Cardiac and behavioural responses to hypoxia and warming in free-swimming gilthead seabreamSparus aurata

Cardiac and behavioural responses to hypoxia and warming were investigated in free-swimming gilthead seabreamSparus aurataequipped with biologging tags in the peritoneal cavity. After suitable recovery in a holding tank, heart rate (fH) and the variance of tri-axial body acceleration (VARm) were logged during exposure to stepwise progressive hypoxia or warming, comparing when either swimming in a tank or confined to individual respirometer chambers. When undisturbed under control conditions (normoxia, 21 °C), meanfHwas significantly lower in tank than respirometers. In progressive hypoxia (100 - 15% oxygen saturation), meanfHin the tank was significantly lower than respirometers at oxygen levels until 40%, with significant bradycardia in both holding conditions below this. Mean VARmwas low and invariant in hypoxia. Warming (21 to 31 °C) caused progressive tachycardia with no differences infHbetween holding conditions. Mean VARmwas, however, significantly higher in the tank during warming, with a positive relationship between VARmandfHacross all temperatures. Therefore, spontaneous activity contributed to raisingfHof fish in the tank during warming. MeanfHin respirometers had a highly significant linear relationship with mean rates of oxygen uptake, considering data from hypoxia and warming together. The highfHof confinedS. aurataindicates that static respirometry techniques may bias estimates of metabolic traits in some fish species. Biologging on free-swimming fish revealed novel information about cardiac responses to environmental stressors, which may be closer to responses exhibited by fish in their natural environment.SUMMARY STATEMENTImplantable biologgers were used to provide the first measurements of cardiac responses to hypoxia and warming in a free-swimming fish, revealing that confinement in respirometer chambers raises heart rate, with consequences for estimates of metabolic rates.