New aerators sought for the Big Eau Pleine

A premier fishing destination in Marathon County is in need of new aeration equipment in order to prevent future fish die-offs, but before that work can be done, county officials are being asked to contribute $175,000 toward the project.

Pictures of massive northern pike and trophy walleyes – alongside images of dead fish washed up on shore – were displayed during a presentation to the Environmental Resources Committee last week by a coalition of groups dedicated to protecting the Big Eau Pleine Reservoir.

Describing the waterway as a unique treasure for the county, the coalition members said the local fishery is at risk due to a steady stream of algaeproducing phosphorus flowing into it from surrounding farmland. One way to prevent large fish kills – like those that happened in 2009 and 2013 – is to break up the winter ice with aerators originally installed in 1980.

Ben Niffenegger, senior environmental specialist at the Wisconsin Valley Improvement Company (WVIC), said the owner’s manuals for the aerators date back to 1962, and the motors are constantly leaking oil, requiring refills every few days just to keep them running.

Estimates for upgrading or replacing the blowers, motors and electrical equipment range from $125,000 to $150,000, based on quotes from contractors, he said. New blower motors are expected to take nearly a year to arrive, so he said the aerators will likely have to “limp along” for at least two more winters even if funding is provided.

Niffenegger was joined by county conservationist Kirstie Heidenreich and two representatives of the Wisconsin DNR, which took over operation of the aerators in 2008 and also monitors the health of the fishery.

Lucas Koenig, the DNR’s fishery biologist for Marathon and Portage counties, said the reservoir “grows some pretty impressive fish.” He touted the high number of large walleye and northern pike present in the waterway even after a 2009 winterkill claimed an estimated 80 percent of the fish population. “Honestly, we’re really lucky to have it,” he said.

Walleye are the predominant species in the reservoir – with an estimated population of just under 25,000 in 2017 based on a mark-recapture survey – but crappies are “a very close second,” he said. The relative abundance of crappie increased by 800 percent between surveys done in 2003 and 2017. The estimated walleye population decreased slightly during that time period, he said, but the overall “biomass” went up because the fish that were captured were larger.

Koenig said the reservoir contains about 3.3 adult walleyes per acre, “which is pretty much what you want to see from a healthy fishery.” He also noted that the percent of walleye measuring 20 inches or more ranked seventh in the state since 2010.

Also, the size structure of northern pike in the Eau Pleine ranked first in the state, which he called “extremely impressive.” Out of 591 northern pike that measured 14 inches or longer, 361 were 28 inches or bigger, 92 were 32 or inches or bigger, 47 were 40 inches or bigger, and two were 44 inches or bigger, he said.

“This is the only spring Fyke netting survey in the entire state of Wisconsin, since 2010, that has captured any pike at all over 44 inches,” he said. “That’s awesome.”

The DNR’s next fish survey is scheduled for 2024, he said.

Koenig said the Eau Pleine’s fishery draws anglers year-round. In January alone, he said the DNR estimates 186,000 “angling hours” have been spent by people fishing on the reservoir.

“If you drive past some of the boat launches this winter, they’re absolutely packed with vehicles,” he said. “There’s a lot of people out there this year. It gets a lot of fishing pressure.”

Niffenegger also talked up the recreational importance of the reservoir, pointing to a 2021 WVIC survey of public landings that counted 50,000 unique boaters and fishermen during the open-water season. This number did not include homeowners living along the waterway.

Big Eau Pleine County Park is by far the most-visited county park, generating close to $167,000 in revenue in 2022, according to the Parks Department. In 2009, however, after the big fish kill, the county’s boat launch revenue dropped from over $6,000 down to about $700 that year.

The Big Eau Pleine watershed, which stretches northwest into parts of Clark and Taylor counties, encompasses a lot of agricultural operations, and runoff from those farms drain into the reservoir via streams and rivers. Phosphorus in that runoff creates large algae blooms, which lead to oxygen depletion and the possibility of widespread winterkills.

After the large fishkill in 2009, a task force made up of state, county and local groups formed to protect the reservoir, and money was raised to upgrade portions of the aeration system in 2010.

The DNR and WVIC work together to measure the dissolved oxygen levels in the reservoir on a weekly basis, cutting test holes every mile for about 18 miles, Niffenegger said.

“It’s a huge cooperative effort,” Koenig said. “A lot of people get together to survey this body of water.”

Niffeneger said the aerators provide a “limited refuge” for fish during periods of low oxygen, opening up about 30 to 60 acres of water across the 6,677 acres in the reservoir.

Heidenreich stressed the importance of partnerships in protecting the reservoir, noting that it takes multiple agencies to collect data and maintain and operate the aerators.

“Once the machines are up and running, it is the partners that keep them going,” she said.

Supervisor Tony Sherfinski, Schofi eld, said he liked the presentation but it was not as “forward-looking as I would like.” He wanted to see more details about the type of equipment that needs to be replaced and a project plan with pricing estimates.

“This is aspirational,” he said. “It’s great, but it’s not actionable.”

Heidenreich said the coalition of groups has obtained cost estimates from contractors and is basing its funding request on a range of pricing options. She said part of the plan is to work with an engineer to go through the options and help make the best decisions.

“At this point, we feel very confident asking for $175,000, but we’re not able to take the next step and start hiring an engineer or contractors to get into the nitty-gritty until we have funding available,” she said.

Sherfinski also asked what was being done to look at solutions beyond just replacing the aerators.

Heidenreich said the longterm solutions involve “regenerative agriculture and soil health,” with the goal of reducing the amount of phosphorus flowing into the reservoir. She said the county is working with farmers, through projects such as the Fenwood Creek Watershed Management Plan, to reduce the thousands of pounds of phosphorus entering the Big Eau Pleine every year.

“We don’t have to invest in infrastructure to feed fish oxygen if we fix what’s happening in the watershed on the land,” she said.

Following the presentation, the committee approved a motion to authorize the use of $175,000 from the Environmental Impact Fund to pay for the aerator repairs and replacements.The recommendation will go to the Human Resources, Finance and Property Committee before going to the full county board for final approval.

“The Big Eau Pleine is a gift to this county,” Heidenreich said. “It’s a gift that we have been given, but we need to preserve it.”