ZEBRA MUSSELS

Although small in size, zebra mussels have had a major impact on the Mississippi River.

ZEBRA MUSSELS. The zebra mussel (Dreissena polymorpha), a small freshwater mollusk,has invaded America's rivers and lakes. Originating in the Balkans, Poland, and the former Soviet Union, they appeared in North America in 1988 in Lake St. Clair, a small water body connecting Lake Huron and Lake Erie. Biologists believe zebra mussels were crried in the ballast water of a ship and later discharged into the Canadian side of Lake St. Clair.

Although not all shells carry the pattern, Zebra mussels are named for the striped pattern of their shells. Usually about fingernail size, they may reach a maximum length of nearly 2 inches. Zebra mussels live four to five years in fresh water at depths of six to twenty-four feet. A female zebra mussel begins to reproduce at two years of age and produces between 30,000 and 1 million eggs annually. About two percent of zebra mussels reach adulthood.

Being small and free swimming, young zebra mussels are easily spread by water currents. Older zebra mussels attach themselves to hard surfaces by an external organ called a byssus. The mussels may attach to boats, pilings, water-intake pipes, crayfish, turtles, other zebra mussels, and native mollusks. While zebra mussels can attach themselves securely, they may also move, and can reattach themselves easily if dislodged by storms.

The animals and algae that are food of zebra mussels are also the food for larval fish and other native species. A large zebra mussel population may cause a decline in other animals, including native fish, mollusks, and birds. The filter-feeding activity of zebra mussels causes a dramatic increase in water clarity in infested lakes and rivers.

Zebra mussels can severely effect native mussels and clams by interfering with their feeding, growth, movement, respiration, and reproduction. Zebra mussels can colonize a clam shell to such a degree that the clam cannot open its shell to eat.

Water and environmental management agencies are working to protect endangered native species from the threat of zebra mussels. The primary emphasis of this effort is education so that boaters and fishermen do not accidently transfer mussel larvae from one water body to another. In some rivers, boaters are prohibited from traveling upstream from infected areas in an attempt to keep the mussels from spreading.

In addition to the impact on wildlife, zebra mussels cause many problems for people. They may severely restrict the water flow to power plants or other municipal or private facilities that rely on fresh water. Impacts include damage to the facilities as well as the cost of removing or controlling the mussels. Zebra mussels may also foul beaches and create boating and navigation hazards. Increased plant growth provides an additional hazard to navigation. Both large and small boats can be severely impacted by increased drag caused by thousands of mussels. Small zebra mussels may get into engine cooling systems, causing overheating and other damage. Navigational buoys have been sunk under the weight of attached zebra mussels. Wood, steel, and concrete are all damaged by prolonged attachment of the mussels.

Once zebra mussels become established in a water body, they are impossible to eradicate with the technology currently available. Many chemicals kill zebra mussels, but these exotics are so tolerant and tough that everything in the water would have to be poisoned to destroy the mussel. Most commercial water users rely on chemicals such as chlorine, filters, or mechanical scraping to remove mussels from their intake pipes and facilities.

Zebra mussels are controlled with a wide variety of methods. Many plants install equipment to preoxidize water at the point of intake, while others rely on different chemical treatments, mechanical controls, or filtration. Physical barriers and chemical coatings are used to prevent zebra mussels from attaching to structures. Removal is accomplished with mechanical scrapers, hot water, air, chemicals, and sound; new methods under investigation. There is no single, ideal solution for all affected facilities.

Zebra mussels do have a positive impact on some native species. Many native fish, birds, and other animals eat young and adult zebra mussels. Migratory ducks have changed their flight patterns in response to zebra mussel colonies. Lake sturgeon feed heavily on zebra mussels, as do yellow perch, freshwater drum, catfish, and sunfish. The increase in aquatic plants due to increased water clarity provides excellent nursery areas for young fish and other animals, leading to increases in smallmouth bass populations in Lake St. Clair and the Huron River.