DNR Investigating Fish Mortality Events In South Central Wisconsin

MADISON, Wis. – The Wisconsin Department of Natural Resources (DNR) is investigating fish mortality events in two different locations in south central Wisconsin. The first is in the Buffalo Lake Watershed in Marquette, Green Lake and Columbia counties. The second is on the Yahara River in Stoughton. Due to recent heavy precipitation, additional reports of fish mortalities are likely, and the DNR will continue to investigate reports as they come in.

On Buffalo Lake, a large number of dead fish were reported to the DNR on July 10. The DNR began investigating the cause on July 11. Fish biologists sampled multiple locations throughout the more than 400 square mile watershed and confirmed low dissolved oxygen levels across the upper water basin and some tributaries.

Dissolved oxygen levels can result from a combination of natural processes and environmental conditions, including heavy, repeated rainfall that carries organic materials from wetlands, agricultural fields and other types of land use to the watershed. When a waterbody has widespread low dissolved oxygen levels and no deeper, cooler water for fish to move to, it can result in fish mortality.

Reports of stained or tea-colored water have been observed, and they are likely caused by tannic acids, a byproduct of decomposing vegetation, being released from flooded wetlands. In addition to changes in watercolor, the public may notice oily sheens on the lake's surface, which occur as bacteria consume excess iron in the water.

All fish species in the watershed have likely been impacted, and anyone recreating on Buffalo Lake or its tributaries may see pockets of dead fish. No dead fish have been found in Puckaway Lake at this time; however, dead fish may appear in the coming days as water moves downstream from Buffalo Lake. 

Although anglers and boaters may see large numbers of dead or dying fish as oxygen levels continue to stabilize, these events usually do not have a lasting impact on the larger fish population.

DNR staff continue to monitor oxygen levels throughout the Buffalo Lake Watershed, and additional sampling will occur over the next several days.

DNR staff are also gathering data on a reported fish mortality in Stoughton on the Yahara River, and no additional information is available at this time.

The DNR appreciates reports from the public to aid in locating fish mortalities such as those occurring in southern Wisconsin. If you see a dying fish (swimming erratically or gulping for air near the water’s surface) or observe large numbers of dead fish, please report it immediately to your local fish biologist or the DNR tip line (1-800-TIP-WDNR or 1-800-847-9367). Do not try to collect or handle dead or dying fish.

Anglers and their pets should never consume dead or visibly dying fish. Always cook harvested fish thoroughly and follow the U.S. Food and Drug Administration’s food safety guidelines.