Exploring antioxidant responses after rapid reoxygenation at low temperature in pacu (Piaractus mesopotamicus) - A hypoxia-tolerant neotropical fish.

Fluctuations in oxygenation and reoxygenation rates are common in aquaculture and natural environments and can cause oxidative stress and contribute to pathological conditions. This study evaluated how reoxygenation rate and temperature influence antioxidant defence and oxidative stress in pacu (Piaractus mesopotamicus) exposed to hypoxia. Juvenile fish (61.7 g) were exposed to 12 h of hypoxia (2 mg O2 L-1) at three temperatures (18, 23, and 28 °C), followed by rapid or gradual reoxygenation (0.5 or 5 h). A normoxic group (7 mg O2 L-1) was maintained at each temperature. At 1 and 12 h after reoxygenation, total antioxidant capacity against peroxyl radicals (ACAP), lipid peroxidation (LPO), and protein thiols (PSH) were determined in the gills and liver. ACAP was lowest at 18 °C after 1 h and at 28 °C after 12 h of recovery, compared with the other two temperatures at the same times (p < 0.05). At both recovery times, gill LPO was lower at 18 °C than at 23 and 28 °C with rapid reoxygenation, while liver LPO was higher at 23 °C than at 28 °C (p < 0.05). Gill PSH was lower at 18 °C than at 23 °C, whereas liver PSH was higher at 23 °C than at 28 °C after recovery (p < 0.05). These organ-specific patterns indicate that intermediate and high temperatures intensify oxidative stress during reoxygenation, whereas 18 °C mitigates it by reducing metabolic demand and reactive oxygen species (ROS) formation. In conclusion, pacu juveniles are hypoxia-tolerant and respond more effectively to rapid reoxygenation at 18 °C.