Polar cod (Boreogadus saida) were acclimated for four months to different temperatures (0, 3, 6, 8°C) and PCO2 (390 and 1170 µatm) conditions. Subsequently, B. saida were exercised in a Brett-type swimming tunnel at their respective acclimation conditions the fourth day after feeding. The swimming protocol involved a careful increase in water speed of 1.9 ± 0.3 cm/sec after 11 min. The onset of burst-type swimming behavior indicated the transition from purely aerobic to partly anaerobic swimming speed (Ugait). The critical swimming speed (Ucrit) was reached as soon as the fish touched the grid for at least 30 sec. Ucrit was further adjusted according to Brett (1964). At each velocity step, burst-type swimming events were counted twice for 30 sec after 5 and 10 min. The maximum burst count (max. BC) as well as the number of total bursts throughout the protocol served to classify the individual anaerobic swimming performance of B. saida. Additionally, individual burst events per velocity step are listed ("Burst counts per velocity step_specimen 1-5", https://doi.pangaea.de/10.1594/PANGAEA.889446). Furthermore, a duration of one second per burst was assumed in order to estimate the contribution of anaerobic metabolism (tsbanaerob) during the time between Ugait and Ucrit ("time spent bursting", tsb).
Immediately after the termination of the swimming protocol, the aerobic performance of B. saida was recorded in a separate intermittent-flow respiration setup. The initial 5 min of the slope of the first ṀO₂ recording was defined as the maximum metabolic rate (MMR) evoked by exercise. Fish remained in the respiration chambers for approximately 48 h. The standard metabolic rate (SMR) was calculated as the 15%-quantile among ṀO₂ records starting from the second night in the respiration chamber. The difference between MMR and SMR characterized the individual aerobic scope of exercise (AS).
The ratio Ucrit/MMR was introduced in order to estimate the energetic efficiency of maximum swimming performance (Emax).
