Abstract.An Aquatector oxygen injection unit was used to supersaturate a hatchery water supply to 200 oxygen saturation (18–20mg/l) and increase the total gas pressure to 120 of saturation. Nitrogen saturation was reduced to near 100. Rainbow trout,Oncorhynchus mykiss(Walbaum), held in the treated water developed signs of gas bubble disease in 4 days, and 50 died within 20 days. We demonstrated that supersaturated total gas pressure due to excessive oxygen saturation causes gas bubble disease in the absence of supersaturated nitrogen gas. It is recommended that users of oxygen injection systems closely adjust the amount of oxygen added to the water to keep the total gas pressure near saturatio
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