Zebra Mussels in the Illinois River: Here Today, Gone Tomorr

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From January, 1996 (Update #26)

A mysterious new chapter was written in 1995 in the ongoing Illinois River zebra mussel saga. Almost no mussels were found this fall in the lower 120 miles of the river. In October, Illinois Natural History Survey (INHS) biologist Scott Whitney sampled sites in the area and found less than one gallon bucket of live zebra mussels. Whitney had found abundant mussels at these sites earlier this summer. "In 1993, our divers were collecting five gallons in five minutes at these same sites," INHS biologist Doug Blodgett said. "We were afraid we'd find something similar this fall." Instead, Whitney found piles of zebra mussel shells, almost all of which were empty - evidence of another significant die- off. INHS biologists observed a zebra mussel population boom during surveys conducted in 1993. These populations radically declined in late 1993 and 1994 along the lower 120 miles of the river, the area regularly surveyed by INHS biologists. "High mortality with densities at one site dropped from over 60,000 per square meter in summer 1993 to less than 600 per square meter by fall 1994," Blodgett said. Another expanding population had been expected earlier this year. "It's obvious zebra mussels were doing well somewhere upriver earlier this year because we calculated over 60 million veligers per second drifting by our station on a couple occasions," Blodgett said. 1995 continued to look like a big mussel year when divers found small, newly settled zebra mussels in densities of 1-5,000 per square meter in July. But between July and October something happened. Sporadic episodes of poor water quality - low dissolved oxygen, heavy silt loads and high water temperatures - could be responsible, Blodgett speculated. As long as reproducing zebra mussels persist upriver, the lower Illinois River will be vulnerable to continuing boom and bust cycles, he believes. - Robin Goettel, Illinois-Indiana Sea Grant.

ID: 199601-6.

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.