Grass Carp
Invasive SpeciesFound in 13 lakes
Grass Carp appears on state infested-waters lists for 13 surface waters. Boaters and anglers moving gear in or out of those lakes carry a clean-drain-dry obligation under state law. 13 lakes — narrow enough that aggressive containment is still on the table. Most upper-Midwest invasives that established here did so quietly over a decade before anyone noticed, so a low number today is not a guarantee for tomorrow.
The presence of Grass Carp does not directly change a lake's letter grade — the grade measures the water itself, not the species living in it. Invasive presence is still worth reading alongside the grade.
Lakes with Grass Carp
| Lake | County | State | Grade | Depth |
|---|---|---|---|---|
| Big Cedar Lake-North Site-Near West Bend | Washington | WI | A | 105 ft |
| Geneva Lake | Walworth | WI | A | 135 ft |
| Oconomowoc Lake | Waukesha | WI | B | 60 ft |
| Oconomowoc Lake Site | Waukesha | WI | B | 60 ft |
| Fowler Lake | Waukesha | WI | B | 50 ft |
| Big Cedar Lake | Washington | WI | B | 105 ft |
| Little Cedar Lake | Washington | WI | B | - |
| Big Cedar Lake Northern Basin Deep Site | Washington | WI | B | 105 ft |
| Spiritwood Lake | Stutsman | ND | C | - |
| Spring Lake | Dakota | MN | NR | 17 ft |
| Nelson Lake | Goodhue | MN | NR | - |
| Robinson Lake | Wabasha | MN | NR | - |
| Grey Cloud Slough Lake | Washington | MN | NR | - |
Frequently Asked Questions
How many lakes have grass carp?
Grass Carp has been documented in 13 graded lakes: 8 in Wisconsin, 4 in Minnesota, 1 in North Dakota.
Does grass carp affect water quality?
Invasive species can significantly affect lake ecosystems. They may alter nutrient cycling, displace native species, and change water clarity. Lakes with grass carp should be monitored for changes in water quality over time.
Invasives Found With Grass Carp
More on invasive species and lake health
Zebra mussel counts from the lakes LakeQuality grades, the counties that show up most often, and the Clean, Drain, Dry rules that still matter.
How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.
Before you buy lakefront, check the water: clarity trend, nutrient impairment, algae advisories, invasive species, and depth — a data-backed checklist.
Source: EPA National Aquatic Resource Surveys, 2026.