Aerobic capacities and swimming performance of polar cod () under ocean acidification and warming conditions.

Polar cod () is an important prey species in the Arctic ecosystem, yet its habitat is changing rapidly: climate change, through rising seawater temperatures and CO concentrations, is projected to be most pronounced in Arctic waters. This study aimed to investigate the influence of ocean acidification and warming on maximum performance parameters of as indicators for the species' acclimation capacities under environmental conditions projected for the end of this century. After 4 months at four acclimation temperatures (0, 3, 6, 8°C) each combined with two  levels (390 and 1170 µatm), aerobic capacities and swimming performance of were recorded following a protocol. At both CO levels, standard metabolic rate (SMR) was elevated at the highest acclimation temperature indicating thermal limitations. Maximum metabolic rate (MMR) increased continuously with temperature, suggesting an optimum temperature for aerobic scope for exercise (AS) at 6°C. Aerobic swimming performance () increased with acclimation temperature irrespective of CO levels, while critical swimming speed () did not reveal any clear trend with temperature. Hypercapnia evoked an increase in MMR (and thereby AS). However, swimming performance (both and ) was impaired under elevated near-future  conditions, indicating reduced efficiencies of oxygen turnover. The contribution of anaerobic metabolism to swimming performance was very low overall, and further reduced under hypercapnia. Our results revealed high sensitivities of maximum performance parameters (MMR, , ) of to ocean acidification. Impaired swimming capacity under ocean acidification may reflect reduced future competitive strength of .