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News Release


August 3, 2006
Matthew Andersen 928-607-4874 mandersen@usgs.gov
Bill Lukas 206-220-4576 wlukas@usgs.gov

Endangered Humpback Chub Population in Grand Canyon Stabilizing

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Recently collected data for endangered humpback chub (Gila cypha) in Grand Canyon suggest that the population of adult fish (age 4+) may be stabilizing after more than a decade of decline, according to biologists with the U.S. Geological Survey's (USGS) Southwest Biological Science Center. Between 2001 and 2005, the number of adult fish appears to have stabilized at an estimated 5,000 fish. In 2005, scientists also detected more juvenile fish (age 1 to 4) and young-of-year fish, or fish hatched in 2005, than previous years.

"The possible stabilization of adult fish numbers is exciting news for the recovery effort because it means that conditions exist in Grand Canyon that allow young fish to reach reproductive age," says Matthew Andersen, USGS Southwest Biological Science Center Supervisory Biologist. "Until recently, the Grand Canyon population was steadily declining because adult fish were dying at a rate of 15% to 20% annually and young fish were not surviving in sufficient numbers to replace adult mortality."

Catch-rate data also indicate an increased abundance of juvenile humpback chub between 2003 and 2005 near the confluence of the Colorado and Little Colorado Rivers, where spawning is known to occur. Increases in juvenile fish during the same period were also apparent for other native species found near the confluence, including bluehead sucker (Catostomus discobolus), flannelmouth sucker (Catostomus latipinnis), and speckled dace (Rhinichthys osculus).

Catch rates for young-of-year humpback chub were higher in 2005 than previous years in middle and lower Marble Canyon. Higher than average catch rates at these locations were unexpected because they are up to 25 river miles above the confluence of the Colorado and Little Colorado Rivers where spawning was thought to be confined. These findings suggest that more favorable conditions for spawning and incubation existed in the Colorado River main channel during 2005.

The primary factors thought to be contributing to the findings are as follows:

The humpback chub was federally listed as endangered on March 11, 1967. The likely factors contributing to the decline of the species in Grand Canyon include changes in flow and reduced water temperature resulting from the regulation of the Colorado River by Glen Canyon Dam, the weakening of young fish by the nonnative Asian tapeworm (Bothriocephalus acheilognathi), and competition with and predation by nonnative fish species.

Specific recovery goals for humpback chub in Grand Canyon are currently being established by the U.S. Fish and Wildlife Service, which has jurisdiction over the humpback chub as a federally endangered species.

The USGS Southwest Biological Science Center's Grand Canyon Monitoring and Research Center (GCMRC) is responsible for the synthesis and analysis of fish data collected by a number of cooperating entities, including U.S. Fish and Wildlife Service and Arizona Game and Fish Department. These activities are undertaken as part of the Glen Canyon Dam Adaptive Management Program, which is administered by the U.S. Department of the Interior.

For more information, download the USGS fact sheet Grand Canyon Humpback Chub Population Stabilizing and images of humpback chub at www.gcmrc.gov/research/humpback_chub/20060802.htm.


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