Objective Physiological and biochemical responses of the gills and liver of Plecoglossus altivelis under acute hypoxic exposure were studied.

Methods Physiological responses of P. altivelis with or without imposed acute hypoxic stress for 6, 24, and 48 h in a tank were monitored. A 6 h reoxygenation period was given for the fish prior to analyses on the structural changes as well as antioxidant and metabolic enzyme activities in the gills and liver.

Results The physiological observation showed in P. altivelis, higher than in benthic fish, a respiration points of head-up at 2.85 mg·L^‒1 and asphyxiation at 1.37 mg·L^‒1 indicating its high sensitivity to hypoxia. Histologically, the hypoxic stress caused gill lamellae bending and swelling, blood cell accumulation, liver vacuolization, hepatic sinusoids dilatation, and cell necrosis in the fish. Prolonged stress significantly elevated the activities of catalase (CAT), and glutathione peroxidase (GSH-Px) in the gills and liver (P＜0.05) to peak in 24 h and declined to some degree in 48 h. The GSH-Px activity in the gills and liver, meanwhile, became significantly higher than control (P＜0.05) and maximized in 24 h followed by a decrease up to 48 h. The pyruvate kinase (PK) and lactate dehydrogenase (LDH) activities as well as the malondialdehyde (MDA) content rose significantly higher than control (P＜0.05) to top in 48 h. After 6 h of reoxygenation, the activities of the abovementioned 4 enzymes and MDA content decreased but still higher than those of control. More important, the damage incurred to the gills and liver did not fully recover after the reoxygenation.

Conclusion Although P. altivelis could resist acute hypoxia by physiologically increasing its antioxidant enzyme activities and anaerobic metabolism, prolonged exposure under the stress would cause irreversible histological and enzymatic alternations in the liver and gills once the threshold of tolerance was exceeded.