Fish Assemblage Recovery Along a Riverine Disturbance Gradient

Artificial fluctuations in streamflow have been documented to alter the composition and structure of stream communities. This study tests the hypothesis that a spatial recovery gradient in 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 was quantified in a 66-km reach of a mid-sized, species-rich river (Tallapoosa River, Alabama) with daily flow fluctuations from hydropower generation. The shoreline fish assemblage in a nearby and similar river (Cahaba River, Alabama) was quantified as a regional reference for the occurrence of fish assemblage gradients. Fish were collected with prepositioned area electrofishers in 240 randomly located sampling sites, and physical habitat was quantified. Using distributional and habitat use information, fish species were categorized as fluvial specialists or macrohabitat generalists (species that occur in a wide variety of aquatic systems). Sampled habitats were similar between rivers and along each study reach. The longitudinal pattern of species occurrence and fish abundance was consistent in the free-flowing river. A longitudinal gradient of increasing abundance and richness of only fluvial specialist species existed downstream of the hydroelectric dam. No similar spatial gradient existed for macrohabitat generalists in either river. Although a fish community recovery gradient was identified, a recovery endpoint was not evident because assemblage change was gradual and possibly incomplete. The preservation and management of riverine fish faunas will partly depend on incorporating spatial recovery into decisions about permitting and siting of anthropogenic changes like hydroelectric dams.