Short-term acclimation at elevated temperatures alters hematobiochemical parameters, erythrocyte and muscle structure, and critical thermal tolerance (CTmax) limit of Nile tilapia, Oreochromis niloticus.

Rising water temperatures pose a significant challenge for the aquaculture industry. These increased temperatures negatively affect fish survival and restrict various physiological processes. This study aimed to assess multiple physiological responses and identify the critical thermal tolerance limits (CTmax) of Nile tilapia subjected to acute thermal stress at different acclimation temperature regimes. In this study, fish were acclimated to three temperature regimes —31 °C, 34 °C, and 37 °C — for 1 week. To determine the CTmax, muscle microstructure was evaluated at the end, and several major hemato-biochemical parameters were measured at the beginning and end of the acute thermal stress, just before the fish lost equilibrium. The results indicated a significant decrease in Hb and RBC levels, while WBC and glucose levels increased notably at 37 °C. The frequencies of erythrocyte cellular abnormalities (ECAs) and erythrocyte nuclear abnormalities (ENAs) were also considerably higher at 37 °C than at the other temperature groups. Muscle fibres were mostly elongated and irregularly shaped at 37 °C compared with the other temperature groups. The CTmax was significantly lower at 37 °C acclimation (41.15 °C) than at 31 °C (44.57 °C) and 34 °C (43.40 °C). Principal component analysis revealed that acute thermal stress at the acclimated temperature (37 °C) significantly affected the hematobiochemical parameters at the endpoint. Furthermore, a negative correlation between CTmax and temperature was observed, indicating that CTmax decreased significantly (p < 0.05) with increasing temperature. From this study, it can be concluded that Oreochromis niloticus should be maintained at temperatures below 37 °C to ensure optimal health conditions.