We quantified fish assemblages in 20 streams containing mill dams in various physical conditions (dams intact, partially breached, or relict with normal flows) in Alabama, USA, during the period from 2006 to 2008. We used a backpack electroshocker to sample three 150-m reaches per stream: 500 to 1000 m downstream of the dam, 0 to 100 m downstream of the dam, and 100 m upstream of the impoundment. Species- and trait-based analyses revealed slightly different, but often complementary, information about fish assemblages. Fish species richness and benthic conditions differed longitudinally among reaches in streams with dams. In streams with breached dams, species richness, but not trait richness, was lower in upstream reaches than in downstream reaches. Overall, species and trait richness were correlated with benthic-habitat variables in streams with relict dams and were significantly correlated with water physicochemical variables in streams with intact and breached dams. Nonmetric multidimensional scaling ordination failed to resolve any discernable site groupings based on species abundance data, and indicator species analysis revealed 1 indicator species, Esox americanus, upstream of relict dams. Fourth-corner trait analysis revealed more trait associations in reaches in streams with breached dams than in those with intact or relict dams. Generalist spawners (nest-guarding polyphils) increased and taxa with a preference for cobble substrates decreased upstream of breached dams. Few longitudinal differences were observed in streams with relict and intact dams. Taken together, dams, particularly those that are breached, appear to exert a strong upstream influence on fish species richness and functional composition and could alter the trophic structure of the entire stream through habitat modifications or limitation of fish movements.
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