The evidence is frozen just beneath the surface of popular Lake Owasso in Ramsey County: Hundreds and perhaps thousands of dead crappies, walleyes, bass and muskies.
The suspected killer is a rare but deadly quirk of nature: Too much oxygen in the water.
“Usually lack of oxygen in winter causes fish kills,’’ said Donn Schrader, Department of Natural Resources fisheries specialist. “In this case it looks like the supersaturation of oxygen in the fish killed them. It’s kind of a freaky thing.’’
He said the incident killed an unknown number of fish, but officials believe the event is over and the fishery — minus a bunch of fish — should recover. “Whatever happened, happened quickly and is past,’’ he said.
The DNR regularly stocks the lake, which straddles Shoreview and Roseville, with muskies and walleyes, but good numbers of northerns and panfish also are present.
Les Hassler, 65, who lives on the lake, strapped on ice skates last week to take advantage of the clear, smooth ice and was shocked to discover dead fish just below the surface.
“There were hundreds and hundreds of panfish, and some nice largemouth bass, walleyes and muskies,’’ he said Monday. “One of the muskies was 48 inches, and I saw eight others almost that size. It’s a shame.’’
Water samples taken over the weekend and on Monday showed oxygen levels of up to 14 parts per million, “which is extremely high,’’ Schrader said. “And that was after the event, so it may have been higher than that.’’
Here’s what the DNR thinks happened: Vegetation and algae in the lake produce oxygen, and when the lake quickly froze, excess oxygen was trapped beneath the ice. The lack of snow meant sunlight penetrated the clear ice, allowing plants to continue the photosynthesis process. Oxygen levels rose too high, killing some fish, mostly in shallow water.
“Obviously it’s not happening on every lake,’’ Schrader said. “The circumstances must have been just right.’’
Some dead fish were removed from the lake and are being tested by the DNR’s pathology lab in St. Paul to determine if something else caused the die-off. But Schrader said it’s unlikely they’ll find anything.
“A similar thing happened on Forest Lake last year,’’ he said. “They checked some fish and didn’t find anything wrong with them.’’
And he dismissed reports that weed control chemicals used on Owasso earlier this year might have caused the fish kill.
“The last treatment was done in August,’’ he said. “There’s no way that chemical would still be viable now.’’
Normally, fish die-offs occur late winter in shallow lakes because of a lack of oxygen. Snow and ice limit the amount of sunlight reaching vegetation, and oxygen is depleted.
Harland Hiemstra, DNR information officer, said the agency had planned to survey muskies, walleyes and northerns in Owasso next spring. Until the survey is done, it’s uncertain how the die-off affected the fish populations, he said.
Added Hiemstra: “Fish kills happen all the time. When it happens on a metro lake that is clear with no snow so you can see these dead fish, it gets your attention. If there had been snow on the lake, it might have gone completely unnoticed.’’