Changing Populations

From May, 1995 (Update #24)

HAVANA, Ill. - Last year's zebra mussel numbers declined 90 percent from 1993 in the lower Illinois River, according to Illinois Natural History Survey (INHS) biologists Rip Sparks, Scott Whitney and Doug Blodgett. During the 1993 explosion, average densities were 61,000 mussels per square meter (/m2). INHS biologists believe low oxygen levels and warm summer temperatures led to the subsequent decline of mussel densities, which averaged 4,000/m2 in August 1994. At the same time, divers found up to 3,000/m2 of empty zebra mussel shells. INHS surveys have shown that zebra mussel populations elsewhere in the Illinois River and upper Mississippi River are building up gradually rather than explosively, and oxygen levels have not yet declined. In the Illinois River at Peoria, the populations increased from about 2,000/m2 in 1993 to 6,000/m2 in July 1994. At Rock Island on the Mississippi, divers found only a 1-2/m2 density in July 1994, and 48/m2 a month later. At another site downstream, divers found no zebra mussels. For further information contact Mussel Project Manager Scott Whitney at (309) 543-6000.

ID: 199505-4.

The Zebra Mussel Update was a 4- to 8-page quarterly national newsletter published by the University of Wisconsin Sea Grant Institute from May 1990 through May 1997. The ZMU documented the spread of the zebra mussel -- an exotic nuisance mussel -- through North America's freshwater environments, especially the Great Lakes, and on efforts to control it.