Changes in the distribution of freshwater mussels (Unionidae) in the upperSusquehanna River basin, 1955-1965 to 1996-1997

In 1955-1965, the streams and rivers of the upper Susquehanna River basin i n New York were surveyed for unionid mussels. We resurveyed unionid populat ions at the same sites in 1996-1997 to determine whether populations had de clined and whether declines could be correlated with inputs of sediments an d nutrients from agriculture, which have been thought to be major threats t o imperiled unionid populations in North America. Despite widespread eviden ce of high nutrient and sediment loading from agriculture and other human a ctivities in the upper Susquehanna basin since at least 1965, we found litt le change in unionid populations between the two time periods. The range of Alasmidonta varicosa contracted while that of Lampsilis cariosa expanded. Overall species richness and the ranges of the other nine species were unch anged. Six species in the basin were formerly listed as Category 2 species or are now considered "Species at Risk" by the United States Fish and Wildl ife Service. Of these, Alasmidonta undulata, A. marginata, Strophitus undul atus and L. cariosa are still abundant and widespread in the basin, Lasmigo na subviridis is scattered and rarely abundant (but apparently stable) and A. varicosa is rare and declining, perhaps because of hybridization with A. marginata. We caution that improperly designed surveys may erroneously sug gest that stable populations are declining.