High oxygen levels suspected as cause of Lake Owasso fish kill

A few of the hundreds of dead fish found belly-up in the frozen shallows of Lake Owasso late last month. The DNR believes high oxygen levels may be to blame. (Submitted photos)

It’s still unclear what killed hundreds, possibly thousands of fish in Lake Owasso over the long Thanksgiving weekend just days after the lake froze over. The prevailing theory is that high oxygen levels in the lake may have caused the fish kill.

“Right now it’s a theory,” Dave McCormack, assistant regional fisheries manager with the Department of Natural Resources said. “We are not exactly sure what caused this.”

McCormack said the DNR treated the lake, located partially in both Roseville and Shoreview, for Eurasian Milfoil and other invasive plant species on Aug. 2, but said the herbicide would kill the plants in days and would not be present in September, let alone late November.

The DNR is doing tests to eliminate other possible causes. DNR fisheries specialist TJ DeBates said there were reports of Garlon herbicide being used in upland areas to kill Buckthorn on the north side of the lake this fall, but he did not believe the application could have been the cause. He said there would have to be a point source, such as a culvert, where the chemical would drain into the lake. He said even then it would only affect a small area, and dead fish were found in shallow areas all around the lakes shoreline.

“We are doing the responsible thing here and looking into the possibility,” he said. “There’s an outside chance, but it’s not likely.”

McCormack said tests show that oxygen levels in Lake Owasso were high at the time, over 14 parts per million. With no snow cover on the frozen lake and abundant algae and vegetation, the process of photosynthesis continued -- boosting oxygen levels. When the lake quickly froze over late last month, oxygen was not allowed to escape, McCormack said.

Several residents living near the shores of the 375-acre lake - drilled holes and removed some of the fish.

Most of the dead were pan fish, McCormack said, but there were also some bass, walleye and muskie -- a few of which were over four feet long. Some of the fish were sent to a pathology lab for testing, he said. However, he did not expect the test results to confirm the cause of death, because the fish had been dead too long.

Large fish kills are uncommon in autumn and early winter, especially from super saturation of gas, DeBates said. 

“This is the first time I’ve seen something like this in my 10-year career with the DNR. It’s somewhat unprecedented, but there have been some cases,” he said.

Large fish die-offs are more common in late winter and early spring, and are more frequently a result of too little oxygen. This can happen after months of heavy snow cover on shallow lakes, which prohibits sunlight from reaching the underwater plants, thus stopping the process of photosynthesis and lowering oxygen levels in lakes. DeBates said the DNR monitors oxygen levels in Lake Owasso in winter, and aerates when levels are too low.

McCormack said the DNR would be able to estimate the number of fish killed after ice-out this spring, when they will return to net the dead fish.

Joshua Nielsen can be reached at jnielsen@lillienews.com or 651-748-7824.

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