ABSTRACT
Aim: The present study was conducted to determine the tolerance to reduced oxygen level (hypoxia) and the behavioural and biochemical responses of mrigal, Cirrhinus mrigala to environmental hypoxia. Methodology: Cirrhinus mrigala were subjected to LC50 test for 96 hr and the mortality were recorded. In a second experiment, the fishes were subjected to a stressful, but safe limit of hypoxia (0.5±0.04 mg l-1) and the behavioral responses and serum metabolites (glucose, lactate, total lipids, free amino acids and ammonia) were evaluated for 96 hrs. Results: A median lethal concentration (LC50, 96 hr) value of 0.25 mg l-1dissolved oxygen) was estimated for mrigal. The upper safe limit was 0.49 mg l-1 DO and the lower lethal limit was 0.19 mg l-1 DO. Gill ventilation frequency increased under severe hypoxic conditions and decreased with exposure time. The serum level of glucose, lactate and total lipids increased significantly (P<0.05) within 24 hr of exposure to hypoxic conditions. Free amino acids and ammonia contents were not altered by a four day exposure to hypoxia. Interpretation: The present study explains the basic metabolic and behavioural mechanism behind the hypoxia tolerance of Indian Major Carp, mrigal assisted by alterations in gill ventilation and metabolic responses.