Thermal limits to the cardiorespiratory performance of Arctic char (Salvelinus alpinus) in a rapidly warming north

The Canadian Arctic is warming at nearly three times the global rate. Consequently, thermal regimes of native cold-adapted species like the Arctic char (Salvelinus alpinus) are being rapidly reshaped. The Arctic char is the most northerly-distributed freshwater fish on Earth and is essential to Inuit food security and culture. Anadromous Arctic char migrate between freshwater habitats and the Arctic Ocean many times throughout their lives, which can expose them to an already extreme range of temperatures (<0 to >21°C). My thesis examined the ability of Arctic char to cope with thermal variation, focusing specifically on cardiorespiratory performance. I used a novel, mobile laboratory in the central Canadian Arctic to assess how acute temperature changes impact cardiac function and aerobic metabolism in migrating Arctic char. Arctic char maintained aerobic performance over an impressive temperature range (4-16°C), but could not recover from exhaustive exercise above 16°C. Furthermore, maximum heart rate, which should increase with acute warming, began to plateau at 16°C, declined at 19°C and became arrhythmic at 21°C. I conducted similar assessments on four other populations to determine how they differed. One population that undertakes physically and thermally challenging migrations had higher maximum aerobic capacity and heat tolerance than another with less harsh migrations. Heart mass was also higher in populations with less challenging migrations. Next, I acclimated hatchery-reared Arctic char to naturally encountered temperatures (2-18°C) to characterize their thermal plasticity. As before, Arctic char maintained high aerobic performance over a broad temperature range (2-14°C) and warm acclimation improved swimming performance and dramatically increased (+35-45%) cardiac heat tolerance. However, mortality occurred with chronic exposure to 18°C. My research indicates that Arctic char have broad thermal performance and acclimation potential that may help mitigate the negative impacts of rapid environmental warming. Nevertheless, I showed that Arctic char already encounter temperatures in the Canadian Arctic that constrain cardiorespiratory performance and impair recovery from exhaustive exercise, which would likely hinder their obligatory return migration. Furthermore, my results suggest that intraspecific diversity in cardiorespiratory physiology, thermal tolerance, and local migration conditions may be important for Arctic char conservation and shape population-specific responses to warming.

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