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from the July/August 2000 issue of People, Land & Water, the employee news magazine of the Department of the Interior

Exotic Crayfish Prey on Rare Southwestern Fishes

Fort Collins, Colorado

To better understand the impacts of non-native crayfish on native fish populations in the Southwest, USGS scientists from the Midcontinent Ecological Science Center and the Arizona Cooperative Fish and Wildlife Research Unit are conducting both laboratory and field experiments on the competitive and predatory interactions between the virile crayfish and four native, nongame fish species: Gila chub, desert sucker, Sonora sucker, and speckled dace.

Virile crayfish, an invasive species, is being investigated by USGS scientists because of its impact on native, nongame fish.

"Crayfish are opportunistic feeders, eating both plants and animals, and can quickly outgrow predation by most fishes," noted USGS scientist Jeanette Carpenter. "Our studies help determine predation rates of crayfish on native fishes and if crayfish compete with native fish for food and cover. We are also interested in knowing if crayfish cause changes in southwestern populations of aquatic invertebrates."

From laboratory results, scientists have learned that juvenile desert and Sonora suckers, both bottom dwellers less than 7 centimeters long, are more vulnerable to crayfish predation than other species. The virile crayfish can attain total lengths of more than 10 centimeters in Arizona. Results also indicate crayfish could out-compete native fish for cover.

Field experiments in Arizona investigated the impacts of crayfish on fish growth as well as abundance and diversity of aquatic invertebrates. Results from field and lab growth experiments showed Gila chub were not affected by crayfish, whereas suckers lost weight when associated with crayfish in the lab. Furthermore, high crayfish densities were associated with decreases in large, slow-moving invertebrates, such as snails and mollusks, and a common aquatic plant species, the water buttercup.

Information from these studies will help managers determine whether to limit the spread of crayfish or work to reduce crayfish numbers in areas where native fish species are being restored. USGS efforts are assisted by the Nature Conservancy, Arizona Game and Fish Department, and the U.S. Forest Service.

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