DECLINE OF NATIVE UNIONID BIVALVES IN LAKE ST CLAIR AFTER INFESTATIONBY THE ZEBRA MUSSEL, DREISSENA-POLYMORPHA

To determine impacts of the zebra mussel, Dreissena polymorpha, on biv alves in Lake St. Clair, densities of Unionidae in 1990 and 1992 (afte r D. polymorpha invasion) were compared with densities in 1986 (before D. polymorpha). Declines in density occurred mainly at sites in the s outheastern portion of the lake where unionids were highly infested wi th D. polymorpha. Unionid densities at highly infested sites declined from 2.4/m(2) in 1986, to 1.8/m(2) in 1990, and to O/m(2) in 1992. Uni onid species with light-weight shells that brood larvae over an extend ed period declined more between 1986 and 1990 than heavy-shelled, shor t-term brooders. Unionid densities at lightly infested sites located m ainly in the northwestern portion of the lake did not decline: mean de nsities were 1.4/m(2) in 1986, 1.6/m(2) in 1990, and 1.4/m(2) in 1992. While the mean number of D. polymorpha per unionid was <1 at these li ghtly infested sites in 1990, it increased to 35 by 1999. Given this r elatively slow increase in D. polymorpha per unionid in this portion o f the lake, continued monitoring of populations should provide a bette r understanding of the relationship between the extent of infestations and unionid mortality.