Alabama has the most diverse crayfish fauna in the world, with more than 90 native species found in the state. Crayfishes play an important role in stream ecosystems by breaking down plant materials, digging burrows that are sometimes used by other stream animals, and serving as predators of or prey for numerous animals, especially some game fishes such as bass and catfish.
However, crayfish populations are declining worldwide, with 48% of North American crayfish species imperiled. Despite their importance, no previous study has examined the impacts of large dams on crayfishes. Dams and their impoundments change stream habitats and block movements of stream organisms, which subsequently changes the species present within the stream and isolates stream populations.
Little Bear and Cedar Creek dams were built in 1975 and 1979, respectively, for flood control. While these dams provide services to humans living in these watersheds, we must understand how they impact stream animals so that we are able to use this valuable resource without losing the amazing stream diversity within the Bear Creek drainage.
USDA Forest Service and University of Mississippi researchers, led by Dr. Zanethia Barnett, compared crayfish diversity, density, and migration within three Bear Creek Drainage streams: Little Bear (with dam), Cedar (with dam), and Rock (without dam) creeks.
Scientists collected a total of seven crayfish species within the Bear Creek drainage: six species from Little Bear Creek, three species from Cedar Creek, and six species from Rock Creek. Two crayfish species (Faxonius validus [powerful crayfish] and F. erichsonianus [reticulate crayfish]) made up more than 95% of collections up and downstream of dams in Little Bear and Cedar creeks. While these two species were present within Rock Creek, they did not dominate every site. There was a gradual shift in species present with movement downstream. Species such as Cambarus striatus (ambiguous crayfish) and F. compressus (slender crayfish) were common in up- and downstream sections of Rock Creek, respectively. These species were rarely encountered in Little Bear and Cedar creeks.
A higher density of crayfishes were collected in Rock Creek than Little Bear and Cedar creeks. Six time as many crayfishes were collected in Rock than Cedar Creek, and three times as many in Rock than Little Bear Creek. Within Little Bear and Cedar Creeks, fewer crayfishes were collected down- than upstream of reservoirs. Half as many crayfishes were collected down- than upstream in Cedar Creek and 35% less crayfish were collected down- than upstream in Little Bear Creek.
Very few crayfishes and often only one crayfish species were collected right below dams. In Little Bear Creek, crayfish density and richness (number of species) increased with distance away from the dam, showing the crayfish assemblage recovered with distance from the dam.
Little Bear and Cedar Creeks had more stable flow and temperature regimes (lower maximum flows and temperatures and lower minimum flows and temperature) when compared to Rock Creek, which can decrease the diversity of crayfishes. Because the timing of extreme high and low flow events is normally seasonal, crayfishes have synchronized their life history events (e.g., timing of mating) with a stream’s natural flow and temperature regimes. Thus, decreasing the variability of stream flow and temperature, as well as the timing of maximum and minimum events, can impact key life history events for crayfishes. This often decreases the abundance of crayfish species that are adapted to natural conditions and sometimes eliminates specialized species. Managing streams to mimic natural flow regimes and releasing water at similar stream temperatures may help maintain diverse stream communities.
There was less vegetation in Cedar and Little Bear Creeks than in Rock Creek. Crayfish often use vegetation as refuge to protect them from predators. In addition, there were also more crayfish predators (e.g., basses) down- than upstream of dams, as well as more crayfish predators downstream of dams than in Rock Creek, which was correlated to lower crayfish abundance and diversity.
Dams reduced crayfish movement throughout stream system, reducing gene flow (movement and mating of crayfishes) between up- and downstream populations. For one species (reticulate crayfish), crayfish downstream of the dam were not able to migrate to upstream sections. This isolates crayfish populations upstream of the dam and increases their risk of local extinction.
Conversely, this species moved freely throughout undammed Rock Creek, with high gene flow between up- and downstream sections. Conservation strategies focused on connecting crayfish populations up- and downstream of dams – such as moving crayfish from downstream of dam to upstream or building fish ladders – may help prevent the reduction and loss of crayfish populations. There is still much to learn about the effects of dams on crayfishes, but it is clear that changes to stream ecosystems by dams have major consequences on crayfishes.
We would like to thank the many landowners in Franklin and Colbert counties for allowing us to access streams through their property. This study could not have been conducted without them!
Sources:
#0563c1;">https://www.srs.fs.usda.gov//pubs/59267
#0563c1;">https://egrove.olemiss.edu/cgi/viewcontent.cgi?article=2547&context=etd
Other articles about research:
#0563c1;">https://www.srs.fs.usda.gov/compass/2020/08/04/dams-crayfish-genetics/
