INITIAL OCCURRENCES OF ZEBRA MUSSELS (DREISSENA-POLYMORPHA) ON FRESH-WATER MUSSELS (FAMILY UNIONIDAE) IN THE UPPER MISSISSIPPI RIVER SYSTEM

The exotic zebra mussel (Dreissena polymorpha) was found attached to n ative freshwater mussels (Unionidae) in La Grange, Peoria, and Alton P ools of the Illinois River and Pools 4, 13, and 26 of the Mississippi River. From 1% to 27% of native bivalves collected within these pools in 1992 had one or more zebra mussels attached with the highest densit ies of zebra mussels found in the Illinois River. At one site in the A lton Pool of the Illinois River, 27% of native mussels had a mean of 2 .2 zebra mussels attached., Densities were lower in the Mississippi Ri ver sites, ranging from 1% to 12%, with the highest density occurring below the confluence of the Illinois and Mississippi Rivers. Of the 25 native species collected, nine had zebra mussels attached. In the Alt on Pool of the Illinois River, Amblema plicata longer than 82 mm had m ore zebra mussels upon them than A. plicata smaller than 82 mm. The la rge numbers of young-of-the-year zebra mussels found in the Illinois a nd Mississippi Rivers indicate D. polymorpha is reproducing and possib ly posing a threat to unionid populations in both rivers.