Consequences of low-head dams to crayfish assemblages and gene flow in Alabama streams

Dams are numerous in many southeastern U.S. streams, with >10,000 in Alabama alone.  Instream physicochemical and biotic impacts of dams can be dramatic, including altering flow and sediment regimes and channel geomorphology as well as reducing longitudinal movement by fishes and other mobile organisms.  Additionally, such barriers can fragment populations, thus decreasing genetic diversity while increasing extinction vulnerability.  In 2006–2007, we quantified crayfishes and their habitats at reaches upstream, immediately downstream (mill sites), and >500m downstream of 22 low-head milldams within 9 Alabama drainages. Ten dams were intact, 7 were partially breached, and 6 were relic with more natural flow regimes.  Preliminary data indicated that crayfish abundance at mill sites with intact dams were lower than sites upstream or further downstream, whereas abundance upstream of breached dams was significantly higher than at mill or downstream sites. In contrast, crayfish longitudinal abundance was similar among sites on streams with relic dams.  In addition, tissue from 30 crayfish specimens from two intact dam sites were used to amplify and sequence a fragment of the mitochondrial cytochrome oxidase 1 (COI) gene, with 10 individuals analyzed from each of the upstream, mill, and downstream reaches.  Initial genetic data suggest dams isolate upstream populations by potentially limiting upstream migration for some species.