Aerobic capacities and swimming performance of Polar cod (Boreogadus saida Lepechin) under ocean acidification and warming conditions

Polar cod ( Boreogadus saida ) is an important prey species in the Arctic ecosystem, yet its habitat is changing rapidly: climate change, through rising seawater temperatures and CO 2 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 B. saida 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 P CO 2  levels (390 and 1170 µatm), aerobic capacities and swimming performance of B. saida were recorded following a U crit protocol. At both CO 2 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 ex ) at 6°C. Aerobic swimming performance ( U gait ) increased with acclimation temperature irrespective of CO 2 levels, while critical swimming speed ( U crit ) did not reveal any clear trend with temperature. Hypercapnia evoked an increase in MMR (and thereby AS ex ). However, swimming performance (both U gait and U crit ) was impaired under elevated near-future P CO 2  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, U gait , U crit ) of B. saida to ocean acidification. Impaired swimming capacity under ocean acidification may reflect reduced future competitive strength of B. saida .