Report - The conservation status of the brook floater mussel, Alasmidonta varicosa, in the Northeastern United States: trends in distribution, occurrence, and condition of populations
The brook floater (Alasmidonta varicosa) occurs along the Atlantic slope from the Canadian Maritimes to Georgia. In Canada it is designated as a Schedule 1 Special Concern Species that is confined to 15 watersheds in Nova Scotia and New Brunswick where it is considered “never abundant, representing between 1-5% of the total mussels present” (Department of Fisheries and Oceans Canada 2016). In the United States it is listed as critically imperiled (S1) in 10 states: New Hampshire, Vermont, Massachusetts, New York, Connecticut, New Jersey, West Virginia, Virginia, North Carolina, and Maryland; imperiled (S1S2) in Pennsylvania; imperiled (S2) in Georgia; imperiled (S3) in Maine (in 2007 Maine amended the status of A. varicosa to threated from special concern); extirpated (SX) in two states (Rhode Island and Delaware), and unranked (SNR) in South Carolina. However, the South Carolina State Wildlife Action Plan 2015, lists A. varicosa as highly imperiled. Here we report on: (1) the biology and life history of A. varicosa, (2) the distribution and condition of all known populations from Maine to Georgia, (3) the human impacts on populations (4) the results of models using environmental factors at both the HUC 12 level and stream level as predictors of population condition, and (5) the results of a survey sent to mussel biologists from Maine to Georgia concerning threats to this species. Alasmidonta varicosa is a strict riverine species that favors low productivity streams and appears to have a low tolerance to eutrophication. It is a small mussel with a moderate life span, moderate age of reproductive maturity and low fecundity. Because it is a host fish generalist, A. varicosa populations are unlikely to be limited by the availability of a particular host fish. Our model results show a strong relationship between the rapid replacement of riparian forests with residential, commercial, agricultural and industrial development and the condition of A. varicosa populations. Protecting and restoring riparian forestlands may be our most practical tool for conserving this species. Survey respondents scored the loss of riparian forests, habitat fragmentation, agricultural runoff of nutrients and toxins, urbanization and development as the most spatially extensive and the most severe threats to A. varicosa populations. Captive propagation, reintroduction and population augmentation may be needed in order to maintain or rescue A. varicosa populations. We document a dramatic contraction in the distribution range of this species. Surveys show that many populations consist of declining numbers of older animals and show little or no evidence of recruitment. Sharp declines in the size and spatial extent of populations as well as population extirpations have occurred throughout the range, however important populations persist in multiple states including Maine, which appears to hold the largest selfsustaining populations range-wide. Dams, impoundments and waters that are heavily polluted isolate many populations throughout the range. We note that current and projected increases in extreme precipitation and drought will seriously impact remaining A. varicosa populations.