Secondary stress responses to hypoxia and re‐oxygenation at different temperatures in pacu (Piaractus mesopotamicus) juveniles

This study aimed to verify the effects of interaction between different temperatures (similar to 18, similar to 23 and similar to 28 degrees C) with different dissolved oxygen levels (similar to 2.0, similar to 4.5 and similar to 7.0 mg O2L-1) and different re-oxygenation rates (0.5 and 5 hr) after hypoxia in pacu (Piaractus mesopotamicus). In experiment 1, fish were exposed to hypoxia, mild hypoxia or normoxia at low, medium or high temperature for 12 hr. In experiment 2, fish were exposed to hypoxia or normoxia at low, medium or high temperature for 12 hr and, after hypoxia, rapid or gradual re-oxygenation was performed with a return to normoxia achieved in 0.5 or 5 hr. In experiment 1, blood glucose and pH levels were higher in the hypoxic treatment. The haematocrit and erythrocyte values were lower in mild hypoxia at low and high temperatures respectively. In experiment 2, blood lactate and glucose levels in rapid re-oxygenation increased after 1 and 12 hr respectively. Twelve hours after rapid re-oxygenation, haematological values were similar or lower than in the normoxia or gradual re-oxygenation. In conclusion, exposure of pacu to hypoxic conditions increased blood glucose and pH levels (experiment 1). Additionally, 12 hr after gradual re-oxygenation, blood variables returned to homeostatic levels (experiment 2).