled earlier work on smaller fish. However, at 19 °C following stress exposure, 81 % died unexpectedly within ≈6 h while surviving fish struggled to recover a baseline metabolic rate. Most noteworthy was that maximum metabolic rates remained similar across temperature whereas smaller Atlantic salmon previously were found to increase their maximum metabolic rates until near-lethal temperatures. As standard metabolic rates also inevitably increases with temperature, aerobic scopes become reduced at 19 °C. Meanwhile lamellar density was unaffected, indicating similar gill surface areas. However, acclimation to 19 °C reduced ventricle roundness and symmetry, while bulbus width to ventricle width ratios increased. These changes presumably reflect adaptive responses to more metabolically demanding environments. Yet the fish appeared unable to supply sufficient oxygen at 19 °C during stress, which we attribute to physiological scaling constraints. Big Atlantic salmon were therefore more susceptible to stress-induced mortality at elevated temperatures, indicating reduced thermal tolerance relative to smaller individuals. This highlights the need to include larger fish in experiments as the underlying basis for thermal tolerance changes across large differences in body size.