U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3093

August 2011

T

An Experiment to Control Nonnative Fish in the Colorado River, 

Grand Canyon, Arizona

he humpback chub (Gila cypha) 

is an endangered native fish 

found only in the Colorado River Basin. 

In Grand Canyon, most humpback chub 

are found in the Little Colorado River 

and its confluence with the Colorado 

River. For decades, however, nonnative 

rainbow trout (Oncorhynchus mykiss) 

and brown trout (Salmo trutta), which 

prey on and compete with native fish, 

have dominated the Grand Canyon fish 

community. Between 2003 and 2006, 

scientists with the U.S. Geological 

Survey and Arizona Game and Fish 

Department experimentally removed 

23,266 nonnative fish from a 9.4-milelong reach of the Colorado River near 

where it joins the Little Colorado River. 

During the experiment, rainbow trout 

were reduced by as much as 90% and 

native fish abundance apparently 

increased in the reach. Concurrent 

environmental changes and a decrease 

in rainbow trout throughout the river 

make it difficult to determine if the 

apparent increase in native fish was 

the result of the experiment.

Conservation of native fish is a key goal 

of the Glen Canyon Dam Adaptive Management Program, a Federal initiative to mitigate 

impacts from dam operations to downstream 

resources in Glen Canyon National Recreation 

Area and Grand Canyon National Park, Arizona. A primary focus of the program’s efforts 

is the endangered humpback chub (Gila cypha), a native fish found only in the Colorado 

River Basin. The Grand Canyon population 

of humpback chub is the largest of the six 

remaining populations of this endangered species. Most humpback chub in Grand Canyon 

are found in the Little Colorado River and its 

confluence with the Colorado River, likely 

because the Little Colorado River provides 

suitable spawning and rearing habitat.

For several decades, the Colorado River fish 

community in Grand Canyon has been dominated in terms of abundance by nonnative 

rainbow trout (Oncorhynchus mykiss) and 

brown trout (Salmo trutta). Rainbow and 

brown trout prey on juvenile native fish and 

also compete with native fish for available 

food and habitat.

Removal of Nonnative Fish

From January 2003 to August 2006, U.S. 

Geological Survey and Arizona Game and 

Fish Department scientists undertook an experiment to remove nonnative fish from the 

Colorado River near its confluence with the 

Little Colorado River. In addition to humpback chub, this area is used by other native 

fish—flannelmouth sucker (Catostomus 

latipinnis), bluehead sucker (Catostomus 

discobolus), and speckled dace (Rhinichthys osculus). The experiment’s goals were 

to evaluate the effectiveness of nonnative 

fish removal efforts in a large river like the 

Colorado River and to understand resulting 

changes in the fish community. 

The scientists conducted 23 trips to remove 

nonnative fish from a reach that extended from 

river mile (RM) 56.3 to RM 65.7, as measured 

downstream from Lees Ferry at RM 0. More 

than 36,500 native and nonnative fish were 

captured in this “removal reach” during the 

study period using “electrofishing,” a nonlethal 

capture method. Following capture, 23,266 nonnative fish were euthanized and 13,268 native 

fish were released alive back into the river. Additionally, a second sampling effort was made 

in an area of high rainbow trout density (RM 

44 to RM 52.1) upstream of the removal reach, 

but all captured fish were released alive back 

into the river. This “control reach” was used to 

determine whether changes in fish abundance 

detected in the removal reach were the result of 

conditions other than removal efforts. 

Findings

In 2003, nonnative fish represented more 

than 95% of fish caught in the removal reach, 

but after July 2005 nonnative fish were less 

than 50% of the catch. There were particularly 

In Grand Canyon, nonnative rainbow trout (Oncorhynchus mykiss; top right) prey on and compete with 

native fish, including the endangered humpback chub (Gila cypha, lower right). Between 2003 and 2006, U.S. 

Geological Survey and Arizona Game and Fish Department scientists experimentally removed nonnative fish 

from the Colorado River near its confluence with the Little Colorado River in the area called the “removal 

reach” (see map). The experiment’s “control reach” and the Lees Ferry reach, a rainbow trout sport fishery, 

are also shown on the map. RM, river mile. 

Paria

Riv er

Glen 

Canyon 

Dam

GLEN CANYON

NATIONAL RECREATION

AREA

Lees Ferry

Little Colorado Riv er

0 20 40 MILES

0 20 40 60 KILOMETERS

Colorado River

Lees Ferry Reach 

(RM -15.7–0)

Control Reach (RM 44–52.1)

Removal Reach (RM 56.3–65.7)

N

GRAND CANYON

NATIONAL PARK

ARIZONA

Map

Area

 Eric Engbretson

Arizona Game and Fish Department
Lewis G. Coggins, Jr., and Michael D. Yard

Edited by James W. Hendley II

Graphics and layout by Judy Weathers

For more information contact:

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research Center

Flagstaff, Arizona

(928) 556-7094

This Fact Sheet and any updates to it are available

online at http://pubs.usgs.gov/fs/2011/3093/

Management Implications

 The results of the experiment 

indicate that it was successful in 

reducing the abundance of nonnative fish, particularly rainbow 

trout, in the 9.4-mile-long removal 

reach. This outcome suggests that 

nonnative fish abundance can be 

reduced in a large segment of the 

Colorado River downstream from 

Glen Canyon Dam. However, 

reduced rainbow trout migration 

rates from the Lees Ferry reach 

and a decline in rainbow trout 

abundance throughout the river 

also contributed to the reduction. 

Removal efforts might not have 

been successful during a period 

of increasing rainbow trout abundance. If removal efforts had been less 

intensive (for example, one removal trip per 

year), then reductions in overall rainbow 

trout abundance would likely have been 

much smaller and possibly undetectable. 

Additionally, concurrent environmental 

changes and the decrease in rainbow trout 

throughout the river make it difficult to confirm that the apparent increase in native fish 

abundance was the result of the experiment. 

Monitoring efforts continue and may be 

able to distinguish among these factors and 

to better inform future management actions.

Reference

Coggins, L.G., Jr., Yard, M.D., and Pine, W.E., 

III, 2011, Nonnative fish control in the 

Colorado River in Grand Canyon, Arizona—An effective program or serendipitous timing?: Transactions of the American 

Fisheries Society, v. 140, no. 2, p. 456–470.

large catches of native flannelmouth sucker 

and humpback chub in September 2005, causing the percentage of nonnative fish to fall to 

a low of 20% that month. The contribution of 

rainbow trout to the catch fell throughout the 

study period, despite high net rainbow trout 

immigration rates during 2003–4, which were 

followed by migration rate declines in 2005–6. 

Rainbow trout decreased from approximately 

90% of the catch in 2003 to less than 10% in 

February 2006, before rebounding slightly in 

August 2006. Generally, the catch of nonnative fish in the removal reach fell throughout 

the study period; however, the number of nonnative cyprinids (fathead minnows and carp) 

caught increased in 2006.

In the control reach, a total of 11,221 fish 

were captured and released during the study 

period, 95% of which were rainbow trout. 

Throughout the study, rainbow trout numbers 

in the control reach declined, particularly after 

the spring of 2005. Rainbow trout numbers in 

the control reach were estimated to be between 

7,000 and 10,000 during 2003–4 and between 

2,000 and 5,000 during 2004–5, suggesting 

that rainbow trout abundance likely declined 

by 50% or more during the study period. This 

finding suggests that a decline in rainbow trout 

abundance thoughout the river occurred independently of removal efforts.

The decline of rainbow trout abundance 

in 2005–6 in the removal and control 

reaches mirrors a similar decline recorded 

for the Lees Ferry reach (RM -15.7 to RM 

0), a recreational trout fishery that is immediately downstream of Glen Canyon 

Dam and the likely source of rainbow trout 

throughout the river. Seasonal flooding of 

the Paria River, a Colorado River tributary just downstream of Lees Ferry, from 

September 2004 to January 2005 may also 

have contributed to the reduced numbers of 

rainbow trout detected downstream. Tributary sediment inputs limit light availability 

and can reduce the production of algae, 

thereby reducing food for the aquatic invertebrates that trout eat. High sediment loads 

likely not only reduced the food available 

to trout but also may limit the ability of 

trout to see their prey.

 Rainbow trout abundance declined in both 

the removal and control reaches but declined 

to a greater extent in the removal reach. Additionally, there is some indication that the 

abundance of juvenile native fish increased 

in the removal reach during the study period, 

suggesting increased survival rates, increased 

reproduction, or both. The most recent assessment of humpback chub data, which includes 

data from 1989 to 2008, shows that more 

juvenile fish survived to adulthood. This trend 

begins with fish spawned in 1998 and appears 

particularly strong for fish spawned in 2003. 

Although early signs that more humpback 

chub are surviving to adulthood are encouraging, it is not possible to know if this improvement is the result of removal efforts or other 

environmental factors. For example, the temperature of water released from Glen Canyon 

Dam increased during the study period to temperatures that may have improved humpback 

chub growth and survival.

Humpback chub 

 (Gila cypha)

Flannelmouth sucker 

 (Catostomus latipinnis)

Bluehead sucker 

 (Catostomus discobolus)

Speckled dace 

 (Rhinichthys osculus)

Brown trout 

 (Salmo trutta)

Rainbow trout 

0 (Oncorhynchus mykiss)

10

20

30

40

50

60

70

80

90

100

Percentage of fish captured

Native fish

Jan−03

Feb−03

Mar−03

Jul−03

Aug−03

Sep−03

Jan−04

Feb−04

Mar−04

Jul−04

Aug−04

Sep−04

Jan−05

Feb−05

Mar−05

Jul−05

Aug−05

Sep−05

Jan−06

Feb−06

Mar−06

Jul−06

Aug−06

Month and year, 2003–6

Nonnative fish

Other nonnative fish

Graph showing the percentages of native and nonnative fish species captured in the experimental removal 

reach during 23 trips that occurred between January 2003 and August 2006. Native fish were returned to the 

river unharmed during the experiment, and nonnative fish were euthanized.