Dane County Office of Lakes & Watersheds
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Madison's Most Unwanted Aquatic Hitchhikers

picture of a zebra mussel picture of several zebra mussels

Zebra Mussel


The zebra mussel is a tiny (1/8-inch to 2-inch) bottom-dwelling clam native to Europe. The mussel takes its name from its striped shell. Zebra mussels were introduced into the Great Lakes system in 1985 or 1986 and first turned up in Lake St. Clair. They were first found in Wisconsin waters of Lake Michigan in 1989. Zebra mussels have been found on Lake Monona and are likely in Mendota and Waubesa.

Problems associated with this invader

The exotics clog water-intake systems of power plants and water treatment facilities, along with the cooling systems of boat engines. According to the Dane County Fisheries Biologist, Kurt Welke, their presence can also damage the aquatic ecosystem. They can severely reduce and may eliminate native mussel species. Because they filter plankton (microscopic plants and animals) from the surrounding water, water clarity may improve, but the long-term effects aren't well understood at this time. Newly hatched larval fish need zooplankton and phytoplankton to survive and zebra mussels, which are incredibly efficient filterers, are competing with native fish for the same food source. Ironically, even though zebra mussels filter out many planktonic species, the nuisance blue-green algae - often the summer bane of the Madison lakes - is not one of them. In simple terms, zebra mussels take out the good stuff, but not the bad stuff.


  • Look like small clams with a "D" shaped shell, usually with dark and light colored stripes.
  • Most are under an inch but can be up to two inches long
  • Usually grows in clusters and are generally found in 6-30 feet of algae-rich water
  • Zebra mussels are the ONLY freshwater mollusc that can firmly attach itself to solid objects: submerged rocks, dock pilings, boat hulls, water intake pipes, etc.
  • More Information
picture of asian carp

Asian Carp

Four species of carp native to Asia (grass, black, bighead and silver) have been introduced to North America. Among these Asian carp, the bighead and silver have gained recent notoriety. Both species have established large populations and impacted native fishes in the Mississippi and Illinois Rivers, and are moving north toward Lake Michigan. It is possible that the silver carp is already in Wisconsin.

picture of eurasian milfoil picture of eurasian milfoil

Eurasian Water Milfoil (Myriophyllum spicatum)


Eurasian Water Milfoil is a submersed aquatic plant native to Europe, Asia and northern Africa. It is one of eight milfoil species found in Wisconsin and the only one non-native to the state. It first showed up in Wisconsin's counties in the 1960's and in the past three decades, has expanded its range to about 310 lakes including seven lakes in Dane County (Crystal Lake, Fish Lake, Kegonsa Lake, Mendota Lake, Monona Lake, Waubesa Lake, Wingra Lake).

Problems associated with this invader

Because of its potential for explosive growth and its incredible ability to regenerate, Eurasian watermilfoil can successfully outcompete most native aquatic plants, especially in disturbed areas. In a number of Wisconsin lakes, Eurasian watermilfoil has formed huge monoculture stands with vast mats of surface foliage that shade-out native aquatic plants and diminish the aesthetic beauty. Recreational activities like swimming, boating and sport fishing are also diminished on Wisconsin lakes infested with Eurasian watermilfoil.


  • Submersed, rooted aquatic plant in shallow waters (less than 30 feet deep)
  • Long branching stems near the surface with soft, feathery leaves
  • Leaves usually attached in whorls of four, but sometimes 3-5 Each leaf has 10-21 pairs of leaflets
  • Leaflets are usually closely-spaced
  • Leaves are limp when out of water
  • Top of plants often turn red Small reddish flowers in mid summer
  • Plants can grow up to 15 feet long
Further Information...

Other aquatic exotic species in Wisconsin

For pictures and info refer to the field guide to aquatic plants and animals.

  • Round goby (Neogobius melanostomus)
  • Sea lamprey (Petromyzon marinus)
  • Rusty crayfish (Orconectes rusticus)
  • White perch (Morone americana)
  • Flowering rush (Botumus umbellatus)
  • Curly-leaf pondweed (Potamogeton crispus)
  • Ruffe (Gymnocephalus cernuus)
  • Spiny water flea (Bythotrephes cederstoemi)
  • Purple loosestrife (Lythrum salicaria)
  • Common carp (Cyprinus carpio)
  • Rainbow smelt (Osmerus mordax)
  • Alewife (Alosa pseudoharengus)

Steps to keep exotics out of our waterways

Boaters using Wisconsin waterways can help prevent transporting exotic plants and animals from one lake or river to another by taking the following precautions with their boats and equipment:

  • Remove plants and animals from your boat, trailer and accessory gear (anchors, centerboards, trailer hitch, wheels, cables and axles) before leaving the water access area.
  • Drain your livewells, bilge water and transom wells before leaving the water access area.
  • Empty your bait bucket on land, never into the water.
  • Wash your boat, tackle, downriggers and trailer with hot water when you get home. Flush water through your motor's cooling system and other boat parts that normally get wet.
  • Learn what exotic organisms look like.
  • Look for the signs posted at boat landings.
  • Click here for an article on seasonal boat and pier removal.
  • More information

Important websites for more information



Yahara Waterways Trail Guide The Yahara Waterways Water Trail Guide is a great resource for exploring our area waters.