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Zebra Mussels “Pulse” in Duluth-Superior Harbor

Mary Balcer

Sea Grant researcher Mary Balcer inserts individual zebra mussels into a “condominium” where they will spend the winter submerged in the Duluth-Superior harbor. Balcer is investigating survival rates of the young zebra mussels.

The mild weather we enjoyed this past year was not only pleasant for northlanders, it may have been beneficial to zebra mussels as well. A surprising increase in the zebra mussel population in the Duluth-Superior harbor was discovered this September. It raises concerns over potential impacts on raw water-dependent industries and the accidental spread of zebra mussels by recreational boaters to Minnesota and Wisconsin inland lakes and rivers.

Nearly a decade after their initial discovery in the harbor, zebra mussels were not considered much of a problem - in stark contrast to the serious impacts they have had on the other Great Lakes, the Mississippi River, and other waters.

Discovery of zebra mussels on the hulls of two recreational boats this fall by Sea Grant staff was the most significant finding of zebra mussels so far. Subsequent reports of zebra mussels on boat lifts, piers, shoreline and on native mussels suggest that the infestation is growing and expanding in the harbor.

Minnesota Sea Grant is collaborating with Mary Balcer, University of Wisconsin-Superior, to assess the extent of the infestation and to determine if the young will successfully over-winter in the harbor. Results of preliminary dives by Balcer show that for the first time thousands of young zebra mussels are living on piers, as well as on rocks along the waterfront. Hopes are that a normal (i.e., cold) winter will take care of the problem. Survival of the young will not be known until next spring.

While the infestation is not necessarily cause for alarm, Sea Grant has issued letters to dozens of raw water industries and operations on the waterfront to raise awareness of the growing problem and to urge operators to take action to lessen potential impacts.

Minnesota Sea Grant is also stepping up awareness efforts to prevent boaters, anglers, and other people from moving zebra mussels from the harbor to inland waters. Boaters are urged to inspect and remove any zebra mussels and aquatic plants before leaving the water access because the harbor now poses a much greater risk for spread to inland waters. Minnesota regulations prohibit carrying zebra mussels or aquatic plants on a public road. Boat owners are required to clean their boats and empty water from live wells and bait buckets before leaving infested waters.

Precautions taken by boaters can work. The spread is not inevitable. Zebra mussels have been in Lake Pepin on the Mississippi River for the last seven years, for example, but haven’t spread to nearby lakes or rivers.

For more information, contact Doug Jensen, Minnesota Sea Grant Exotic Species Information Center Coordinator, at 218.726.8712; or by e-mail.


By Doug Jensen
December 1998

Return to December 1998 Seiche



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