Wild juvenile Arctic char (Salvelinus alpinus) show seasonal acclimatization of cardiac and whole-animal heat tolerance

Arctic fishes experience extreme seasonality, yet how seasonal acclimatization shapes their ability to cope with thermal variation remains poorly understood. The Arctic char (Salvelinus alpinus) is one Arctic fish of immense cultural, subsistence, and economic value and encounters temperatures ranging from below 0{degrees}C to above 21{degrees}C. Thermal physiology commonly varies across life stages, but has been largely unexamined in wild juvenile Arctic char. We performed winter and summer field-based assessments of wild juvenile Arctic char thermal habitat use, whole-animal and cardiac heat tolerance, and mechanisms shaping the acute thermal sensitivity of heart rate ([f]H). Average body temperature increased from 1.2{degrees}C in winter to 8.9{degrees}C in summer, and critical thermal maximum (CTmax) increased correspondingly from 21.4 to 25.9{degrees}C. We assessed the intrinsic and maximum [f]H ([f]Hint and [f]Hmax) of anaesthetized fish during acute warming using cholinergic blockade (atropine) and either {beta}-adrenergic blockade (sotalol; [f]Hint) or stimulation (isoproterenol; [f]Hmax). [f]H increased with acute warming in both seasons and treatments. However, in summer, [f]H was more responsive to warming (Q10 from 5 to 15{degrees}C: 2.4 vs. 2.0 in winter), reached a higher peak, and continued to increase while maintaining rhythmicity up to higher temperatures, with the onset of heat-induced cardiac arrhythmia (Tarr) occurring at 23.6{degrees}C vs. 18.7{degrees}C in winter. Contrary to expectations, {beta}-adrenergic blockade had little effect on these cardiac parameters. During acute warming, winter-acclimatized fish exhibited slower atrioventricular conduction (longer PR interval) and slower ventricular depolarization (longer QRS duration), which may underlie their lower cardiac heat tolerance and the reduced responsiveness of [f]H to warming. Combined, our results show that wild Arctic char exhibit strong seasonal plasticity of acute thermal performance, demonstrating the value of field-based physiological assessments and the need to consider plasticity in evaluating responses to environmental change.