Global warming affects the metabolism of ectothermic aquatic breathers forcing them to migrate and undergo high-latitudinal distribution shifts to circumvent the temperature-induced mismatch between increased metabolic demand and reduced water oxygen availability. Here the authors examined the effects of temperature on oxygen consumption rates in an Arctic stenotherm, the Greenland halibut Reinhardtius hippoglossoides, and calculated the optimal temperature for maximum aerobic scope, AS(T_opt,AS), which was found to be 2.44°C. They also investigated cardiac performance as limiting the oxygen transport chain at high temperatures by measuring maximum heart rate (f_Hmax) over acute temperature increases and found various metrics related to f_Hmax to be at least 3.2°C higher than T_opt,AS. The authors’ measured T_opt,AS closely reflected in situ temperature occurrences of Greenland halibut from long-term tagging studies, showing that AS of the species is adapted to its habitat temperature, and is thus a good proxy for the species' sensitivity to environmental warming. The authors did not find a close connection between f_Hmax and T_opt,AS, suggesting that cardiac performance is not limiting for the oxygen transport chain at high temperatures in this particular Arctic stenotherm. The authors’ estimate of the thermal envelope for AS of Greenland halibut was from −1.89 to 8.07°C, which is exceptionally narrow compared to most other species of fish. As ocean temperatures increase most rapidly in the Arctic in response to climate change, and species in these areas have limited possibility for further poleward-range shifts, these results suggest potential severe effects of global warming on Arctic stenotherms, such as the Greenland halibut. The considerable economic importance of the species raises concerns for future fisheries and species conservation of Arctic stenotherms in the Northern Hemisphere.