Fish predation of zebra mussels attached to Quadrula quadrula (Bivalvia: Unionidae) and benthic molluscs in a Great Lakes coastal wetland

Although the introduced zebra mussel, Dreissena polymorpha, has eliminated most native unionid populations within the lower Great Lakes, some recent surveys have found diverse unionid communities in several Lake Erie coastal wetlands. In 2004, we tested whether fish predators reduced zebra mussels on Quadrula quadrula (Unionidae) in a Lake Erie coastal wetland. In June, we placed unionids and colonization plates in small-mesh exclosures (2.5-cm X 2.5-cm mesh), large-mesh exclosures (5-cm X 10-cm mesh), and open exclosures (2.5-cm X 2.5-cm mesh with two 40-cm X 40-cm openings). Zebra mussels and other benthic molluscs were sampled in October, and zebra mussel numbers on Q. quadrula outside exclosures were significantly higher in October than in June. Densities of zebra mussels on Q. quadrula and colonization plates were much higher in small-mesh and large-mesh treatments that excluded large fish than open treatments. Mean (+/- 1 SE) densities of zebra mussels/unionid were also higher inside (1041 +/- 103) than outside (6.9 +/- 1.5) exclosures in October. These results indicate that large-bodied molluscivores (e.g., common carp, freshwater drum, channel catfish) can limit zebra mussel numbers in coastal wetlands. Densities of other molluscs (Sphaeriidae, Corbiculidae, Gastropoda, and Unionidae) were not different in sediments of exclosures and uncaged areas, suggesting that fish can have a greater impact on numbers of attached zebra mussels than benthic molluscs.