Oxygen consumption and ventilatory frequency responses to gradual hypoxia in common dentex (Dentex dentex): Basis for suitable oxygen level estimations

This study established different oxygen level categories (optimal, sub-optimal, dangerous and lethal) for Dentex dentex according to ventilatory frequency (Vf) and oxygen consumption (MO2) in response to gradually decreasing dissolved oxygen levels for different fish masses (117–745 g) and water temperatures (13.9–28.1 °C). Initial and maximum ventilatory frequency (Vf0 and Vfmax, respectively) and oxygen saturation at which ventilatory frequency was altered (SVf) ranged from 31.3 to 71.3 beats/min, 58.0 to 129.7 beats/min and 48.1 to 81.7%, respectively. D. dentex maintained a constant MO2 rate, until a critical oxygen saturation (Scrit) was reached. The Scrit and lethal oxygen saturation (LS50) ranged from 26.5 to 40.6% and 12.3 to 18.9%, respectively. Vf0, Vfmax, Scrit and LS50 were correlated positively and significantly with temperature, but only Vfmax was correlated with body mass, pointing to a higher Vf response in small than large fish. In the range of fish masses and temperatures tested, our findings suggest an optimal dissolved oxygen saturation of above 70–75% (constant Vf and MO2), a sub-optimal value of between 70 and 35% (altered Vf but not MO2), with anything below 35% being dangerous (altered Vf and MO2). We conclude that suitable oxygen levels for D. dentex are similar with respect to other species of interest in aquaculture.