Is fish predation of zebra mussels contributing to survival of unionids in Lake Erie coastal wetlands?

Although exotic zebra mussels (Dreissena polymorpha) have eliminated unionids within many of the lower Great Lakes, several recent surveys have found  unionid communities in unimpounded Great Lake coastal wetlands.  We conducted unionid surveys in three coastal wetlands along Lake Erie and found communities comprised of  6 to 15 species.  Common species collected were Quadrula quadrula, Leptodea fragilis, Pyganodon grandis, and Toxolasma parvus.  We found juvenile unionids and also mature individuals >5 years in age, indicating populations are being replenished by current unionid reproduction.   Fish surveys in one of these wetlands found 23 species including several molluscivores including common carp, channel catfish, pumpkinseed, and round goby.  At an unimpounded coastal wetland in Ottawa National Wildlife Refuge, Ohio, we constructed 5-m diameter wire mesh enclosures to test how fish predation affects zebra mussel recruitment on unionid shells.  In June 2007, we placed live, cleaned Q. quadrula and L. fragilis in each enclosure.  Average zebra mussel recruitment on Q. quadrula was significantly greater where fish were excluded (~70 zebra mussels/unionid) than in uncaged control areas (~8 zebra mussels/unionid) in both July and September 2007.   However, average zebra mussel recruitment on L. fragilis was not different inside and outside exclosures on either date (60-80 zebra mussels/unionid in each treatment in September 2007) .  This suggests that the impact of fish predation varies among unionid species.  Enclosure studies initiated in 2008 are testing the impacts of carp predation on zebra mussels attached to unionid shells.