Effects of small dams on stream fish assemblages across Alabama, USA

From 2006 to 2008, we quantified fish assemblages in 22 streams containing milldams of various physical conditions (dams intact, partially breached, or relict with normal flows) within Alabama, USA.  Using a backpack electroshocker, three 100-m reaches were sampled per stream: 500-1000 m downstream of the dam, 0-100m downstream of the dam, and 100 m upstream of the impoundment.  There was no difference in total catch, richness, or diversity among streams with contrasting dam conditions or longitudinal differences among reaches of a given stream.  However, there were feeding guild differences, with a larger proportion of piscivores and smaller proportion of invertivores in streams with breached dams than those with intact or relict dams. In addition, fewer narrow endemic species occurred in streams with breached vs. intact dams.  Non-metric multidimensional scaling ordinations revealed that mean Euclidean distance from downstream to milldam to upstream reaches was larger in breached than relict or intact dams, suggesting greatest upstream–downstream assemblage dissimilarity in breached streams.  Taken together, breached dams appear to exert a stronger influence on stream fish assemblage structure than relict or intact dams, potentially by causing shifts toward general piscivorous species, reductions in invertivore proportions, and increased longitudinal differences in fish assemblages.