ROCHESTER — Winona-area advocates and city staff are taking tentative first steps to clean up Lake Winona by reducing the lake's common carp population.
Lake Winona has been on Minnesota's impaired waters list since 2014 because of its high concentration of phosphate nutrients. At the same time, the lake has housed common carp for decades — the invasive species has been in area waterways for more than a century.
Local officials decimated Lake Winona's carp population in the early 1970s, but the fish returned through the Mississippi River.
Carp are omnivores that feed off vegetation at the bottom of the lake, which captures phosphates and acts as food for other fish. Too many pesky carp can complicate treatment efforts in the future by depleting much-needed lake plants and stirring up even more lake sediment — leading to more issues if the lake can't properly absorb phosphate nutrients.
"The best thing we can do is manage them and keep the population down so they're not causing problems," said John Howard, a sustainability coordinator with the city of Winona.
Samples taken by Winona State University students from 2011 to 2019 show the population continues to grow, likely surpassing what the lake can healthily support, according to Prof. Neal Mundahl.
A 2020 report recommends first addressing the carp before taking further steps, such as using the compound alum to lower the lake's acidity.
Healthy Lake Winona, a group dedicated to improving the lake, is applying for a $50,000 Clean Waters Program grant from the Minnesota Soil and Water Board to fund a one-year study of where the local carp spawn.
If they secure a grant later this spring, the group will work with Winona to hire Carp Solutions, a company that specializes in tagging and tracking carp. City officials will tailor plans based on results of the study to cull carp over the next several years.
The carp's spawning grounds likely are close to Lake Winona, not in it, because bluegill in the lake often feed on carp eggs. Mundahl suspects the carp spawn in wetlands or a shallow lake nearby, one that freezes out most fish in wintertime.
"They migrate out of lakes typically to spawn and then ... the adults return back to where they were originally," Mundahl said.
Howard said Winona will look for other funding sources if it doesn't get state funding, in part to address the lake's current problems and in part to develop strategies for other invasive species such as Asian carp or silver carp.
"That's just another pressure we'd want to factor in," Howard said.