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Aquatic invasive species (AIS) are plants, animals, or pathogens that are introduced by human action to an area where they do not naturally occur. Once they establish breeding populations, they spread rapidly in their new environments. AIS lack natural predators and competitors, which contribute to their population explosions. Learn more about AIS in Green Lake below.

Protecting Green Lake from Invasive Species

Protecting Green Lake from Invasive Species

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Zebra Mussels (Dreissena polymorpha) – small, fingernail sized organisms that can attach to any hard surface in water. Adults are approximately 1/4 inch to 1 1/2 inches long, and feature D-shaped shells with alternating yellow and brownish stripes.


Native to Eastern Europe, zebra mussels were first introduced into the Great Lakes through ballast water that was drained from international cargo ships. As effective filter feeders, they can remove large levels of plankton and other small organisms, creating increased water clarity levels. However, increased water clarity is not always an indicator of a healthy ecosystem, and the increased clarity can cause increased aquatic plant growth, creating an imbalanced ecosystem.

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Eurasian Watermilfoil (Myriophyllum spicatum) – once commonly sold as an aquarium plant, this invasive species was first introduced as early as the 1940’s. Similar in appearance to northern watermilfoil, a native species, Eurasian watermilfoil can be best identified by counting the number of leaflets. Eurasian watermilfoil will have 12 or more leaflets, compared to the 6 or less on native species.


Once established, Eurasian watermilfoil can quickly become the dominant plant in a local ecosystem, blocking sunlight from native species and spreading easily from one area to another. Often introduced by boaters who go from one boat landing to another without properly cleaning and removing plant debris from boats and trailers.

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Curly-Leaf Pondweed (Potomogeton crispus) – often the first pondweed to appear in spring, Curly-Leaf Pondweed thrives in cold waters, such as those found throughout Wisconsin and other Midwestern states. Due to its preference for cold water, Curly-Leaf Pondweed begins growing while ice may still cover most lakes, gaining a head start over other native plants. This unique life-cycle allows it to outcompete native plants for resources, but it also means that it dies off earlier than most other species.


This creates a unique problem because the decomposition of Curly-Leaf Pondweed in late summer can decrease dissolved oxygen levels, destroying native fish populations. In addition it can create a sludgy mess that is a headache for boaters and lakefront home owners.



Rusty Crayfish (Orconectes rusticus) – once commonly used by anglers as an excellent fishing lure, the Rusty Crayfish has now been effectively introduced throughout the Midwest. Dominant and aggressive, these crustaceans will outcompete other species for resources, devastating the natural food chain and degrading overall biodiversity.

While it is now prohibited to use Rusty Crayfish for fishing, capturing them is encouraged, so long as they are not introduced into another body of water. They are edible and make a scrumptious meal when prepared in a variety of ways. Plus, you have the added benefit of removing them from areas that they would otherwise negatively impact!

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Purple Loosestrife (Lythrum salicaria) – a wetland plant from Europe and Asia, purple loosestrife was first introduced as an ornamental landscape plant. Having no native predators or diseases to control its population, and with each plant producing over 1 million seeds per year, Purple Loosestrife spread broadly across the United States.

Having established itself as a dominant species in many wetlands, Purple Loosestrife damaged habitats that were vital to other plants and animals. Thankfully, it was discovered that beetles that were native to the same region that Purple Loosestrife originated, could be bred and released, destroying many of the plants while leaving native species alone. The Purple Loosestrife eating organisms, Galarucella beetles, are often bred and released as part of a control program in Green Lake County.



Carp (Cyprinus carpio) – carp destroy important vegetation, stir up the lake bottom, fuel algae growth,and muck up the water. Every spring, the GLA joins forces with our partners to remove invasive carp that harm the health of the lake. Fewer carp means a lake that is better protected against nutrient and phosphorus pollution.

How did they get here? In the 1800's the U.S. Government introduced carp to many lakes seeing them as a cheap and easy food source. However, the carp population soon grew out of control since one female carp can lay one million eggs per year!

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