Physiological and histopathological responses of Pangasianodon hypophthalmus to ammonia exposure at varying thermal regimes

Abstract

This study evaluated the effects of ammonia exposure on the physiological responses, vitality, and histopathology of Pangasianodon hypophthalmus at two temperatures. A 2 × 2 factorial design was applied using two total ammonia nitrogen (TAN) levels (0 and 10 mg/L) and two temperature regimes (28 °C and 32 °C) over a 4-week period. At the end of the experiment, clinical signs, histopathology, hematology, stress indicators (glucose and cortisol), liver function enzymes, and oxidative stress markers (malondialdehyde levels and antioxidant enzymes) were assessed. Fish exposed to 10 mg/L TAN at 32 °C exhibited severe clinical signs, including dermal erosion, muscle necrosis, and respiratory distress. Ammonia exposure at 32 °C significantly reduced red blood cell (RBC) counts, hemoglobin, and hematocrit. Furthermore, elevated temperatures exacerbated ammonia-induced stress, evidenced by significant increases in cortisol, blood glucose, and malondialdehyde levels, alongside altered liver and antioxidant enzyme activities. Histopathological analysis confirmed significant damage to the gill filaments, hepatopancreas, and renal tissues, with severity increasing alongside water temperature. These results indicate a synergistic effect between ammonia exposure and water temperatures, where higher temperatures reduce the threshold for ammonia tolerance, triggering respiratory distress, systemic oxidative stress, and metabolic failure. These findings underscore the critical need for strict water quality management in tropical aquaculture, particularly in regions like Egypt, as rising global temperatures due to climate change may transform currently sub-lethal ammonia levels into potent lethal stressor for P. hypophthalmus. However, the study is limited by controlled laboratory conditions and relatively short experimental duration, suggesting the need for long-term investigations.