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Aquatic Invasive Species

Latest AIS Management Update | 2022 AIS Prevention & Management Plan


Common Carp and Other Rough Fish


Species and Origin:The common carp is a large omnivorous fish. They have large scales, a long dorsal fin base, and two pairs of long barbels (whiskers) in its upper jaw. Native to Europe and Asia, it was intentionally introduced into Midwest waters as a game fish in the 1880s.
(Be aware of a native look-a-like: the native fish bigmouth buffalo (looks like a carp without barbells)


  • Common carp are one of the most damaging aquatic invasive species due to its wide distribution and severe impacts in shallow lakes and wetlands

  • Their feeding disrupts shallowly rooted plants muddying the water

  • They release phosphorus that increases algae abundance

  • Carp induced declines in water quality causes declines of aquatic plants needed by waterfowl and fish

Status: They are established in 48 states. They are distributed in hundreds of waters in the southern two-thirds, and a few waters in the northern third of Minnesota. See US map.

Means of spread: The incidental inclusion and later release of live bait spreads common carp.

Where to look: They live in lakes, rivers, and wetlands and are often seen in spring when they spawn in shallow waters.

Regulatory Classification: It is a regulated invasive species (DNR), which means introduction into the wild is prohibited. Fish caught while angling may be returned to the same water body.


  • Fish barriers: The CLFLWD owns and operates 3 fish barriers. Two of the three are mechanical barriers located at the inlet and outlet of Bone Lake. One is an electric barrier located in the tributary stream draining from Shields Lake into Forest Lake.
  • Winter aeration: The CLFLWD owns and operations a winter aeration system on Moody Lake. The purpose of the aeration system is to control bullhead populations within the lake. This is accomplished by increasing oxygen levels within the lake during the winter months in order to increase game fish winter survival rates. Game fish predate and provide competition to the bullhead population.
  • Fish surveys and carp population estimates: In 2015 the CLFLWD worked with St. Mary's University to conduct fish surveys and carp population estimates on Bone Lake, Moody Lake and Shields Lake. Blue Water Science performed mini-trap net fish surveys and submerged obstacle evaluations for Bone Lake and Moody Lake.

District lakes currently with nuisance levels of common carp and/or other rough fish:

For more information, visit MN DNR's webpage on common carp.

Curly-leaf Pondweed


Common names: Curly cabbage, crisp pondweed.

Location: Grows from  shore to depths of up to 15 feet.

Description: Leaves are somewhat stiff and crinkled, approximately 1/2-inch wide and 2 to 3 inches long; leaves are arranged alternately around the stem, and become more dense toward the end of branches; produces winter buds can be confused with claspingleaf pondweed.

Hints to identify: Has small "teeth" visible along edge of leaf; begins growing in early spring before most other pondweeds; dies back during midsummer; the flower stalks, when present, stick up above the water surface in June; appears reddish-brown in the water, but is actually green when pulled out of the water and examined closely. Easily confused with claspingleaf pondweed, which has no "teeth" are their edges.

Importance of plant: Provides some cover for fish; several waterfowl species feed on the seeds; diving ducks often eat the winter buds.

Management: Curlyleaf pondweed is not native to the United States and often causes problems due to excessive growth. When control is necessary, herbicides and harvesting can be effective. In order to reduce mid-summer die-back and phosphorus release, the CLFLWD manages curly-leaf pondweed on lakes within the District that have nuisance levels of growth. In 2018 the CLFLWD treated curly-leaf pondweed with herbicide on Forest Lake. Curly-leaf on Comfort Lake and Bone Lake was not treated due to low levels of growth and/or small acreages of nuisance growth in the spring. Hand pulling events have been organized with each lake association in order to mechanically manage both curly-leaf pondweed and Eurasian watermilfoil on these lakes.

District lakes currently containing curly-leaf pondweed:

Full list of MN infested waters.

For more information, visit MN DNR's webpage on curly-leaf pondweed.


Eurasian Watermilfoil


Eurasian watermilfoil was accidentally introduced to North America from Europe. Spread westward into inland lakes primarily by boats and also by waterbirds, it reached Midwestern states between the 1950s and 1980s.

In nutrient-rich lakes it can form thick underwater stands of tangled stems and vast mats of vegetation at the water's surface. In shallow areas the plant can interfere with water recreation such as boating, fishing, and swimming. The plant's floating canopy can also crowd out important native water plants.

A key factor in the plant's success is its ability to reproduce through stem fragmentation and runners. A single segment of stem and leaves can take root and form a new colony. Fragments clinging to boats and trailers can spread the plant from lake to lake. The mechanical clearing of aquatic plants for beaches, docks, and landings creates thousands of new stem fragments. Removing native vegetation creates perfect habitat for invading Eurasian watermilfoil.

Eurasian watermilfoil has difficulty becoming established in lakes with well established populations of native plants. In some lakes the plant appears to coexist with native flora and has little impact on fish and other aquatic animals.

Likely means of spread: Milfoil may become entangled in boat propellers, or may attach to keels and rudders of sailboat. Stems can become lodged among any watercraft apparatus or sports equipment that moves through the water, especially boat trailers.

Management: In 2021 the CLFLWD coordinated surveys of Eurasian watermilfoil, but not treatment.

District lakes currently infested with Eurasian watermilfoil:

Full list of MN infested waters.

For more information, visit MN DNR's webpage on Eurasian watermilfoil.


Flowering Rush



Appearance: Perennial aquatic herbaceous plant. It grows 1-4' high on an erect stem along shores in shallow water. In deeper water it grows submerged without producing flowers. Flowering rush is very difficult to identify when not in flower. It closely resembles many native shoreland plants, such as the common bulrush.

Leaves: Leaves are sword-shaped, triangular in cross section.

Flowers: Pink flowers are arranged in umbels (umbrella-shaped).

Seeds: Populations in the eastern U.S. produce seeds. Only one Minnesota population (Forest Lake) produces viable seeds.

Roots: Reproduces by vegetative spread from its rootstock in form of bulb-lets. Both seeds and bulb-lets are dispersed by water current. The flowering rush population in Forest Lake is known to produce viable seeds meaning it can reproduce and spread by both bulb-lets and seeds.

Management: The CLFLWD applies for grants and manages flowering rush on Forest Lake, as it is currently the only lake within the District that is infested. The herbicide that will be used for management in 2018 is called Tribune, and the active ingredient is Diquat. The product label containing all safety information can be found here: Tribune (Diquat) Product Label.

See also DNR aquatic plant regulations for permits to remove flowering rush.

District lakes currently infested with flowering rush:

Full list of MN infested waters.

For more information, visit MN DNR's webpage on flowering rush.

Zebra Mussels


Species and Origin: Zebra mussels are small, fingernail-sized animals that attach to solid surfaces in water. Adults are 1/4 to 1-1/2 inches long and have D-shaped shells, often with alternating yellow and brownish colored stripes. Female zebra mussels can produce 100,000 to 500,000 eggs per year. These develop into microscopic, free-living larvae (called "veligers") that begin to form shells. After two to three weeks, the microscopic veligers settle and attach to any firm surface using tiny fibers called "byssal threads." Zebra mussels are native to Eastern Europe and Western Russia and were brought over to the Great Lakes in the ballast water of ships. Populations of zebra mussels were first discovered in the Great Lakes in 1988.

Impacts: Zebra mussels can be a costly problem for cities and power plants when they clog water intakes. Zebra mussels also cause problems for lakeshore residents and recreationists; for example, they can:

  • attach to boat motors and boat hulls, reducing performance and efficiency,

  • attach to rocks, swim rafts and ladders where swimmers can cut their feet on the mussel shells, and

  • clog irrigation intakes and other pipes.

Zebra mussels also can impact the environment of lakes and rivers where they live. They eat tiny food particles that they filter out of the water, which can reduce available food for larval fish and other animals, and cause aquatic vegetation to grow as a result of increased water clarity. Zebra mussels can also attach to and smother native mussels.

Status: Zebra mussels have spread throughout the Great Lakes, parts of the Mississippi River, and other rivers and inland lakes. They are established in Minnesota and were first found in the Duluth/Superior Harbor in 1989. See the infested waters list for more information on water bodies in Minnesota where zebra mussels have been found or water bodies that are closely connected to zebra-mussel-infested waters.

Management: Zebra mussels were discovered in Comfort Lake in July 2017, in Forest Lake in July 2015, and in Bone Lake in June 2019. After a rapid response assessments were performed following each discovery, it was determined that the infestations were too widespread for treatment to be feasible on either Comfort or Forest, however Bone Lake was approved for a pilot study. More information on the Bone Lake zebra mussel pilot study can be found in the report linked below. The CLFLWD continues to work with volunteers to monitor each lake for population density estimates, and other District lakes for early detection purposes.

District lakes currently infested with zebra mussels:

Full list of MN infested waters.

For more information, visit MN DNR's webpage on zebra mussels.

Purple Loosestrife


Purple loosestrife is a wetland plant from Europe and Asia. It was introduced into the east coast of North America in the 1800s. First spreading along roads, canals, and drainage ditches, then later distributed as an ornamental, this exotic plant is in 40 states and all Canadian border provinces.

Purple loosestrife invades marshes and lakeshores, replacing cattails and other wetland plants. The plant can form dense, impenetrable stands which are unsuitable as cover, food, or nesting sites for a wide range of native wetland animals including ducks, geese, rails, bitterns, muskrats, frogs, toads, and turtles. Many rare and endangered wetland plants and animals are also at risk.

Currently there are about 2,000 purple loosestrife infestations recorded in 77 of Minnesota's 87 counties. Of those sites, the majority (70%) are lakes, rivers, or wetlands. Inventory totals indicate that Minnesota presently has over 58,000 acres infested with purple loosestrife.

Likely means of spread: Seeds escape from gardens and nurseries into wetlands, lakes, and rivers. Once in aquatic systems, seeds are easily spread by moving water and wetland animals.


The treatment of purple loosestrife on Lake Keewahtin has been, and will continue to be, a multi-year effort. In 2016 and 2017 Loosestrife beetles (Galerucalla sp.) and weevils (Nanophyes mamoratus) were collected from White Bear Lake and released into known patches of purple loosestrife around Lake Keewahtin. Unfortunately, the population of beetles and weevils did not establish throughout the lake and the District turned to other treatment methods. In September 2020 the District surveyed purple loosestrife growth on the lake. That survey was used to obtain a treatment permit from the DNR and determine treatment areas for 2021. Treatment with the herbicide Triclopyer was conducted in September 2021 (Survey and Treatment Map). A follow-up survey is scheduled to be conducted this August to determine the effectiveness of the treatment and inform future management decisions.

District lakes currently infested with purple loosestrife:

Full list of MN infested waters.

For more information, visit MN DNR's webpage on purple loosestrife.

Species to Watch


The following species are not known to be present within the CLFLWD.

What You Can Do


Help prevent the spread

Read about what you can do to prevent the spread of AIS here


Learn to identify invasive species versus native species

MN Aquatic Invasive Species Research Center AIS Identification


Manage invasive species and other aquatic plants along your shoreline

The MN Department of Natural Resources oversees all chemical treatments of AIS and other aquatic plants in the state. An aquatic plant management permit (APM) is required to clear areas of aquatic plants for surface water access. The permit is generally issued to lake homeowners or groups of lake homeowners who want to maintain a clear area near their property for recreation. More information can be found by contacting your regional DNR Aquatic Plant Management Specialist, or by visiting The DNR also offers a step-by-step guide for identifying appropriate aquatic plant control options: