Effects of feeding and hypoxia on cardiac performance and gastrointestinal blood flow during critical speed swimming in the sea bass Dicentrarchus labrax

Previous studies have shown that if European sea bass are exercised after feeding, they can achieve a significantly higher maximum metabolic rate (MMR) than when fasted. They can meet combined metabolic demands of digestion (specific dynamic action, SDA) and maximal aerobic exercise, with no decline in swimming performance. If, however, exposed to mild hypoxia (50% saturation), bass no longer achieve higher MMR after feeding but they swim as well fed as fasted, due to an apparent ability to defer the SDA response. This study explored patterns of cardiac output (QA) and blood flow to the gastrointestinal tract (QGI) associated with the higher MMR after feeding, and with the ability to prioritise swimming in hypoxia. Sea bass (mean mass ~ 325 g, forklength ~ 27 cm) were instrumented with flow probes to measure QA and QGI during an incremental critical swimming speed (Ucrit) protocol in a tunnel respirometer, to compare each animal either fasted or 6 h after a meal of fish fillet equal to 3% body mass. Feeding raised oxygen uptake (MO2) prior to exercise, an SDA response associated with increased QA (+ 30%) and QGI (+ 100%) compared to fasted values. As expected, when exercised the fed bass maintained the SDA load throughout the protocol and achieved 14% higher MMR than when fasted, and the same Ucrit (~ 100 cm s-1). Both fed and fasted bass showed pronounced increases in QA and decreases in QGI during exercise and the higher MMR of fed bass was not associated with higher maximum QA relative to when fasted, or to any differences in QGI at maximum QA. In hypoxia prior to exercise, metabolic and cardiac responses to feeding were similar compared to normoxia. Hypoxia caused an almost 60% reduction to MMR and 30% reduction to Ucrit, but neither of these traits differed between fed or fasted bass. Despite hypoxic limitations to MMR and Ucrit, maximum QA and patterns of QGI during exercise in fasted and fed bass were similar to normoxia. Estimating GI oxygen supply from QGI indicated that the ability of bass to prioritise aerobic exercise over SDA when metabolically limited by hypoxia was linked to an ability to defer elements of the SDA response occurring outside the GI tract.