Physiological and Pathological Responses of Grass Carp (Ctenopharyngodon idella) to Total Dissolved Gas Supersaturation During Exposure and Subsequent Recovery

The total dissolved gas (TDG) supersaturation induced by discharged floods severely threatens fish survival during the flood season. However, investigations into the physiological and pathological effects of TDG supersaturation on fish remain limited, and even less is known about the recovery from TDG-induced injuries at the physiological and pathological levels. The biochemical and histopathological responses of fish injured by TDG exposure, and whether injured fish can return to normal conditions, remain unclear. In this study, on the basis of a combination of in situ investigations and laboratory studies, we investigated the physiological and pathological responses to TDG supersaturation stress in grass carp, as well as their recovery after TDG exposure. The results revealed that 96 h of TDG exposure caused epithelial cell hyperplasia and necrosis in the gill tissues of grass carp, as well as mucosal epithelial cell necrosis in the intestine. However, the damaged gills failed to achieve restoration after a 336 h recovery period. No pathological changes occurred in the liver. Moreover, the present results revealed that under the above exposure conditions, TDG supersaturation significantly decreased catalase (CAT) activity and increased superoxide dismutase (SOD) activity, whereas no significant alterations in alanine aminotransferase (ALT) or aspartate aminotransferase (AST) activity were detected. The CAT and SOD activities of the species returned to normal levels after 24 h of recovery. Thus, our results suggest that the recovery of the impaired antioxidant system requires 24 h for species under relatively low TDG levels (≤120%), whereas histopathological recovery requires a longer period. These findings can be used to understand the potential effects of TDG exposure on fish and provide valuable information for formulating reservoir discharge plans.