Zebra Mussels

Ten things you should know about zebra mussels:

1. Where did Zebra Mussels come from?
Zebra Mussels are originally from the Caspian Sea region of Northern Iran, Azerbaijan, southern Russia, Kazakhstan, and Turkmenistan. Due to extensive trade between this region and Europe during the 17th and 18th centuries they were able to spread. Zebra mussels were carried with the trade vessels to Europe and spread throughout the continent, reaching the Isle of Great Britain by the 1830's.

When large ships head out on to the open ocean, they need to carry weight, called ballast, to remain stable. If the ship is not transporting cargo, that ballast is water. This water is picked up in the homeport carried with in the ship and dumped out as they prepare to load cargo. So whatever is in the ballast can travel thousands of miles before being released into new waters.

Zebra mussels were first discovered in Detroit's Lake St. Clair in 1988. They would have been brought there in ballast water, survived, and reproduced to a number where they were noticed. Since then they have spread by attaching to boats and floating with currents throughout the Great Lakes and Mississippi watersheds. This is where they are found today as well as some scattered lakes.

2. Why do they like to stick to hard things like bridges and boats?
They don't want to get washed away, so when they find a good place with plenty of food and good water (oxygen, pretty clean, etc.), they want to stay put so they attach.

3. How do they attach to things?
They use little sticky threads call a byssus, or byssal threads to attach to things. The byssus grows out of the "back" or hinge of the mussel.

4. What do they eat?
Phytoplankton: very small plants, like algae, that float in the water. Zebra mussels are filter feeders they take in water and remove what food they want. Two important thing to note: Lots of zebra mussels can eat lots of food, so other animals might not have as much as they need, and what zebra mussels don't eat, like zooplankton, or little floating animals, they wrap up in liquid and spit onto the river/lake bottom. Little insects and fish can't eat this spit. So both eaters of phytoplankton and zooplankton may be impacted.

5. Is there anything in their shell?
The shell is made up of lots of calcium and protein, kinda like an eggshell. The "insides" or body of a zebra mussel contains mussel tissue and various organs used for respiration, digestion, and reproduction.

6. How do they travel from place to place?
When zebra mussels are larvae, less than 3 weeks old, they float in the water and move with the current. After three weeks, they settle down in the river or lake to find a good hard surface upon which to attach. Generally, they will never move again, unless, of course, this good hard surface is a boat that moves them or they get knocked off. Humans and their boats are really the only way zebra mussels can move upstream, or across land. Help prevent their use of boats...

7. Are they harmful to humans?
Not directly, however, one doesn't want to step on their broken shells as they can be sharp. When zebra mussels die in large numbers, they can, like any animal, be a health hazard and smelly.

8. Are they harmful to plants and animals?
In some respects, zebra mussels would be good for plants. As the exotic mussels eat, they filter particles out the water. This allows sunlight to reach deeper in the water, and will lead to increased plant growth. However, in the presence of an explosion in numbers, zebra mussels will attach to anything, including plants, reducing the plant's health.

As for animals, if they enjoy increases in water clarity and plant cover-they may do ok. But animals that compete with zebra mussels, like our rich assortment of native mussels, will find it tough going. Zebra mussels can take much of food away from other animals. They reduce the oxygen in the water by critical amounts, and they colonize areas on the river bottom so nothing else can move in. In addition, zebra mussels attach themselves to native mussel so the natives can't eat, breathe or reproduce. In other areas of the country, native mussels have been wiped out entirely-we don't want this to happen here on the St. Croix. It's always easier to prevent an invasion than to mop up afterwards. Please help us by making sure your boat or bait bucket is free of exotic species.

9. When did they arrive at the St. Croix and what is the National Park Service doing?
Since 1992, the National Park Service has led efforts designed to halt or slow the spread of zebra mussels (Dreissena polymorpha) into the St. Croix Riverway. Prevention activities include education and information, access management, monitoring, planning for remediation and research. Zebra mussels have been found attached to a small number of boats in the St. Croix since 1995. Individual mussels have been discovered scattered in small numbers on the bottom along the lower 25 miles of river. While these animals have had an individual presence on the river for a number of years, a reproducing population had never been discovered on the St. Croix. Unfortunately, that changed during the summer of 2000. For the first time, zebra mussel reproduction was found within the river, along the lower 16 miles or so. The source of reproduction has yet to be pinpointed, but the resulting settlement of juvenile mussels was very disturbing. Park Service and U.S. Fish and Wildlife SCUBA Divers spent three weeks last summer exploring the Lower St. Croix Riverway in an effort to fully understand the extent of the situation.

Armed with the unfortunate knowledge that zebra mussels have arrived, the National Park Service has begun the task of evaluating every aspect of its prevention and control plan. Rules designed to stop the spread of zebra mussels from infested to noninfested waters will be strictly enforced. The river may indeed be at a juncture in its ecological history--what actions are taken in the future may be critical to the plants and animals that call the river home. Of additional importance is considering a recreating public that enjoys the many wonderful aspects of this nationally protected waterway. The management issues surrounding zebra mussels are complex and may be controversial, but are important for the well being of the St. Croix National Scenic Riverway for decades to come.

10. Where else can I get information about Zebra Mussels?
This website also provides information on quagga mussels. Quagga mussels are very similar to zebra mussels and have been found in the Mississippi River adding an additional threat to the St. Croix and Namekagon Rivers.

http://www.seagrant.umn.edu/ais/zebramussels_threaten This site also provides information on other exotic species like rusty crayfish and sea lampreys.


Zebra Mussel Activities Reports:
Provide information related to zebra mussel monitoring within the St. Croix River Watershed. They document what has been done to prevent the spread of zebra mussels and current status within the Riverway. Activities include those primarily funded by the U.S. Army Corps of Engineers and may include native mussel work on the river. The St. Croix River long-term zebra mussel density study, conducted at eight locations from Stillwater to Prescott, is contained in these reports.

2013 Zebra Mussel Activities Report (960 KB)

2012 Zebra Mussel Monitoring Report (753 KB)

2011 Zebra Mussel Monitoring Report (754 KB)

2010 Zebra Mussel Monitoring Report (1.55 MB)

2009 Zebra Mussel Monitoring Report (470 KB)

2008 Zebra Mussel Monitoring Report (461 KB)

St, Croix National Scenic Riverway Zebra Musse Activities 2007 (pdf 214KB)

2007 Zebra Mussel Density Data (360 KB)

2006 Quantitative Assessment of Zebra Mussels (204 KB)

2005 Quantitative Assessment of Zebra Mussels (269 KB)

2004 Zebra Mussel Density Data (423 KB)


A multiagency task force has been assembled to respond to the threat zebra mussels pose to the St. Croix River. Members meet annually to review and update a response plan based on current conditions. Presented are the annual spring action plans:

2017 Action Plan (PDF accessible)
2015 Action Plan (pdf 55.8 KB)
2013 Action Plan (pdf 55KB)
2012 Action Plan (pdf 55KB)
2010 Action Plan 2010 (pdf 54 KB)
2008 Action Plan 2008(pdf 53 KB)
2005 Action Plan 2005 (pdf 70 KB)

In 2005 and 2006, the task force provided their aquatic invasive species activities to the U.S. Fish and Wildlife Service:
2006 Standardized Reporting for the Aquatic Invasive Species, Interstate Management Plans: The St. Croix National Scenic Riverway (pdf 665 KB)

Last updated: April 30, 2019

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