Fish assemblage recovery along a riverine disturbance gradient

Artificial fluctuations in streamflow have been documented to alter the composition andstructure of stream communities. This study tests the hypothesis that a spatial recovery gradientin fish assemblage structure exists downstream of a hydroelectric dam, and that recovery can be identified by the presence and abundance of species largely restricted to flowing-water habitats(fluvial specialists). A longitudinal gradient of change in a shoreline fish assemblage wasquantified in a 66-km reach of a mid-sized, species-rich river (Tallapoosa River, Alabama) withdaily flow fluctuations from hydropower generation. The shoreline fish assemblage in a nearbyand similar river (Cahaba River, Alabama) was quantified as a regional reference for theoccurrence of fish assemblage gradients. Fish were collected with prepositioned areaelectrofishers in 240 randomly located sampling sites, and physical habitat was quantified. Usingdistributional and habitat use information, fish species were categorized as fluvial specialists ormacrohabitat generalists (species that occur in a wide variety of aquatic systems). Sampledhabitats were similar between rivers and along each study reach. The longitudinal pattern ofspecies occurrence and fish abundance was consistent in the free-flowing river. A longitudinalgradient of increasing abundance and richness of only fluvial specialist species existeddownstream of the hydroelectric dam. No similar spatial gradient existed for macrohabitatgeneralists in either river. Although a fish community recovery gradient was identified, a recoveryendpoint was not evident because assemblage change was gradual and possibly incomplete.The preservation and management of riverine fish faunas will partly depend on incorporatingspatial recovery into decisions about permitting and siting of anthropogenic changes like hydroelectric dams. http://dx.doi.org/10.2307/194192