Thousands of fish die in Fitzroy River after floods TESTS on the Fitzroy River following the deaths of thousands of fish have found oxygen levels too low for them to survive. Environment Minister Andrew Powell said the Rockhampton Regional Council test found dissolved oxygen levels were between 1.7 and 1.8 mg/l. The optimal dissolved oxygen level for fish is 5 mg/l. Below 2 mg/l they are not likely to survive," Mr Powell said. Fish need oxygen to live; it's as simple as that. The fact is that this incident, as unfortunate and distressing as it is, is a natural phenomenon which is a result of the natural disaster that hit Queensland last week. We have similar reports of fish kills up and down the coast, in Gladstone and the Boyne River. Environment Department staff also were investigating but low oxygen levels were most likely the result of large amounts of organic material being washed into river by heavy rain. "The organic material decomposed and consumed oxygen, which made it impossible for fish to survive," he said.