Consequences of low-head dams on crayfish distribution and gene flow in Alabama stream
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Dams are numerous in many eastern US streams, and instream physicochemical and biotic impacts of dams can alter flow and sediment regimes and channel geomorphology as well as reducing longitudinal movement by fishes and other mobile organisms. In addition, dams can fragment populations, thus decreasing genetic diversity while increasing extinction vulnerability. I quantified freshwater crayfish abundance and their habitats at reaches upstream (1000-5000 m), immediately downstream (mill reaches), and >500 m downstream of 22 low-head milldams within 9 Alabama drainages in 2006–2008. Eleven dams were intact, 5 were partially breached, and 6 were considered relict with more natural flow regimes. On streams with intact dams, crayfish abundance was lower at mill reaches than at reaches upstream of impoundments or further downstream, whereas on streams with breached dams abundance was higher at upstream reaches than at mill or downstream reaches. In contrast, longitudinal patterns in crayfish abundance were similar among sites on streams with relict dams. Predatory fish abundance was higher at mill reaches on streams with intact dams than at sites upstream or further downstream, suggesting that predatory fish aggregations at dam reaches were responsible for low crayfish abundance. Genetic diversity and population connectivity of 2 crayfish species (Cambarus striatus and C. coosae) also was quantified from upstream, mill, and downstream reaches of 2 focal, intact dam sites by sequencing a fragment of the mitochondrial cytochrome oxidase I (COI) gene. Cambarus striatus in Sandy Creek showed evidence of upstream population isolation with movement limited to downstream migration across the dam, whereas C. coosae in Hatchett Creek showed no evidence of population structure. Our results suggest that small low-head dams and their reservoirs can alter abundance and impede longitudinal migration of some freshwater crayfishes.