DNR investigating two Wisconsin fish kills that could be the result of heavy rains

The Wisconsin Department of Natural Resources is investigating two separate fish kills discovered recently in south central Wisconsin, and the culprit could be heavy rains.

A large group of dead fish was reported to the agency July 10 in Buffalo Lake, which is in Marquette County. The other was on the Yahara River in Stoughton, which links the lakes in and around Madison and eventually drains to the Rock River, then the Mississippi.

No additional information is available about the Yahara River fish kill at this time, the DNR said in a Wednesday news release.

As for Buffalo Lake, DNR fish biologists sampled multiple locations throughout the 400-square mile area that drains to the lake, including Marquette, Green Lake and Columbia counties, and found low dissolved oxygen levels in the upper watershed and some tributaries.

Dissolved oxygen is the amount of oxygen in a body of water that's available to fish and other aquatic life. Low dissolved oxygen levels can have a number of causes, according to the DNR — but one is heavy, repeated rainfall washing organic materials from wetlands, farm fields and other lands into the water.

Wisconsin has experienced plenty of rain recently, including some heavy downpours that have caused flash flooding and other problems.

Bacteria break down those organic materials, sucking up oxygen in the water. When the oxygen levels in a body of water get depleted and there's no deeper, cooler water for fish to move into, they can die, according to the DNR.

Reports of stained or tea-colored water in the area have also come in, the news release said, which is likely happening because tannic acids — a byproduct of decomposing vegetation — are being released from flooded wetlands.

Anyone out on Buffalo Lake or its tributaries may see pockets of dead fish, the DNR said, and dead fish may appear in the coming days in Lake Puckaway, a 5,000-acre nearby lake.

More reports of fish kills are likely due to the recent rains, the news release said, but such events don't usually have a lasting impact on the larger population.

The agency reported another fish kill Thursday in a northeastern Wisconsin creek that drains to the Fox River. It was caused by a manure spill in the area of eastern Outagamie County and western Brown County. In May, a manure spill from a western Wisconsin dairy caused a fish kill more than a mile long.

If you see a dying fish swimming erratically or gulping for air near the water's surface, or observe large numbers of dead fish, report it to your local fish biologist or the DNR tip line, 1-800-847-9367. Anglers and their pets should not consume dead or dying fish.

Madeline Heim is a Report for America corps reporter who writes about environmental issues in the Mississippi River watershed and across Wisconsin. Contact her at 920-996-7266 or mheim@gannett.com.