Endangered Grand Canyon Native Fish Starts to Recover

The humpback chub, Gila cypha, is a freshwater fish that may live to 40 years of age and is found only in the Colorado River Basin. The humpback chub was placed on the federal endangered species list in 1967. Only six populations of humpback chub are currently known to exist, five above Lees Ferry, Arizona, and one in Grand Canyon, Arizona.

The upward trend reverses population declines from 1989 to 2001. Scientists estimate the number of adult chub in the Grand Canyon population at between 6,000 and 10,000, with the most likely number being 7,650 fish.

"USGS scientists and their cooperators are actively pursuing research that will increase our understanding of why native fish populations are increasing," said Matthew Andersen, a USGS supervisory biologist.

"Experimental flows from Glen Canyon Dam and above average water temperatures as the result of drought conditions may have supported native fish," said Andersen. "Removal of some nonnative fish species in select locations may also have helped."

An endangered humpback chub (Photo courtesy Glen Canyon Adaptive Management Program)

Declines in humpback chub numbers are believed to have been caused by dams on the Colorado River which altered water flow and water temperature and allowed introduced fish species which eat small humpback chub to proliferate.

It is not easy to determine what is causing the rebound because several natural and human-caused changes have taken place between 2000 and 2008, Andersen said.

The experimental removal of large numbers of rainbow trout and brown trout from the area near the confluence of the Colorado and Little Colorado Rivers, the area where most Grand Canyon chub are found, may have benefited the endangered chub because these fish prey on young chub and compete with the adult chub for food.

Between 2003 and 2006, the rainbow trout population in the Colorado River near the Little Colorado River was reduced by more than 80 percent.

Native fishes, including humpback chub, are thought to have benefited from drought-induced warming beginning in 2003. Before then, water temperatures in the main channel of the Colorado River have been too cold for humpback chub to successfully reproduce near the Little Colorado River.

A series of experimental releases of water from behind the Glen Canyon dam took place between 2000 and 2008 that may have benefited humpback chub and other native fish.

Humpback chub hatched in 1999 may have prospered as the result of in-stream warming as the result of the 2000 low summer steady flow experiment, which raised water temperatures in lower sections of Grand Canyon, the USGS scientists say.

Grand Canyon native fish populations have experienced recent improvements, which is not the case elsewhere in the Colorado River Basin.

Populations of the flannelmouth sucker, Catostomus latipinnis, and bluehead sucker, Catostomus discobolus, both native Colorado River fish, are stable and appear to have increased in the reach upstream and downstream from the mouth of the Little Colorado River. In this area, scientists have found juvenile and adult fish of both species, suggesting that more successful reproduction is occurring.

"The Grand Canyon is the one bright spot in the Colorado River Basin for native fishes, which is excellent news," said Andersen.

The likely factors that contributed to the historical decline of Grand Canyon native fish 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 parasites such as Asian tapeworm, and competition with and predation by nonnative fish species, he said.

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 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.

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