Last week's die-off of fish on Lake Agnes in Alexandria, originally thought to be caused by a naturally present bacteria, may be because fish weren't able to get enough oxygen, the DNR fisheries office in Glenwood says.

When Bill McKibbin, acting regional director of the Glenwood office, recently returned from vacation, he took a look at Lake Agnes himself. He also reviewed the public's reports of the die-off and found that although the fish kill involved almost all crappies, it also involved some perch, bluegill and white suckers. He also found that members of the public reported the fish as gasping for air.

That indicated that the fish kill probably wasn't due to the columnaris bacteria, but likely because oxygen rapidly left the water. That happens, McKibbin said, when dying algae robs the water of oxygen. Lake Agnes has an abundance of algae due to high phosphorus levels. Phosphorus enters the lake from the Alexandria Lake Area Sanitary District, which cleans sewage water, then pours it back into the chain of lakes. It also enters the water from stormwater runoff, and when lakeshore owners fertilize their lawns.

McKibbin said the algae died off during recent cloudy and stormy weather, when it was unable to conduct photosynthesis. Fish kills are more common in the spring during spawning season and rapid changes in water temperature.

The Glenwood office has been busy this year tracking down reports of fish kills, he said.

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"I've worked for the DNR for almost 20 years in Glenwood here, and this is by far one of the busiest years we've ever had regarding fish kill reports," McKibbin said. "Most of our fish kills were reported in the springtime when water temps jumped about 10 degrees in four days."

So far this year, the office has received documented reports of fish kills in 10 locations in lakes in Douglas, Pope, Grant and Stevens counties, McKibbin said. In a normal year, they receive one to three such reports. It's been mostly a problem in shallow lakes, that are warmer, lower and weedier this year.

The fisheries office has encountered its own problems with fish kills this year, as walleye have died off in their shallow fingerling ponds. It remains to be seen if that will cause a problem for stocking lakes, he said.

"I've got friends, other managers across the state, and this is a pretty common experience in the southern two-thirds of the state," he said.

Crappies in Lake Agnes were already stressed. A 2017 report found lots of crappies, and they appeared stunted, McKibbin said.

Too many crappies means they are more stressed and more susceptible to fish kills. This die-off thinned out the numbers of 3-5 inch crappies, "Which isn't necessarily a bad thing," McKibbin said, as the surviving fish may be able to grow faster.