Theory on massive fish kill at Lake Iroquois: 'Lake turnover'

Theory on massive fish kill at Lake Iroquois: 'Lake turnover'

LAKE IROQUOIS — If there was a silver lining to last weekend's massive fish kill at Lake Iroquois near Loda, it was that the 80-acre lake had not yet been stocked with new fish this year.

Thousands of fish were found dead Sunday morning — everything from carp to bass to catfish to walleye to crappies to blue gill.

"I saw some of every species that I'm aware of that were still in Lake Iroquois that were part of the kill. It didn't discriminate based on species at all," said Lake Iroquois resident Mike Johnson, head of the Lake Iroquois Association Bass Club.

What caused the fish kill remained unknown Monday. An inspector for the Illinois Environmental Protection Agency collected water samples at the lake Monday to be tested later this week.

"I believe he's primarily looking at the outfall area of the lake to determine if there was something that entered the lake that could have caused the fish kill, or otherwise looking to see if it was lake turnover that reduced the oxygen levels," said IEPA spokesman Kim Biggs.

Lake Iroquois resident Jim Shearl, chairman of the association's recreation committee, said that "as best as we can determine," the fish kill was caused by the fish not getting enough oxygen due to a phenomena called "inversion," also known as "lake turnover."

Shearl said the lake's surface layer likely became "very hot" during recent "extremely hot and humid weather." Then on Friday, a cold snap occurred, causing the surface water to cool down, become more dense and sink, in turn causing the water and sediment on the lake's bottom to rise.

"The oxygen was not available in the water for the fish," Shearl said, because "the lake turned over."

Shearl noted that following the fish kill, the lake's water appeared "very muddy," which lines up with that theory.

A group of marine biochemists that the association uses to maintain oxygen and PH levels at its 5-acre fishing pond checked that pond on June 29, Shearl said. Around that time, the group had received reports of lake inversion elsewhere, he said.

"So this is something that doesn't happen very often, but it is common," Shearl said.

When the marine biochemists tested the oxygen levels at the small fishing pond, they predicted that the pond would "turn over." Yet it appears it was the lake, not the pond, that did so.

Shearl said the pond likely avoided a fish kill of its own because mechanical aerators had been installed in it this spring. The lake does not have any aerators, Shearl said.

"There's motors that create air that emits out into the (pond) that causes the water to move," Shearl explained. "And that aeration was enough to keep that fish population (in the pond) safe."

The fish kill was discovered Sunday morning. Later that morning, Shearl organized volunteers to help remove the dead fish.

"By 10 a.m., we had 30 people at our swim beach volunteering and cleaning up, picking up fish and putting them into an end-loader on a tractor," Shearl said. "Then we put (the fish) into a dump trailer and hauled them away to a proper disposal area.

"The volunteers worked for four hours on the swim beach. Then, after lunch, they moved to another cove and worked another four hours."

The cleanup continued Monday with about a dozen volunteers, Shearl said. He estimated it would likely continue for another week, noting that he expects more dead fish to surface in coming days. Larger dead fish often take longer to float to the surface.

"It's not done yet," Shearl said.

Will Brumleve is editor of the Ford County Record, a News-Gazette Media community newspaper. For more, visit

Sections (2):News, Local