and Control of the Zebra Mussel
National Atlas of the United States®
A small freshwater mollusk called the zebra mussel (Dreissena
polymorpha), has been steadily invading America's rivers and
lakes. Zebra mussels originated in the Balkans, Poland, and the
former Soviet Union. They first appeared in North America in 1988
in Lake St. Clair, a small water body connecting Lake Huron and
Lake Erie. Biologists believe the zebra mussels were picked up in
a freshwater European port in the ballast water of a ship and were
later discharged into the Canadian side of Lake St. Clair.
J. E. Marsden, Lake
Michigan Biological Station
Zebra mussels get their name from the striped pattern of their shells,
though not all shells bear this pattern. They're usually about fingernail
size but can grow to a maximum length of nearly 2 inches. Zebra
mussels live 4 to 5 years and inhabit fresh water at depths of 6
to 24 feet. A female zebra mussel begins to reproduce at 2 years
of age, and produces between 30,000 and 1 million eggs per year.
About two percent of zebra mussels reach adulthood.
S. van Mechelen,
University of Amsterdam, The Netherlands
Young zebra mussels are small and free swimming, and can be easily
spread by water currents. Older zebra mussels attach themselves
to hard surfaces by an external organ called a byssus, which consists
of many threads. The mussels may attach to boats, pilings, water-intake
pipes, and other hard surfaces, as well as to 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.
Zebra mussels upset ecosystems, threaten native wildlife, damage
structures, and cause other serious problems. Millions of dollars
are spent each year in attempting to control these small but numerous
||Zebra Mussel Threats
D. Jude, Center for Great Lakes Aquatic Sciences
Threat to Other Species
Zebra mussels are filter feeders. An adult zebra mussel filters up
to a quart of water per day, which multiplied by millions of mussels
means that the mussels may be filtering all the water in a lake or
stream in a day. The animals and algae that are the food of zebra
mussels are also the food for larval fish and other native species,
so 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 related and frequently 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. For example, zebra mussels can colonize a clam shell
to such an extent that the clam cannot open its shell to eat. Some
native mussels have been found with more than 10,000 zebra mussels
attached to them. In addition to colonizing native mussels and clams,
zebra mussels may attach to slow-moving species such as crayfish
Exotic Species Graphics Library
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 to education so that boaters
and fishermen do not inadvertently 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.
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.
However, these native species do not feed heavily enough on zebra
mussels to keep the populations under control.
Threat to Navigation, Boating, and Industry
In addition to the impact on wildlife, zebra mussels cause many problems
for people. They may colonize water intake pipes, severely restricting
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.
Zebra mussels will attach to almost any hard surface, either natural
or manmade. On boats, they may attach to the hull, motor, or any
item immersed in the water. 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.
In addition to threatening boats, zebra mussels pose a threat to
navigational buoys, piers, docks, and other structures in the water.
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.
Threat to Outdoor Recreation
Shells of zebra mussels foul beaches and near-shore swimming areas.
Bare feet are at risk from the sharp shells, and clean up costs are
high. Due to changes in fish populations, zebra mussels also adversely
impact recreational fishing.
M. Parsons, Michigan
||Removal and Control of the Zebra Mussel
Cost of Removal and Control
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
R. Peplowski, Detroit
While accurate cost figures are not currently available, it is
known that the cost of dealing with zebra mussels varies widely,
depending on the type of facility, the length of infestation, and
the control methods chosen. Frequently the highest costs are to
retrofit a facility for effective control; control costs usually
drop dramatically once retrofits are in place. Because nuclear power
plants use large quantities of water they tend to have the highest
associated costs per plant, followed by industrial plants, fossil
fuel power plants, and drinking water facilities. For many plants,
costs average hundreds of thousands of dollars per year.
Methods of Removal and Control
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 are constantly under investigation. There is no single, ideal
solution for all affected facilities.