The humpback chub (Gila cypha) is a large and unique freshwater fish found only in the Colorado River Basin. Known to be somewhat rare when it was first described in the 1940s, the humpback chub was placed on the first Federal list of endangered species in 1967. Modeling of the adult humpback chub population suggests that there were many more adults in Grand Canyon in 1989 than in 2001. The decline is likely due to predation from nonnative fish, competition for food with nonnative fish, and habitat alterations brought about by operation of Glen Canyon Dam. Of the six known populations of humpback chub, five are found above Lees Ferry, Arizona, and one is in Grand Canyon, Arizona. The USGS Grand Canyon Monitoring and Research Center (GCMRC) oversees monitoring and research activities for the Grand Canyon population under the auspices of the Glen Canyon Dam Adaptive Management Program.
Recent USGS estimates for humpback chub in Grand Canyon showed a 50 percent increase in adult fish between 2001 and 2008. Scientists estimate that the number of adult humpback chub in Grand Canyon is between 6,000 and 10,000. The most likely number is estimated at 7,650 fish. This recent resurgence reverses the steadily declining trend of adult humpback chub between 1989 and 2001. More than 90% of the Grand Canyon humpback chub population occurs in and near the Little Colorado River.
Factors contributing to the increase of humpback chub in Grand Canyon are hard to determine. However, scientists believe that both human-caused and natural events have taken place that may have improved conditions for humpback chub, including experimental water releases from Glen Canyon Dam, removal of nonnative fish through mechanical removal near the Little Colorado River confluence, and drought-induced warming of the Colorado River. GCMRC, U.S. Fish and Wildlife Service, and Arizona Game and Fish Department scientists will continue to monitor the Grand Canyon population of humpback chub, in order to identify the conditions associated with recent improvements in population and recruitment trends.
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