Effects of acute cold stress on the physiological and biochemical dynamics of Nile tilapia (Oreochromis niloticus Linn.): in vivo and in vitro studies.

This study was aimed at examining the physiological and biochemical response of live Nile tilapia (NT) (Oreochromis niloticus Linn.) and Nile tilapia blood cells (NTBC) to abrupt ambient temperature reductions. NT was exposed to water temperatures between 24 °C and 14 °C during the experiment, while NTBC was subjected to temperatures between 24 °C and 12 °C, with a 2 °C temperature reduction at each stage. Physiological parameters of NT were assessed, including core body temperature, heart rate, ventilation rate, body coloration score, and oxygen saturation. In addition, biochemical parameters of NTBC were measured, i.e., total antioxidant capacity, hydrogen peroxide, malondialdehyde, nitric oxide, and mitochondrial activity. It was found that lowering the water temperature from 24 °C to 14 °C caused a drop in core body temperature and ventilation rates (P < 0.05), while heart rates rose at 18 °C (P < 0.05) before falling. Body coloration scores became higher at 20 °C and grew more intense as the water temperature fell further (P < 0.05). Mitochondrial activity peaked at 18 °C (P < 0.05), while malondialdehyde rose significantly between 16 °C and 12 °C (P < 0.05). Total antioxidant capacity decreased at 18 °C (P < 0.05) and remained low to 12 °C. Nitric oxide dropped at 20 °C and 12 °C (P < 0.05), while hydrogen peroxide peaked at 14 °C (P < 0.05). These findings indicate that NT is highly sensitive to temperatures below 20 °C, which increases oxidative stress and requires enhanced antioxidant defenses below 18 °C. Finally, the results align between in vivo and in vitro experiments.