A RANGE-WIDE ASSESSMENT OF POPULATIONS OF ALASMIDONTA HETERODON, AN ENDANGERED FRESH-WATER MUSSEL (BIVALVIA, UNIONIDAE)

The purpose of this study was to estimate the densities and sizes of t he major remaining populations of Alasmidonta heterodon, a unionid mus sel listed as endangered by the United States Fish and Wildlife Servic e. We studied 13 streams from New Hampshire to North Carolina. At 2-9 reaches on each stream, we assessed A. heterodon populations using bot h timed searches and quadrats, and measured the shell lengths of all l iving A, heterodon that we found. All populations of A. heterodon that we studied had 3 characteristics: 1) low density, with mean densities typically <0.01-0.05/m(2); thus, low or declining density per se may threaten populations of A. heterodon; 2) recent reproduction, as shown by the presence of young animals or gravid females; and 3) vulnerabil ity to loss from small ranges, low population densities, linear ranges , or a combination of these 3 factors. Furthermore, several of the pop ulations that we studied included 100s to 10s of 1000s of animals, so these populations probably were too large to be strongly affected by m any of the conservation problems of small populations (e.g., inbreedin g, demographic stochasticity). These populations were large enough to provide animals for experimental studies, attempts to reestablish hist orical populations, and the like. Other populations, especially those in the Neuse basin of North Carolina, had exceedingly low densities, a nd may have contained fewer than 100 adults. The most robust populatio ns of A. heterodon probably were those in the Connecticut River (New H ampshire), Ashuelot River (New Hampshire), Neversink River (New York), Po River (Virginia), and Shelton Creek/Tar River (North Carolina)-and perhaps the Little River (North Carolina).