Road Ecology Center

Several studies have measured the influence of road crossings on fish movement and on fish communities within the Ouachita National Forest. In an initial study, passage of more than a hundred darters through a baffled pipe and over a grouted rip-rap ramp was documented over nine weeks. A broader study of fish movements associated with nine crossings ranging from natural-bottomed fords to piped crossings showed that natural ford and box culverts allowed unrestricted fish passage, but other designs were associated with reduced passage or none at all. Six piped crossings were examined in more detail including three that were modified in an attempt to improve fish passage. Fish were less likely to move across reaches with these low-water bridges compared to nearby natural reaches without low-water bridges. Average species richness was higher for fish communities downstream of the crossings compared to upstream (12.5 versus 6.3). Two rip-rapped low-water crossings were the only ones where upstream fish passage was detected. In a study of leopard darters, only one individual was detected moving downstream through a low-water crossing and none were found moving upstream. In an extensive study of twenty-one randomly selected low-water crossings, species richness was greater downstream versus upstream (9.4 versus 7.1, respectively). Total abundance (total number of all individuals of all species) was also significantly lower in the combined upstream reaches versus the combined downstream reaches. New box culvert installations indicated limited success in upstream passage, though detection of marked fish was quite low. Watershed-scale road and crossing densities were not significantly related to diversity and abundance of warmwater fishes or smallmouth bass density and biomass. Another study looking strictly at the effects of low-water crossings on stream geomorphology found stream widening upstream, stream incision downstream and changes in substrates when compared to a representative reach without a crossing. Work continues in designing, constructing and monitoring crossings that will pass fish.