The physiological response of the Arctic key species Polar cod, Boreogadus saida, to hypoxia in a warming ocean: critical oxygen levels and swimming performance.

Global warming has already caused a loss of nearly 50% Arctic sea-ice coverage since the 1980s. This sea-ice loss strengthens summer stratification of the ocean’s water column and, consequently, hypoxic zones may form in the deep-water layers. Our study investigated the response of an Arctic keystone species, the Polar cod, Boreogadus saida, to hypoxia and warming. We measured the metabolic capacity (standard, routine and maximum metabolic rates, aerobic scope, and critical oxygen saturations (Pcrit and Pcmax)) and swimming performance of Polar cod under progressive hypoxia at 2 °C and after warm acclimation at 10 °C using intermittent-flow and swim tunnel respirometry. We observed stable patterns that were similar in both temperatures: Polar cod displayed oxygen-regulating behaviour under progressive hypoxia, with standard metabolic rates never below aerobic baseline metabolism at 2 °C and a very stable aerobic scope for both temperatures. Our study revealed that Polar cod can handle oxygen saturations down to a Pcrit of 12.56 ± 1.59% air saturation at typical habitat temperatures. At 10 °C, Pcrit rose to 24.37 ± 1.65% air saturation. We suggest that low-oxygen areas could potentially serve as protective zones for Polar cod for the next decades as long as bottom water temperatures still remain cold in summer. However, since hypoxia tolerance will decrease with warmer temperatures, this physiological advantage over invading species and predators will be progressively lost.