Abstract:When high dams discharge high velocity flows

large amount of air was taken into the water downstream of the dams

and the intense aeration between water and air leads to the generation of total dissolved gas (TDG) supersaturation. It gives threatens to the fishes’ survival especially those valuable and rare fishes. Experiments were conducted to measure the acute and chronic lethality on Leptobotia Elongata to TDG supersaturation. The fishes were placed in distinct tanks with different concentration of TDG-supersaturated water. The result shows that the median lethal times (LT50) of the Leptobotia Elongata were 152.81h

64.02h

14.89h when TDG supersaturation levels were 120%

130% and 140% respectively. Throughout the trail

no fish died when exposed to 110% TDG. Furthermore

surviving fishes which had been pre-exposed to 110% and 120% TDG for 96h were sustaining re-exposed to two TDG supersaturation levels (130% and 140%). The resulting median lethal times were 8.72h and 7.14h which are 41.4% and 52.05% shorter than the former 140% level. And the median lethal times were 47.77h and 15.52h which were 25.56% and 75.81% shorter than the former 130% level as well. It appeals that the low TDG supersaturation level (110%) still has chronic lethality on Leptobotia Elongata.