Local Effects of Culverts on Habitat Features and Fish Assemblages in Blue Ridge Streams

I studied environmental conditions, fish assemblages, and culvert features in 11 headwater streams (Tennessee and Conasauga river drainages) in Cherokee National Forest (Blue Ridge ecoregion), Tennessee, May-August 2008. Culvert-related effects on instream habitat and fish assemblages were measured at 10 stream sites: five sites contained culverts with artificial bottom substrates and five sites contained culverts with natural bottom substrates. On each stream, paired sampling reaches (reach length ∼35 times mean reach width, drainage area <11 km2) were established 50 m upstream and 50 m downstream of the culvert. Reaches downstream of culverts with artificial substrates (compared to upstream reaches; related-samples t test) exhibited greater water depths, lower gravel/sediment depths, and higher percentages of bedrock and boulder substrates (P < 0.10). Reaches downstream of natural substrate culverts (compared to upstream reaches) exhibited faster current velocities (P < 0.10). Fish abundance (predominantly western blacknose dace Rhinichthys obtusus) was consistently higher downstream than upstream (P < 0.10), regardless of the culvert type. In addition, reaches downstream of artificial substrate culverts exhibited reduced species evenness compared to upstream reaches. Mark-recapture experiments on two streams documented fish movements through a natural substrate culvert (similar to those in reference areas); however, movements through a perched pipe culvert were not detected. Data collected in the present study suggest that culverts had localized effects on instream habitat and fish assemblages, and that certain types of culverts may impede fish dispersal.