| Summary: | Mass specific oxygen uptake in juveniles of Wallago attu under three concentrations of dissolved oxygen viz., saturated, 100% (SW) DO 7.5-7.0 mg l-1, normoxic, 60% (NW) DO 4.5-4.0 mg l-1 and hypoxic, 30% (HW) DO 2.5-2.0 mg l-1 at water temperature of 28±10C was measured in the laboratory. Two size groups of juveniles, with body weight 152.5±10 g and 187.5±5 g, were exposed to three concentrations of dissolved oxygen in closed respirometer system. The volume of oxygen uptake decreased with increase in body weight of fish. Breathing frequency (BF) increased with lowering of DO level in both the weight groups. Under different retention periods and number, mean oxygen uptake was 506.5±4.1, 285.1±4.6 and 120.1±5.1 mg kg-1 h-1 respectively in the smaller size group, with BF of 53-96 min-1, while in the bigger size group(187.5±5 g), oxygen consumption was 473.2±22.4, 272.7±9.7 and 78.1±1.2 mg kg-1 h-1 respectively with a breathing rate (BR) of 49.6 - 78.1 min-1. With an increase of 35±7.5 g body weight, mean reduction of 12.4-42.0 mg kg-1 h-1 DO was recorded. An abrupt behavioural change was observed below DO concentration of 4.0-3.5 mg l-1. Oxygen uptake value showed declining trend with lowering of DO concentration. Based on these results, oxygen uptake has been estimated at 27.8 mg 100 g juvenile-1 h-1 at DO saturation level of 60% in water.
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