Crayfish – also called crawfish, crawdads, or mudbugs, depending on where you live – look like tiny lobsters, and live in freshwater rivers and streams. Crayfish need instream cover to hide from predators – and from larger, cannibalistic kin. They also use cover to find food, to shelter while incubating eggs, and to keep themselves from being washed away in floods.
Susan Adams, U.S. Forest Service researcher, examined different types of cover in the Yazoo River basin of Mississippi to see whether crayfish used large pieces of household trash for shelter when natural cover was limited. Adams is a fisheries research scientist at the Forest Service Southern Research Station Center for Bottomland Hardwoods. Her study was recently published in Environmental Management.
Rivers and streams in the Yazoo River basin have undergone major changes due to historic land use and river management. Many streams and rivers have become deeply incised, partly as a result of channelization and dredging in the watersheds. Water moves faster through such streams, picking up more sediment while either flushing out or burying large wood and other forms of cover. “There is some habitat variation, especially in headwater streams,” says Adams. “However, most streams in north-central Mississippi are highly degraded and have simplified habitat with very little instream cover.” In some streams, trash – old tires, televisions, refrigerators, car parts, and other discarded household items – provides the most complex and stable cover available.
In marine environments, certain types of large trash have been intentionally sunk to create artificial reefs that provide shelter and hard substrates for marine life, but very few studies have examined whether small freshwater animals use trash as an artificial reef. “This study was designed to see whether crayfish use trash as cover,” says Adams, “We also looked at whether crayfish size influenced their choice of shelter.”
Adams selected eight study sites in the Yazoo River basin and established 294 plots that were evenly split between three cover classes – natural cover such as pieces of wood or masses of leaves and roots, large pieces of trash, or no cover. Adams caught a total of 413 crayfish, most of them (253) in plots with natural cover, followed by trash (154) and no cover (8). Across all sites, the percentage of plots containing crayfish was similar between garbage and natural cover plots, but significantly lower in plots without any cover.
“This study suggests that while crayfish do use trash, it is probably not an adequate surrogate for natural cover,” says Adams. “However, large trash in streams appears to be functioning as an artificial reef, especially for larger crayfish.” Large female crayfish have the highest reproductive value, so a stable or increasing crayfish population may depend on suitable habitat for the largest individuals. Smaller crayfish used trash too, but they were more likely to be found in natural cover such as rootmats, and became more likely to use trash as they grew bigger.
“Ultimately, the conclusions extend beyond whether or not trash is beneficial as cover, and show that crayfish in the region depend heavily on instream cover,” says Adams. Maintaining natural cover in deeply incised and channelized streams can be difficult. In incised streams, the river channel often becomes so wide that during low flows the roots of streamside plants – an important type of natural cover for small crayfish – lose contact with the water. However, as long as rivers are not dredged and land use does not dramatically change hydrology, secondary floodplains will eventually develop along incised banks to help provide the stable, complex cover that crayfishes, fishes, and other animals depend on.
For more information, email Susan Adams at firstname.lastname@example.org