Impoundments and the decline of freshwater mussels: A case study of an extinction gradient

One major factor lending to the imperilment of freshwater mussels (Bivalvia , Unionidae) has been the large-scale impoundment of rivers. We examined th e distribution and abundance of mussels at 37 sites along a 240-km length o f the Little River in southeastern Oklahoma, U.S.A., which is affected of b oth mainstem and tributary reservoirs. We observed a mussel extinction grad ient downstream from impoundments in this river: with increasing distance f rom the mainstem reservoir there was a gradual, linear increase in mussel s pecies richness and abundance. Mussel species distributions were significan tly nested, with only sites furthest from the impoundment containing relati vely rate species. Below the confluence with the inflow from the second res ervoir these same trends were apparent but much weaker, and overall mussel abundance was greatly reduced. Our results suggest that considerable stream lengths are necessary to overcome the effects of impoundment on mussel pop ulations, and such information should be considered in conservation and man agement plans.