October 8, 2025 –OTTER TAIL COUNTY, MN – Otter Tail County officials announced that the Minnesota Department of Natural Resources (DNR) has positively identified two invasive aquatic species in Anna Lake, located north of Underwood off County Highway 35. The invasive species discovered include starry stonewort and zebra mussels.
The initial discovery was reported on Thursday, September 11, when the DNR confirmed the presence of both species at the end of the dock at the public access in approximately 4-6 feet of water. Following the confirmation, a comprehensive survey of the lake was conducted on Tuesday, September 16, by Spencer McGrew, Otter Tail County AIS Specialist, and Mark Ranwieler, Aquatic Invasive Species Specialist, Northwest Region, DNR based out of Fergus Falls. The survey found no additional starry stonewort beyond the initial discovery site at the boat ramp.
In response to the initial findings, county officials moved quickly to implement a rapid response and management strategy. Otter Tail County retained the services of Professional Lake Management (PLM) of Brainerd to conduct specialized treatment of the affected area.
“We are working with our partners to move forward with a rapid response plan,” said McGrew. “We’d like to remind water users to be diligent in making sure their equipment is clean, drained and dry, and that they dispose of unwanted live bait in the trash.”
The county received approval for an Invasive Aquatic Management Permit (IAMP) from the DNR to treat a rectangular half-acre area of water near the Anna Lake public access. The treatment utilized Environmental Protection Agency (EPA) and Minnesota Department of Agriculture-approved chemicals, including Copper Sulfate and Hydrothol 191.
The treatment was conducted on October 2. The estimated cost of the effort is $285.00.
Starry stonewort is an invasive alga that can form dense mats on lake bottoms, while zebra mussels are small mollusks that can rapidly reproduce and damage aquatic ecosystems. Both species can significantly impact water quality, native wildlife, and recreational activities.
Lake users are reminded to clean, drain, and dry all watercraft and equipment when moving between water bodies to prevent the spread of invasive species. The county supports two self-service watercraft rinse stations in Battle Lake (Lions Park) and Perham (Paul Miller Park), and DNR-operated units can be found on the DNR website at dnr.state.mn.us/invasives/watercraft_inspect/courtesydecon.html
County officials will provide updates on the treatment progress as the project develops.