J. Great Lakes Res. 28(3):479–489 Internat. Assoc. Great Lakes Res., 2002 NOTE A Refuge for Native Freshwater Mussels (Bivalvia: Unionidae) from Impacts of the Exotic Zebra Mussel (Dreissena polymorpha) in Lake St. Clair David T. Zanatta 1,* , Gerald L. Mackie 2 , Janice L. Metcalfe-Smith 3 , and Daelyn A. Woolnough 2 1 Aquatic Ecosystem Impacts Research Branch National Water Research Institute 867 Lakeshore Road, P.O. Box 5050 Burlington, Ontario, Canada L7R 4A6 2 Department of Zoology University of Guelph Guelph, Ontario, Canada N1G 2W1 3 Aquatic Ecosystem Impacts Research Branch National Water Research Institute 867 Lakeshore Road, P.O. Box 5050 Burlington, Ontario, Canada L7R 4A6 ABSTRACT. The introduction and spread of the exotic zebra mussel (Dreissena polymorpha) through- out the Great Lakes has decimated native unionid populations. However, significant communities have continued to survive in several nearshore areas of Lake Erie. This study documents the discovery of a “refuge” site for unionids in Lake St. Clair. Ninety-five sites in various areas around the lake were sur- veyed between 1998 and 2001, and 2,356 live unionids of 22 species were found alive at 33 of these sites. Almost all sites (31) were in shallow (mainly < 1 m) waters of the St. Clair delta, in habitats similar to refugia in Lake Erie, i.e., nearshore areas with firm sandy substrates and marshy bays with soft, muddy sediments. Species richness ranged from 1 to 12 species per site, and relative abundance ranged from 2 to 302 unionids per person-hour of sampling effort. Densities at nine sites ranged from 0.03 to 0.07 per m 2 . Five species considered to be at risk were found alive. Infestation rates at sites near the St. Clair delta ranged from 0 to 286 zebra mussels per unionid, which is slightly higher than rates at other known refuge sites. The community is now dominated by thick-shelled species such as Fusconaia flava and Lampsilis cardium, which are known to be least susceptible to zebra mussels. Further studies are needed to deter- mine if unionid populations in the delta are stable, and to understand the mechanisms responsible for unionid survival at this and other refugia. Such information could be used to predict the locations of other natural sanctuaries and to guide their management for the preservation of the Great Lakes unionid fauna. INDEX WORDS: Unionidae, freshwater mussels, zebra mussels, Lake St. Clair, refuge. INTRODUCTION North America historically supported the greatest diversity of freshwater mussels (Mollusca: Unionacea) in the world. Nearly 300 species and subspecies, representing one-third of the world’s * Corresponding author. E-mail: dave.zanatta@ec.gc.ca freshwater mussel fauna, are native to this continent (Williams et al. 1993, Bogan 1993). Forty of Canada’s 53 freshwater mussel species occur in the lower Great Lakes drainage basin, and 32 of these have been recorded from Lake St. Clair (Metcalfe- Smith et al. 1998). The introduction and spread of the exotic zebra mussel ( Dreissena polymorpha) throughout the Great Lakes in the late 1980s deci- 479