Zebra Mussels

Zebra mussels Madison lakes

About Zebra Mussels in Madison lakes

What are zebra mussels?

You may have heard about zebra mussels in Madison lakes; what are zebra mussels?

Zebra mussels are the only freshwater mussels that can attach themselves to solid surfaces underwater. Their yellowish or brownish shells, can be up to two inches long. Zebra mussels are D-shaped and have light and dark colored stripes.

This prolific invader deprives native fish and other aquatic life of food and habitat, and is a costly nuisance to boaters and swimmers. Zebra mussels are known to reach densities as high as 700,000 per square meter.

Zebra mussel
Zebra mussels have light and dark colored stripes and can grow up to two inches long

Where do they come from?

They are native to Europe and Asia, and arrived in the U.S. in the late 1980s. They most likely reached North America as larvae in the ballast water of ships traveling from freshwater ports in Eurasia to the Great Lakes.

How are they spread?

Microscopic zebra mussel larvae float in the water, allowing them to spread through connected water systems. They are also known to attach to boats, which may be a major cause of their spread to unconnected inland lakes.

Have they been found in the Yahara lakes?

Yes. Zebra mussels were discovered in fall of 2015 in Lake Mendota by an undergraduate University of Wisconsin limnology class. Scientists found zebra mussels aged up to three years old, which indicated the species has been established in the lake for a while. Currently, zebra mussels can be found densely colonizing submerged rocks, logs, and other firm surfaces throughout lake. According to the DNR, they are also in Lake Monona.

Zebra mussels
Zebra mussels can reach high densities

How are lakes and recreational users affected?

Zebra mussels tend to have negative effects on the water systems they inhabit. They feed on suspended zooplankton, plants, and other debris. They are highly efficient at filtering food from water, and can significantly deplete zooplankton populations. This creates increased competition for fish that also rely on these food sources. Zebra mussels’ sharp shells can also impact barefooted swimmers.

Zebra mussels high filtration capacity usually results in increased water clarity. UW-Madison researchers believe the population in Lake Mendota is sufficient to filter the lake’s volume of water within a few weeks. While this may seem like a good thing, it allows for greater sunlight penetration to the bottoms of our fertile lakes. As a result, large growths of rooted aquatic plants and “filamentous” algal mats generally form on the lakebed.

Filamentous algae is not a public health threat like the potentially toxic cyanobacteria (blue-green algae). Algae that forms on the lakebed may look stringy and form fibrous mats. It can have the consistency of cotton candy and can be seen draping over or attached to aquatic plants like green fuzz. As these filamentous algae start to die and break apart, they float to the surface and wash into shore, contributing to foul smells as they decay.

Zebra mussels are known to consume all types of algae but one. Cyanobacteria, which continues to be a large problem for Madison’s lakes and which poses a risk to humans, is not eaten by zebra mussels. In fact, cyanobacteria may even do better in water systems where zebra mussels are present.

Can we get rid of zebra mussels?

Unfortunately, once zebra mussels are established, very little can be done to control them. Backflushing, industrial vacuums, physical removal, and oxygen deprivation are sometimes used to reduce populations. Bacterial and chemical applications that kill zebra mussels are also occasionally utilized. However, these methods are unlikely to succeed in Madison’s major lakes, such as Lake Mendota, given their large size. While some types of fish, waterfowl, and other animals are known to consume zebra mussels, their impacts on zebra mussel populations are usually negligible.

Zebra mussels Madison lakes
Zebra mussels found on a cinder block in Lake Monona by a Clean Lakes Alliance volunteer monitor. Photo by Tom Smith.

How do their populations usually change over time?

Once zebra mussels are introduced into a lake, it may take years before they reach noticeable densities. Infested lakes with the right conditions (i.e., rocky vs. soft bottoms) often experience an initial explosion in population as the zebra mussels have few natural predators to keep them in check. Populations may then subside before eventually reaching a more steady state.

According to Wisconsin DNR, 250 lakes and rivers in the state are known to have zebra mussels. In Dane County, the infected lakes include Mendota, Monona, Wingra, Waubesa, Kegonsa, Koshkonong, and Upper Mud Lake, as well as the Wisconsin River. The population of zebra mussels in Madison lakes is growing rapidly. Because zebra mussels start out as microscopic, free-swimming larvae, they can easily find their way into connected waterbodies.

What can I do?

Boaters should carefully inspect their equipment and remove any attached mud or plants before leaving or entering a new waterbody. Boaters should also drain bilge water, livewells, and bait buckets that can harbor invasive organisms. In addition, it is important not to release aquarium plants, fish, or other animals into the water. By taking these steps, you are helping to prevent the spread of nuisance species to other lakes, or introducing new problems to YOUR lake.

You can also help by participating in efforts to reduce phosphorus runoff. This includes keeping soil, leaves, fertilizers, and other phosphorus-containing material out of your street and storm gutters. Keeping one pound of phosphorus out of the lakes can prevent the growth of up to 500 pounds of algae. Maintaining clean construction sites, planting water-absorbing rain gardens, and raking fall leaves out of the street gutter in front of your home are all examples of ways we can all be part of the solution for cleaner lakes.

Become a Friend of Clean Lakes today to help protect water quality in Madison’s lakes.

Search our website for something else: