Here’s a release from the USGS:

Adult endangered humpback chub (Gila cypha) in Grand Canyon, Arizona, increased by about 50 percent between 2001 and 2008, according to analysis recently conducted by the U.S. Geological Survey. The upward trend reverses population declines from 1989 to 2001. The estimated number of adult chub in the Grand Canyon population is between 6,000 and 10,000, with the most likely number being 7,650 individuals.

The humpback chub is a freshwater fish that may live up to 40 years and is found only in the Colorado River Basin. The humpback chub was placed on the Federal list of endangered species on March 11, 1967. Only six populations of humpback chub are currently known to exist, five above Lees Ferry, Arizona, and one in Grand Canyon, Arizona.

“USGS scientists and their cooperators are actively pursuing research that will increase our understanding of why native fish populations are increasing,” said Matthew Andersen, 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. Removal of some nonnative fish species in select locations may also have helped.”

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

The experimental removal of large numbers of rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) from the area near the confluence of the Colorado and Little Colorado Rivers may have benefited humpback chub. Rainbow and brown trout are thought to prey on young fish and compete with humpback chub for food. Between 2003 and 2006, the rainbow trout population in the Colorado River near the Little Colorado River, the area where most Grand Canyon chub are found, was reduced by more than 80%.

Native fishes, including humpback chub, are thought to have benefited from drought-induced warming beginning in 2003. Before 2003, 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. Humpback chub require a minimum temperature of 16°C (60.8°F). As the level of the Lake Powell has dropped, warmer water found closer to the surface of the reservoir has reached the release structures. In 2005, water temperatures in the mainstem Colorado River near the Little Colorado River exceeded 17°C (62.6°F), the warmest temperatures recorded in this section of the river since the reservoir filled in 1980.

A series of experimental releases from Glen Canyon 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 substantial in-stream warming as the result of the 2000 low summer steady flow experiment. As a result of the experiment, peak water temperatures in lower sections of Grand Canyon exceeded 20°C (68.5°F) in the summer of 2000, compared with typical peak temperatures of 15-18°C (59-64°F).
Grand Canyon native fish populations have experienced recent improvements, which is not the case elsewhere in the Colorado River Basin. In addition to humpback chub, the flannelmouth sucker (Catostomus latipinnis) and bluehead sucker (Catostomus discobolus), both native Colorado River fish, are stable, 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 Matthew Andersen, USGS supervisory biologist.

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 (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.

More information is available by visiting the Status and Trends of the Grand Canyon Population of Humpback Chub factsheet and open file report.