It took less than 10 years. Nonnative
zebra mussels from Europe first appeared in the Mississippi
River in 1991, and today the exploding zebra mussel population
has carpeted some parts of the Mississippi River bed with
10,000 to 20,000 mussels per square yard.
Spread largely in bilge water of commercial Mississippi
River barge traffic, the mussels' hard shells can cut
humans, dogs, and other animals that come in contact
with them. Zebra mussels are the only freshwater mollusks
who can attach themselves to solid objects such as submerged
rocks, dock pilings, and boat hulls. They can clog intake
pipes at power plants and require expensive treatments
to remove them.
Native mussel species are the losers in this war.
Almost 70 percent of the nation's 297 native mussel
species are endangered, threatened, or potentially warrant
federal protection. Zebra mussels have decimated native
mussel species along the Mississippi and pushed the
Higgin's eye pearly mussel to the brink of extinction.
Not only do the zebra mussels compete with the native
mussels for food, they attach to their shells, preventing
their reproduction and smothering them, according to
Ron Benjamin, who works with the Wisconsin Department
of Natural Resources as fisheries supervisor for the
Mississippi River.
This week crews from Wisconsin, Iowa, and the federal
government will fan out along a 500-mile stretch of
the Mississippi River to look for zebra mussels and
clues about how to fight this exotic invasive species
and save the native species.
They'll use bilge pumps and buckets to collect water
samples that will be analyzed at an Illinois laboratory
for veligers, the microscopic, larval form of zebra
mussels. One cup of river water can contain as many
as 100 of these young invaders.
"We're trying to study how veligers are distributed
in the river, where they settle, how they fare from
year to year, and what they do to an ecosystem over
time," said John Sullivan, a Wisconsin Department of
Natural Resources water-quality specialist for the Mississippi
River. "We're in the infancy of studying zebra mussels
and how they affect our ecosystems."
This is the fourth year crews have looked for the
veligers. They are part of a larger effort to understand
and cope with the invading mollusks that includes the
U.S. Army Corps of Engineers, the U.S. Fish & Wildlife
Service, and other states' agencies.
A native of eastern Europe and western Asia, the thumbnail-sized
zebra mussels were first discovered in U.S. waters in
1988. They likely arrived in the ballast of ocean-going
vessels that emptied their tanks in Great Lakes ports.
Today zebra mussels have spread to all of the Great
Lakes and major river systems in the Midwest, moving
through waterways by attaching to boats and barges.
This spring, biologists with the U.S. Fish and Wildlife
Service and the Minnesota and Wisconsin Natural Resource
Departments began working to bring the federally endangered
Higgins' eye pearly mussel back from extinction.
Since May, scuba divers and groups of workers armed
with syringes and plastic buckets have been collecting
Higgins' eye pearly mussels. They intend to produce
young Higgins' eyes at the service's Genoa National
Fish Hatchery in Genoa, Wis., for eventual release back
into their natural habitat.
"Our goal with the hatchery project is to take adult
Higgins' eye mussels from areas already infested with
zebra mussels, raise young mussels in the hatchery,
and then release them in areas where they should be
safe from that threat," said Pam Thiel, fisheries biologist
with the Fish and Wildlife Service. "We hope we can
keep this species going in the hatchery and in some
remaining suitable natural habitats to prevent zebra
mussels from eliminating the Higgins' eye."
Fisheries supervisor Benjamin said, "Zebra mussels
have the ability to impact our ecosystems. They like
the algae fish like, so they're competing with our fish
for food. But they don't like the blue-green algae that
can foul many lakes and rivers, and in fact, they're
adding to the problem because they release ammonia-
and phosphorus-containing waste products that may fuel
blue-green algae blooms," he said.
In fact, respiratory demand and waste products released
by 30 acres of zebra mussels requires roughly the same
amount of dissolved oxygen required to break down the
organic material discharged by the Twin Cities Metropolitan
Wastewater Treatment Plant, water-quality specialist
Sullivan said.
That oxygen demand contributed to unusually low dissolved
oxygen concentrations in portions of the Mississippi
River during the early summer periods of 1997 and 1998,
stirring fears of fish kills.
This week's sampling for larval zebra mussels is important
in helping verify that the relocated native mussels
are living in hospitable sites. The sampling will involve
collecting water from below the locks and dams stretched
along the 500 miles from the Hastings Dam below St.
Paul, Minn. to Keokuk, Iowa, and on large tributaries,
including the Chippewa, the Black, the Wisconsin, and
the St. Croix, Sullivan said.
These hardworking biologists are warning people to
clean their boats and trailers carefully to prevent
zebra mussels from moving into new lake and riverine
systems. Larvae can be carried in bilge water, bait
buckets, and fish holds. Adults can easily hitch a ride
on the bottom of boat hulls or on barges.
Fisheries experts have set up two experimental projects
to relocate vanishing native mussel species to parts
of the Upper Mississippi River with lower zebra mussel
densities.
Benjamin said, "We need to find a place to move enough
mussel species out of harm's way so (that) if we ever
do get a handle on zebra mussels and how to control
them, we have the genetic material to put the ecosystem
back together."
Fisheries authorities on the West Coast are alert
to the possibility that zebra mussels may have reached
the Columbia River. The U.S. Coast Guard and the Fish
and Wildlife Service, working closely with the office
of U.S. Sen. Ron Wyden of Oregon, have released $275,000
for a three-year study of aquatic nuisance species in
the lower Columbia River.
The Columbia River is considered to be at risk for
invasion by potentially damaging nonnative species such
as zebra mussels. The study will be conducted by the
Pacific States Marine Fisheries Commission in collaboration
with Portland State University, the University of Washington,
and Oregon State University.
