Zebra Mussels

Ten Things To 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 the spread of zebra mussels by cleaning your boat.

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 (NPS) 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 22 miles. The source of reproduction has yet to be pinpointed, but the resulting settlement of juvenile mussels was very disturbing. Scuba divers from NPS, U.S. Corps of Engineers, and the U.S. Fish and Wildlife spent three weeks exploring the Lower St. Croix Riverway in an effort to fully understand the extent of the situation.

To track changes overtime and better understand the invasion of zebra mussels in the Riverway, annual measurements of densities within the known infestation zone (the lower 22 miles of the river) have been collected since 2004. Ten cinder blocks were dropped to the bottom on the river at strategic locations from Stillwater, Minnesota to Prescott, Wisconsin, reflecting the range of habitats and hydrology found in the infestation zone. Tracked for over a decade, density data for zebra mussels provides population trends at these locations. NPS continues to monitor these sites with the support of the U.S. Army Corps of Engineers.

In 2020, numbers of zebra mussels detected on cinder blocks increased at most locations dramatically. Cinder blocks at Prescott (Pool 4) saw the heaviest recruitment of any of the pools in 2020. Since 1996, NPS has instituted a Zebra Mussel Control Point prohibiting boat travel (includes motor and non-motorized boats) upstream from river mile 29.5 (at the High Bridge) to prevent the spread of zebra mussels. These aquatic invasive pests remain a significant threat with the potential to drastically alter the river ecosystem and devastate native mussel populations.

10. Where else can I get information about Zebra Mussels?
This website 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.

St. Croix 360 - Zebra Mussel Numbers Skyrocket in Lower St. Croix River
This site provides information on current zebra mussel status in the river.

Last updated: July 20, 2023

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