U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007-3113

December 2007

Grand Canyon Humpback Chub Population Improving

Figure 1. The humpback chub (Gila cypha) is an endangered freshwater fish found only in the Colorado River Basin. Analysis of recently 

collected data indicates that the number of adult fish (age 4+ years) in 

Grand Canyon increased to 6,000 following years of decline (photograph 

courtesy of Arizona Game and Fish Department).

The humpback chub (Gila cypha) (fig. 1) is a long-lived, 

freshwater fish found only in the Colorado River Basin. Physical adaptations—large adult body size, large predorsal hump, 

and small eyes—appear to have helped humpback chub evolve 

in the historically turbulent Colorado River. A variety of factors, 

including habitat alterations and the introduction of nonnative 

fishes, likely prompted the decline of native Colorado River 

fishes. Declining numbers propelled the humpback chub onto 

the Federal list of endangered species in 1967, and the species 

is today protected under the Endangered Species Act of 1973. 

Only six populations of humpback chub are currently known to 

exist, five in the Colorado River Basin above Lees Ferry, Ariz., 

and one in Grand Canyon, Ariz.

The U.S. Geological Survey’s (USGS) Grand Canyon 

Monitoring and Research Center oversees monitoring and 

research activities for the Grand Canyon population under 

the auspices of the Glen Canyon Dam Adaptive Management 

Program (GCDAMP). Analysis of data collected through 2006 

suggests that the number of adult (age 4+ years) humpback chub 

in Grand Canyon increased to approximately 6,000 fish in 2006, 

following an approximate 40–50 percent decline between 1989 

and 2001 (Coggins, 2007). Increasing numbers of adult fish 

appear to be the result of steadily increasing numbers of juvenile 

fish reaching adulthood beginning in the mid- to late-1990s and 

continuing through at least 2002 (Coggins, 2007). 

Background

The majority of Grand Canyon humpback chub are found 

in the Little Colorado River (the largest tributary to the Colorado River in Grand Canyon) and in the Colorado River near 

its confluence with the Little Colorado River (Paukert and others, 2006). The vast majority of these fish are produced in the 

Little Colorado River because mainstem Colorado River temperatures are generally too cold for spawning. Humpback chub 

require a minimum temperature of 16oC (60.8oF) to reproduce 

(Hamman, 1982). Today, because the release structures of Glen 

Canyon Dam are well below the surface of Lake Powell, where 

the warmth of the sun cannot penetrate, Colorado River temperatures in Grand Canyon range from 7oC to 12oC (44.6oF–53.6oF). 

Humpback chub have been affected not only by changes 

in hydrology since the construction of the dam but also by 

competition with and predation by nonnative fishes. Fishes 

originating in many parts of the world are currently found in the 

Colorado River, including introduced species of trout, catfish, 

carp, and bass. Nonnative fish parasites, such as the Asian 

tapeworm (Bothriocephalus acheilognathi) and anchor worm 

(Lernaea cyprinacea), which infect some humpback chub, have 

also found their way into the Colorado River. 

Since 1989, when scientists began repeated monitoring 

efforts, the population of adult humpback chub in Grand Canyon has declined steadily until about 2001 (fig. 2.), when the 

population appeared to stabilize. More recently, the population 

appears to have increased, reaching an estimated 6,000 adult 

fish in 2006. Current estimates suggest that there has been an 

approximate increase of 20–25 percent in adult fish abundance 

since 2001 (Coggins, 2007). This increase in adult fish numbers since 2001 appears to be the result of increasing recruitment that began in the mid- to late-1990s and has continued 

through at least 2002.

Improved Assessment Procedures 

USGS scientists have been working to improve their ability 

to estimate the status and trends of the Grand Canyon population of humpback chub. They evaluated the age-structured 

mark-and-recapture models used to estimate adult humpback 

chub numbers and made refinements to ensure that current estimates are most consistent with the data (Coggins, 2007). Model 

innovations included accounting for the impacts of seasonal and 

spatial variation in water temperature on fish growth. A new 

method was also developed for estimating the relation between 

fish age and length (Coggins, 2007). Scientists also concluded 
Year

Adult Abundance (x1000)

198919901991199219931994199519961997199819992000200120022003200420052006

4

5

6

7

8

9

10

Figure 2. Adult (age 4+ years) humpback chub population estimates 

(1989–2006) for the Grand Canyon population. Error bars represent 95 

percent confidence intervals (from Coggins, 2007). 

that only large changes in recruitment caused by management 

actions can be readily detected, given the uncertainty in current 

assessment procedures. 

Discussion

The phenomena that may be causing recent increases in 

adult humpback chub numbers in Grand Canyon are uncertain. 

Scientists hypothesize that the fish may have benefited from several factors, including the experimental water releases from Glen 

Canyon Dam, removal of nonnative fish, and drought-induced 

warming. 

Humpback chub produced in 1999, which are now adults, 

may have benefited from substantial in-stream warming as the 

result of the 2000 low steady flow experiment. In the summer 

of 2000, as a result of the experiment, peak water temperatures 

in some parts of Grand Canyon exceeded 20oC (68.5oF), which 

represents a temperature increase when compared with typical 

peak temperatures of 15oC–18oC (59oF–64oF) in recent years. 

Additionally, experimental high flows released in 1996, 1997, 

2000, and 2004 may have disadvantaged nonnative fish and 

improved humpback chub habitat (Valdez and others, 2001).

Beginning in 2003, large numbers of rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) were removed 

from the area near the confluence of the Colorado and Little 

Colorado Rivers. Rainbow and brown trout are thought to 

compete with humpback chub for food and to prey on young 

fish. Between 2003 and 2006 the rainbow trout population in 

the Colorado River near the Little Colorado River was reduced 

by more than 80 percent (U.S. Geological Survey, unpub. data, 

2007). Other nonnative fish species were also removed as part of 

this effort.

Since 2003, water temperatures below Glen Canyon Dam 

increased as the result of drought conditions. As drought has 

reduced flows into Lake Powell, the level of the reservoir has 

dropped, allowing warmer water found closer to the surface of the 

reservoir to reach the release structures. In 2005, water temperatures in the mainstem Colorado River near the Little Colorado 

River exceeded 17oC (60.8oF), the warmest temperatures recorded 

since the reservoir filled in 1980 and approximately the minimum 

temperature needed by humpback chub to successfully reproduce. 

Native fish are thought to benefit from warmer water releases; 

however, there is great concern that warmer water temperatures 

may also benefit nonnative warmwater fish like the channel catfish (Ictalurus punctatus) and other voracious predators.

Scientists do not know if experimental activities, the 

natural warming of the river, or other factors such as improved 

rearing conditions in the Little Colorado River are most responsible for increases in recruitment and survival of humpback 

chub. Continued monitoring of humpback chub, other species, 

and habitat conditions will be necessary to establish cause-andeffect relationships. 

The Glen Canyon Dam Adaptive Management Program 

was established to monitor and analyze the effects of dam operations on downstream resources and to use these assessments to 

recommend to the Secretary of the Interior adjustments intended 

to improve the values for which the Glen Canyon National Recreation Area and Grand Canyon National Park were established. 

Fieldwork related to humpback chub research was conducted 

cooperatively by the USGS and GCDAMP partners, including 

the Arizona Game and Fish Department and the U.S. Fish and 

Wildlife Service.

References

Coggins, L.G., 2007, Abundance trends and status of the Little 

Colorado River population of humpback chub; an update considering 1989–2006 data: U.S. Geological Survey Open-File 

Report 2007-1402, 53 p. 

Hamman, R.L., 1982, Spawning and culture of humpback chub: 

Progressive Fish-Culturist, v. 44, no. 4, p. 213–216.

Paukert, C.P., Coggins, L.G., and Flaccus, C.E., 2006, Distribution and movement of humpback chub in the Colorado River, 

Grand Canyon, based on recaptures: Transactions of the 

American Fisheries Society, v. 135, p. 539–544.

Valdez, R.A., Hoffnagle, T.L., McIvor, C.C., McKinney, T., and 

Leibfried, W.C., 2001, Effects of a test flood on fishes of the 

Colorado River in Grand Canyon, Arizona: Ecological Applications, v. 11, no. 3, p. 686–700.

More Information

Matthew E. Andersen

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research Center

Tel: 928-556-7094 

E-mail: mandersen@usgs.gov

Website: www.gcmrc.gov