Critical Thermal Tolerance and Physiological Stress in the Estuarine Catfish, Mystus gulio

ABSTRACT

Understanding critical thermal tolerance and physiological stress responses is crucial to evaluating fish resilience to environmental changes. This study evaluated the critical thermal maximum (CTmax) and minimum (CTmin) under normoxic (sufficient oxygen supply; dissolved oxygen (DO) > 6.0 mg/L) and hypoxic (deficient oxygen supply; DO < 2.0 mg/L) conditions in long‐whiskered catfish ( Mystus gulio ), a commercially important, climate‐sensitive estuarine species from the Bay of Bengal. Under normoxia, CTmax and CTmin were recorded as 41.8 ± 0.24°C and 11.6 ± 0.33°C, respectively, while under hypoxia, CTmax and CTmin were 36.9 ± 3.44°C and 13.5 ± 0.36°C, respectively. Dissolved oxygen levels decreased notably at CTmax under hypoxia, while free CO ₂ levels showed the opposite trend. At CTmax and CTmin, oxygen saturation was significantly reduced ( p  < 0.05) under hypoxia compared to both normoxia and the starting point. Oxygen consumption and opercular respiratory rates rose significantly at CTmax under hypoxia. Hemato‐biochemical parameters revealed notable changes, with significantly lowered levels of hemoglobin and red blood cell counts, while levels of glucose, cholesterol, and white blood cell count increased significantly under hypoxic conditions in both CTmax and CTmin. Erythrocytic cellular and nuclear abnormalities were more prevalent under hypoxia in both CTmax and CTmin. Water quality parameters also fluctuated with temperature changes. M. gulio recovered successfully immediately before CTmax and CTmin when oxygen was restored, and the temperature increment was stopped. These findings demonstrate the reduced thermal tolerance of M. gulio under hypoxia and highlight significant physiological and cellular stress responses, revealing key mechanisms in the species’ adaptive capacity under environmental stressors.