Prominent vulnerability of red hybrid tilapia (Oreochromis spp.) liver to heat stress-induced oxidative damage

Many aquatic species are vulnerable to warming water temperatures. Nonetheless, data on the physiological impact of global warming on cultured freshwater fish species are limited compared to marine species. This study investigated the effect of heat stress on oxidative stress response in red hybrid tilapia, Oreochromis spp. (total length 37.0 ± 3.2 cm, body weight 500.0 ± 15.0 g). Fish were assigned randomly and exposed to gradual increment of water temperature (1°C. 8 h-1) from 28 to 31°C in aerated and thermoregulated fibreglass tanks for 2 weeks. Oxidative stress response was determined based on malondialdehyde levels and activities of antioxidant enzymes: superoxide dismutase, catalase and gluthathione s-transferase in the muscle, liver and kidney on day 1, 7 and 14. The heat-stressed group showed significantly increased malondialdehyde levels in the muscle, liver and kidney in parallel to the exposure period. The highest malondialdehyde level was observed in the liver tissue. Activity of superoxide dismutase was significantly decreased in the muscle over the course of the exposure period, in contrast to liver and kidney. Catalase activity was significantly higher in the muscle and liver, while gluthathione s-transferase activity was significantly increased in the muscle and liver but decreased in the kidney. The level of malondialdehyde strongly correlated with superoxide dismutase, catalase and gluthathione s-transferase activities in the liver compared with the muscle and kidney. Microscopic examination of liver showed congestion and hepatocytes with karyorrhexis indicating progress of necrosis. Our results suggest that liver is more susceptible to heat stress-induced oxidative damage compared with muscle and kidney. Red hybrid tilapia showed narrow upper thermal tolerance, implicating high vulnerability to the rise in water temperature.