By Katie Hein, UW Trout Lake Station*

Eurasian watermilfoil is an invasive aquatic plant now widespread in Wisconsin lakes. Although it often persists at low levels, it can form dense surface‑topping mats that outcompete native vegetation, reduce gamefish abundance and growth, impede recreation, and lower property values. To lessen these impacts, Wisconsin has invested heavily in control efforts — more than $9.3 million annually.

Yet management actions can bring their own ecological costs. Herbicide treatments may kill larval fish and native aquatic plants, and they do not always reduce Eurasian watermilfoil abundance. In some lakes, the plant declines without intervention. Evaluating the tradeoffs among management options is therefore complex and better understanding how Eurasian watermilfoil functions as habitat after it becomes established could improve decision‑making.

Researchers at UW Trout Lake Station received funding from the Wisconsin Department of Natural Resources to investigate how Eurasian watermilfoil compares with native aquatic plants as habitat for invertebrates and fish. The idea emerged from workshops with lake enthusiasts and managers who noted that fishing along Eurasian watermilfoil edges can be surprisingly productive.

In summer 2025, Dr. Katie Hein, a UW aquatic research scientist, conducted a pilot study on Little Spider Lake with undergraduate students Caroline Gruber and Madeline Hetland. Snorkeling in plots dominated by Eurasian watermilfoil and in plots dominated by fern‑leaf pondweed, they found — unexpectedly — higher fish abundance and diversity in the Eurasian watermilfoil beds, including large schools of juvenile bluegill.

Juvenile bluegill swimming in Eurasian watermilfoil on Little Spider Lake

Over the next two summers, master’s student Abby Buzdon will expand this research. She and three undergraduate students have already begun sampling lakes this season. The team will characterize fish populations using electrofishing and traps, then establish six Eurasian watermilfoil plots and six native‑plant plots in each lake. Within these plots, they will sample zooplankton, larval aquatic insects such as dragonflies and midges, and fish. They will also compare largemouth bass diets between the two habitat types and deploy loggers that record water temperature and dissolved oxygen every ten minutes. Sampling has begun on Upper Gresham Lake, with a goal of studying six lakes in total.

This research will clarify how Eurasian watermilfoil compares to native aquatic plants as habitat for invertebrates and fish. The findings will help inform ecological considerations in aquatic plant management and support more nuanced decisions about when, where, and how to intervene.

A great background resource from WDNR for readers interested in learning more about Eurasian watermilfoil and other aquatic plants is available here.

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