U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3109

July 2006

Grand Canyon Humpback Chub Population Stabilizing

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Figure 1. The humpback chub (Gila cypha) is an endangered freshwater fish found only in the Colorado River Basin. Recently collected data 

indicate that the number of adult fish (age-4+) in Grand Canyon stabilized 

between 2001 and 2005 after years of decline (photograph courtesy of 

George Andrejko, 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. To 

survive in the famously turbulent Colorado River, the species 

developed some unusual adaptations, including a large adult 

body size, large predorsal hump, and small eyes. A number of 

factors have contributed to the decline of humpback chub and 

other native Colorado River fish. In 1967, the humpback chub 

was added to the federal list of endangered species and 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, Arizona, 

and one in Grand Canyon, Arizona.

Monitoring and research of the Grand Canyon population 

of humpback chub is overseen by the U.S. Geological Survey’s 

Grand Canyon Monitoring and Research Center (GCMRC) 

under the auspices of the Glen Canyon Dam Adaptive Management Program (GCDAMP). Recently collected data indicate 

that the number of adult (age-4+) humpback chub in Grand 

Canyon stabilized between 2001 and 2005 after more than a 

decade of decline. 

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 the Colorado River near its 

confluence with the Little Colorado River. Small numbers of 

humpback chub are found elsewhere in Grand Canyon, but successful reproduction has only been documented for those fish 

found in or near the Little Colorado River. 

Reproduction has been restricted to the Little Colorado 

River because of changes in the mainstem Colorado River after 

the completion of Glen Canyon Dam. For example, prior to the 

dam, the water temperature of the Colorado River fluctuated seasonally from 0oC to 29oC (30–80oF). Today, because the release 

structures of the dam are well below the surface of Lake Powell, 

the water that leaves the dam is cold, with an average temperature of 8oC (46oF). Water temperatures in the main channel of 

the Colorado River have been too cold for humpback chub to 

successfully reproduce except near the Little Colorado River.

Recent Findings

Since scientists began monitoring efforts in 1989, the population of adult humpback chub in Grand Canyon has declined 

steadily until recently (fig. 2.). The death of 15% to 20% of 

adult fish each year and a low rate of juvenile fish surviving into 

adulthood contributed to the decline. Adult mortality rates and 

the failure of juvenile fish to reach adulthood have both been 

attributed to changes in Little Colorado River and Colorado 

River hydrology, the weakening of young fish by the nonnative 

Asian tapeworm (Bothriocephalus acheilognathi), and competition with and predation by nonnative fish species.

Between 2001 and 2005, however, conditions appear to 

have improved and the number of adult fish stabilized at an 

estimated 5,000 fish (fig. 2). Additionally, near the confluence of the Colorado and Little Colorado Rivers, catch-rate 

data from the monitoring program indicate an increased abundance of juvenile humpback chub between 2003 and 2005. 

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).

Elsewhere in Grand Canyon, catch rates for humpback 

chub produced in 2005 were higher than previous years in 

middle and lower Marble Canyon (U.S. Geological Survey, 

unpub. data, 2006). 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 usually occurs. These findings suggest that 

more favorable conditions for spawning and incubation existed 

in the Colorado River main channel during 2005.
Discussion

The exact causes of the stabilization of the adult population and increased numbers of young humpback chub cannot 

be specified at this time. However, humpback chub in Grand 

Canyon are thought to have benefited from several changes, 

including the experimental removal of nonnative fish, experimental water releases, and drought-induced warming. 

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 prey on young fish. Since 

2003, the rainbow trout population in the Colorado River near 

the Little Colorado River has been reduced by more than 60%. 

The removal effort will continue through 2006.

Humpback chub produced in 1999 may also have benefited 

from substantial in-stream warming as the result of the 2000 

low summer steady flow experiment. The experiment held Glen 

Canyon Dam releases constant at 8,000 cubic feet per second 

from June through August 2000 and included two habitat 

maintenance flows (high, steady dam releases). As a result, in 

the summer of 2000, 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 15–18oC (59–64oF) in recent years. Humpback chub 

habitat may also have been improved as the result of experimental floods conducted in 1996, 1997, 2000, and 2004.

Since 2003, water temperatures below the dam have also 

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), a 

voracious predator.

Scientists are not yet able to determine the relative importance 

of the various factors that may be contributing to recent improvements. More work will be required to understand how nonnative 

fish, temperature, and the operation of Glen Canyon Dam interact 

to affect the humpback chub population in Grand Canyon.

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 GCMRC 

and GCDAMP partners, including the Arizona Game and Fish 

Department and the U.S. Fish and Wildlife Service.

References

Coggins, L.G., Pine, W.E., III, Walters, C.J., Van Haverbeke, 

D.R., Ward, D., Johnstone, H.C., 2006, Abundance trends and 

status of the Little Colorado River population of humpback 

chub: North American Journal of Fisheries Management, v. 

26, p. 233–245.

 Gloss, S.P., and Coggins, L.G., 2005, Fishes of Grand Canyon 

in Gloss, S.P., Lovich, J.E., and Melis, T.S., eds., 2005, 

The state of the Colorado River ecosystem in Grand Canyon: 

U.S. Geological Survey Circular 1282, 220 p.

Melis, T.S., Martell, S.J.D., Coggins, L.G., Pine, W.E., III, and 

Andersen, M.E., 2006, Adaptive management of the Colorado River ecosystem below Glen Canyon Dam, Arizona: 

using science and modeling to resolve uncertainty in river 

management, in Specialty Summer Conference on Adaptive 

Management of Water Resources, Missoula, Mont., 2006, 

CD-ROM Proceedings (ISBN 1-882132-71-8): Middleburg, 

Va., American Water Resources Association.

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.

More Information

Matthew E. Andersen

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research Center

2255 N. Gemini Drive

Flagstaff, Arizona, 86001

Tel: 928-556-7094 

E-mail: mandersen@usgs.gov

Website: www.gcmrc.gov

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

(1989–2005) for the Little Colorado River. Upper and lower bounds 

are 95% Bayesian credible intervals. When confidence intervals are 

considered, the model indicates that the population has stabilized.