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

Wild juvenile Salvelinus alpinus show strong seasonal acclimatization: summer fish raise CTmax from 21.4 to 25.9°C and delay heat-induced cardiac arrhythmia to 23.6°C versus 18.7°C in winter.

• Why it matters: Arctic seasonality creates large thermal swings, but it is unclear how wild juvenile fish adjust cardiac and whole-animal heat tolerance across seasons to cope with changing temperatures.
• What they did: Winter and summer field assessments measured habitat use and whole-animal and cardiac heat tolerance, including intrinsic and maximum heart-rate sensitivity ([f]Hint, [f]Hmax) during acute warming with atropine plus either sotalol or isoproterenol.
• The result: Heart rate increased with warming in both seasons, but summer showed greater responsiveness (Q10 2.4 vs 2.0) and higher peaks, while winter fish had longer PR and QRS intervals, consistent with lower cardiac heat tolerance.

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