Arctic lake trout (Salvelinus namaycush) show evidence of seasonal acclimation of cardiac adrenergic sensitivity but not heat tolerance.

Many Arctic fishes experience prolonged periods of extreme cold and large thermal variation over both rapid and seasonal time scales which challenge critical physiological functions. In the central Canadian Arctic, we caught wild adult lake trout (Salvelinus namaycush) acclimatized to winter and summer temperatures to determine the extent to which they seasonally adjust cardiac thermal performance and adrenergic control. We assessed the intrinsic and maximum heart rate (f_Hint and f_Hmax) of anaesthetised fish through cholinergic blockade and either adrenergic blockade (f_Hint) or stimulation (f_Hmax) during acute warming. Contrary to expectations, S. namaycush showed largely consistent heart rate responses to acute warming between seasons and adrenergic treatments. The f_Hmax increased with acute warming and reached a peak (peak f_H) of 83 and 93 beats min^-1 at temperatures (T_peak) of 19.6 and 20°C and arrhythmia occurred at temperatures (T_arr) of 22.7 and 22.3°C in winter and summer, respectively. However, in the winter, adrenergic stimulation was important to achieve peak f_H at high temperatures; adrenergic blockade in winter did not affect T_peak (18.3°C) or T_arr (21°C) but lowered peak f_H to only 68 beats min^-1 (p < 0.05). Despite limited seasonal differences in f_H, when compared at common, cool to moderate temperatures, winter acclimated fish did exhibit slower atrioventricular conduction (longer PR interval), slower ventricular depolarization (longer QRS duration) and a shorter overall systolic duration (lower QT interval). Overall, S. namaycush exhibited no seasonal plasticity in cardiac thermal tolerance and modest seasonal changes in adrenergic control at high temperatures and in cardiac electrical activity. This limited thermal plasticity may constrain their ability to cope with seasonally distinct thermal challenges including extreme heat events and increase their reliance on other coping or avoidance mechanisms when thermal refugia are available (e.g. behavioural thermoregulation). An increased need for avoidance behaviour could limit the accessibility of key habit for foraging and spawning which occurs in late summer in this region (Dubos et al., 2024).