A refuge for native freshwater mussels (Bivalvia: Unionidae) from impacts of the exotic zebra mussel (Dreissena polymorpha) in Lake St. Clair

The introduction and spread of the exotic zebra mussel (Dreissena polymorpha) throughout 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 surveyed 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 m². 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 determine 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.