Patricia Morrison, U.S. Fish and Wildlife Service, Ohio River Islands National Wildlife Refuge, PO Box 1811, Parkersburg, WV 26102

Abstract. The Ohio River is home to over 50 species of native freshwater mussels, including ten federally endangered species. Zebra mussels Dreissena polymorpha first entered the Ohio River in 1991. Since 1995, a cooperative of federal, state and private partners have been working together to maintain a network of monitoring sites along the mainstem Ohio River to track the status of zebra mussels and the condition of native mussels in infested areas. Zebra mussel densities continue to build at most of the sites. Some of the sites are experiencing a significant decline in native mussel density, condition and survival. Native mussel mortality (expressed as percent fresh-dead animals) is approaching 40% at three of the lower river sites. In addition to the objective of providing a river-wide assessment of the zebra mussel problem, the data gathered provides critical information needed for management of the imperilled native fauna: identifying populations most at risk, and the "threshhold" for rescue or relocation of at-risk species.