U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3035

April 2009

Status and Trends of the Grand Canyon Population 

of Humpback Chub

The humpback chub (Gila cypha) is a 

freshwater fi sh that may live as long as 40 

years and is found only in the Colorado 

River Basin. Habitat alterations, including 

changes caused by dams and reservoirs and 

the introduction of a wide variety of nonnative fi sh, have contributed to population 

declines that caused the humpback chub 

to be placed on the Federal list of endangered species in 1967. Only six populations 

of humpback chub are currently known 

to exist, fi ve above Lees Ferry, Arizona, 

and one in Grand Canyon, Arizona. 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.

Reaching 20 inches in length, the 

humpback chub possesses features that 

distinguish native Colorado River fi shes—

large adult body size, small depressed 

skull, large predorsal hump, and small 

eyes. These features are assumed to be 

adaptations to life in the historically turbulent and seasonally sediment-laden Colorado 

River. Today the species is confi ned to isolated deep canyon stretches of the Colorado 

River and its tributaries. In Grand Canyon, 

most humpback chub are found in the Little 

Colorado River and at its confl uence with 

the Colorado River.

The Grand Canyon population reproduces primarily in the Little Colorado 

River because mainstem Colorado River 

temperatures are generally too cold for 

spawning and juvenile rearing. Humpback 

chub require a minimum temperature of 

16oC (60.8oF) to reproduce. Except during 

drought conditions, the water-intake structures at Glen Canyon Dam are well below 

the surface of Lake Powell, where the water 

is not warmed by the sun. Colorado River 

temperatures in Grand Canyon generally 

range from 7oC to 12oC (44.6oF–53.6oF). 

Humpback chub have been affected 

not only by dam-induced changes but also 

through predation by and competition with 

nonnative fi sh. Fish originating in many 

parts of the world are currently found in 

the Colorado River, including introduced 

species of trout, catfi sh, carp, and bass. 

Also present in the Colorado River are 

nonnative fi sh parasites, such as the Asian 

tapeworm (Bothriocephalus acheilognathi) 

and anchor worm (Lernaea cyprinacea), 

which infect some humpback chub and 

may be a threat.

Monitoring the Humpback Chub 

Population

The USGS uses a method called the AgeStructured Mark-Recapture (ASMR) model, 

which combines capture data and statistical modeling, to estimate humpback chub 

U.S. Geological Survey scientists estimate that the adult population of humpback chub (Gila 

cypha) in the Grand Canyon increased 50 percent between 2001 and 2008. (Photograph 

courtesy Arizona Game and Fish Department)

The Colorado River Basin supports one of the most distinctive fi sh communities in North America, including the 

federally endangered humpback chub 

(Gila cypha). One of only six remaining populations of this fi sh is found in 

Grand Canyon, Arizona. U.S. Geological 

Survey scientists and their cooperators 

are responsible for monitoring the Grand 

Canyon population. Analysis of recently 

collected data indicates that the number 

of Grand Canyon adult humpback chub—

fi sh 4 years old and older and capable of 

reproduction—increased approximately 

50 percent between 2001 and 2008. When 

possible model error is considered, the 

estimated number of adult chub in the 

Grand Canyon population is between 

6,000 and 10,000. The most likely number 

is estimated at 7,650 individuals.

Humpback Chub
1990

4

Year

Adult abundance, in thousands 6

8

10

12

1992 1994 1996 1998 2000

Abundance of Humpback Chub

2002 2004 2006 2008

population and recruitment trends. This 

method provides a relatively strong indication of the population size trend; however, 

it offers somewhat less confidence for the 

single population estimate. The data show 

a steadily declining trend of adult humpback chub from 1989 to 2001 (Coggins and 

Walters, 2009). From 2001 until 2008, the 

population reversed, increasing to an estimated 7,650 adult fish in 2008, an increase 

of approximately 50 percent (Coggins and 

Walters, 2009). 

The ASMR model has been verified 

as an appropriate approach for estimating trends in humpback chub population 

size by a series of independent reviewers. 

Although the current adult population size 

is estimated at approximately 7,650 individuals, this estimate is very sensitive to 

assumptions about humpback chub growth 

and mortality rates. Considering a range 

of probable values for these parameters, it 

is likely that there are between 6,000 and 

10,000 adult chub in the Grand Canyon 

population. This level of uncertainty suggests that managers should focus on the 

temporal trend in humpback chub abundance rather than estimates of absolute 

abundance.

Causes of the Population Increase

The factors contributing to the estimated 

increases of adult humpback chub numbers 

in Grand Canyon are not easy to determine. 

Between 2000 and 2008, both humancaused 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, and droughtinduced warming of the Colorado River.

Experimental Water Releases

A series of experimental releases from 

Glen Canyon Dam took place between 

2000 and 2008 that may have benefited 

humpback chub and other native fish. For 

example, during the summer of 2000, 

releases from Glen Canyon Dam were constrained during a low steady flow experiment, which caused peak downstream 

water temperatures in Grand Canyon to 

exceed 20oC (68.5oF). Nearshore water 

temperatures during the summer of 2000 

exceeded the minimum temperature that 

humpback chub need to reproduce and may 

have created conditions that allowed fish 

produced in 1999 to grow more rapidly 

than in normal, colder temperatures. Additionally, a series of experimental releases 

from the dam, including three high-flow 

experiments, may have disadvantaged nonnative fish and improved humpback chub 

habitat.

Removal of Nonnative Fish

The removal of large numbers of 

nonnative fish, particularly rainbow trout 

(Oncorhynchus mykiss) and brown trout 

(Salmo trutta), from the area near the 

confluence of the Colorado and Little 

Colorado Rivers may also have benefited 

the humpback chub population in that 

area. Not only can trout prey on juvenile 

humpback chub, but also they rely on the 

same food sources—aquatic and terrestrial invertebrates, algae, and small 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.

Drought-Induced Warming

Drought conditions are also thought to 

play a role in the recent rebound in humpback chub numbers. Water temperatures 

below Glen Canyon Dam increased starting 

in 2003, as drought caused the level of 

Lake Powell reservoir to drop, allowing 

warmer surface water of the reservoir to be 

released downstream. In 2005, water temperatures in the mainstem Colorado River 

near the Little Colorado River exceeded 

17oC (60.8oF), the warmest temperatures 

recorded there since the reservoir filled 

in 1980 and approximately the minimum 

temperature needed by humpback chub to 

successfully reproduce. Although native 

fish likely benefit from warmer water 

releases, there is great concern that warmer 

water temperatures may also benefit nonnative warmwater fish such as channel catfish 

(Ictalurus punctatus) and other predators.

In the future, USGS, U.S. Fish and 

Wildlife Service, and Arizona Game and 

Fish Department scientists will continue to 

monitor the Grand Canyon population of 

humpback chub, trying to isolate the conditions responsible for recent improvements 

in humpback chub population and recruitment trends. Research activities will examine, among other questions, how the March 

2008 high-flow experiment affected habitat 

used by native fish and whether high flows 

disadvantage nonnative species.

Matthew E. Andersen

Edited by Peter H. Stauffer

Layout by David R. Jones

Estimated adult humpback chub abundance (age 4+) from Age-Structured Mark-Recapture 

(ASMR) model incorporating uncertainty in assignment of age. Error bars represent minimum 95-percent confidence intervals (from Coggins and Walters, 2009) and do not consider 

uncertainty in growth or mortality rates.

Reference

Coggins, L.G., Jr., and Walters, C., 2009, Abundance trends and status of the Little Colorado 

River population of humpback chub—an update 

considering data from 1989-2008: U.S. Geological Survey Open–File Report 2009–1075, 18 p. 

For more information contact:

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research Center

Flagstaff, AZ 86001

928–556–7094 or

mandersen@usgs.gov

This Fact Sheet and any updates to it

are available online at 

http://pubs.usgs.gov/fs/2009/3035/