Low O2 avoidance is associated with physiological perturbation but not exhaustion in the snapper (Pagrus auratus: Sparidae)

It is already known that the New Zealand snapper (Pagrus auratus, Sparidae) does not avoid hypoxia until reaching an oxygen partial pressure (PO2) of 3.1 ± 1.2 kPa at 18 °C. Avoidance at this level of PO2 and temperature is below the critical oxygen partial pressure of the species (Pcrit = 5.8 ± 0.6 kPa, 43.5 ± 4.5 mm Hg) and is therefore expected to result in major physiological stress. Results from the current study showed that avoidance was associated with numerous physiological perturbations, including a significant endocrine response, haematological changes, osmoregulatory disturbance and metabolic adjustments in the heart, liver and muscle. Snapper clearly experienced physiological stress at the point of avoidance but they were not however in a state of physiological exhaustion since some fuel reserves were still available. In addition to avoidance, snapper also showed a subtle reduction in swimming speed — this energy-saving response may have helped snapper minimise the physiological challenge of low O2 residence. It is therefore concluded that snapper can reside in water below their Pcrit threshold for brief periods of time and, without any evidence of physiological exhaustion at the point of avoidance, fish should recover quickly once normoxia is selected. Lastly, with signs of anaerobic metabolism in cardiac tissue at the point of avoidance, we tentatively suggest that snapper may leave hypoxia to protect heart function.