Small dams as barriers to freshwater mussels (Bivalvia, Unionoida) and their hosts

The distributions of two unionoids (fragile papershell Leptodea fragilis and pink heelsplitter Potamilus alatus) were examined in five North American Midwest river systems in relation to the location of dams on the rivers. These dams were non-navigational (without locks), lacked fish ladders, and varied in height from 1 to 17.7 m. Both species were restricted in their distribution to the river downstream of the darns. This suggests that the host fish(es) of these species was unable to move upstream of these obstacles. Both unionoids are believed to parasitize the freshwater drum Aplodinotus grunniens. Several endangered unionoid species also may use this fish, or other dam-limited fishes, as hosts. Dams, even lowhead structures, may contribute to the overall depletion of unionoids by artificially restricting their distributions and isolating populations from each other. Management practices for endangered fishes and mussels must take into account these physical obstacles.