This study is an attempt to gain an integrated understanding of the interactions between temperature, locomotion activity and metabolism in the European sea bass (Dicentrarchus labrax). To our knowledge this study is among the few that have investigated the influence of the seasonal changes in water temperature on swimming performance in fish. Using a Brett-type swim-tunnel respirometer the relationship between oxygen consumption and swimming speed was determined in fish acclimatised to 7, 11, 14, 18, 22, 26 and 30°C. The corresponding maximum swimming speed (U_max), optimal swimming speed (U_opt), active (AMR) and standard (SMR) metabolic rates as well as aerobic metabolic scope (MS) were calculated. Using simple mathematical functions, these parameters were modelled as a function of water temperature and swimming speed. Both SMR and AMR were positively related to water temperature up to 24°C. Above 24°C SMR and AMR levelled off and MS tended to decrease. We found a tight relationship between AMR and U_max and observed that raising the temperature increased AMR and increased swimming ability. However, although fish swam faster at high temperature, the net cost of transport (COT_net) at a given speed was not influence by the elevation of the water temperature. Although U_opt doubled between 7°C and 30°C (from 0.3 to 0.6 m s^-1), metabolic rate at U_opt represented a relatively constant fraction of the animal active metabolic rate (40-45%). A proposed model integrates the effects of water temperature on the interaction between metabolism and swimming performance. In particular the controlling effect of temperature on AMR is shown to be the key factor limiting maximal swimming speed of sea bass.

The uptake of anthropogenic CO₂ by the ocean has been suggested to impact marine ecosystems by decreasing the respiratory capacity of fish and other water breathers. We investigated the aerobic metabolic scope of the spiny damselfish, Acanthochromis polyacanthus, from the Great Barrier Reef, Australia when exposed for 17 days to CO₂ conditions predicted for the end of the century (946 μatm CO₂). Surprisingly, resting O₂ consumption rates were significantly lower and maximal O₂ consumption rates significantly higher in high-CO₂-exposed fish compared with control fish (451 μatm CO₂). Consequently, high-CO₂-exposed fish exhibited an unexpected increase in absolute (38%) and factorial aerobic scopes (47%). Haematological and muscle water changes associated with exercise were not affected by CO₂ treatment. Thus, contrary to predictions, our results suggest that elevated CO₂ may enhance aerobic scope of some fish species. Long-term experiments are now required to assess the response to elevated CO₂ further, because developmental and transgenerational effects can be dramatic in fish. Ultimately, understanding the variability among species regarding the effects of CO₂ on aerobic scope will be critical in predicting the impacts of ocean acidification on marine communities and ecosystems.