Native and Nonnative Fish Populations of the Colorado River 

are Food Limited—Evidence from New Food Web Analyses

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3039

June 2013

Fish populations in the Colorado River downstream from Glen 

Canyon Dam appear to be limited by the 

availability of high-quality invertebrate 

prey. Midge and blackfly production 

is low and nonnative rainbow trout 

in Glen Canyon and native fishes in 

Grand Canyon consume virtually all of 

the midge and blackfly biomass that is 

produced annually. In Glen Canyon, the 

invertebrate assemblage is dominated 

by nonnative New Zealand mudsnails, 

the food web has a simple structure, 

and transfers of energy from the base 

of the web (algae) to the top of the 

web (rainbow trout) are inefficient. 

The food webs in Grand Canyon are 

more complex relative to Glen Canyon, 

because, on average, each species in the 

web is involved in more interactions and 

feeding connections. Based on theory 

and on studies from other ecosystems, 

the structure and organization of Grand 

Canyon food webs should make them 

more stable and less susceptible to large 

changes following perturbations of the 

flow regime relative to food webs in Glen 

Canyon. In support of this hypothesis, 

Grand Canyon food webs were much 

less affected by a 2008 controlled flood 

relative to the food web in Glen Canyon.

Background 

Between Glen Canyon Dam and Lake 

Mead, the Colorado River flows through 

Glen Canyon, Marble Canyon, and Grand 

Canyon. Immediately downstream from the 

dam in Glen Canyon, the Colorado River is 

characterized by cold water and almost no 

suspended sediment, whereas Marble and 

Grand Canyons are progressively warmer 

and more sediment laden (fig. 1). 

Food is a resource that often limits 

the distribution or abundance of animal 

populations. In streams and rivers, 

aquatic invertebrates are an important 

food resource for fish populations. Thus, 

to identifythe likely causes of Colorado

River fish population response to adaptive 

management experimentation, it is critical 

consumption by fish to understand the 

efficiency of energy transfer within the 

food web. Fish consumption was calculated 

from measurements of the food contents 

in the stomachs of fish, and annual 

consumption estimates were developed 

using bioenergetics modeling. Here, we 

summarize the results of our food web 

analyses, which were conducted between 

2006 and 2009 and were recently reported 

in the peer-reviewed literature. 

Previous Research 

Early research on food webs in Glen, 

Marble, and Grand Canyons described 

spatial and temporal trends in the dominant 

species of algae and invertebrates. 

Other studies linked those trends to 

characteristics of the nonnative rainbow 

trout (Oncorhynchus mykiss) population in 

Glen Canyon. However, those studies did 

not evaluate the response of the food web 

to significant management actions such as 

controlled flood releases, administratively 

referred to as high flow experiments. 

Although the feeding habits of nonnative 

to also describe responses by invertebrate 

prey resources. Food webs depict the 

flow of energy through an ecosystem by 

mapping who eats whom and in what 

quantity and, as such, they are a useful tool 

for investigating fundamental biological 

linkages in the Colorado River. 

Food webs can be quantified by 

expressing all components of the web in 

a common currency: biomass produced 

and biomass consumed. Invertebrate 

production, for example, is the creation 

of new invertebrate biomass over time, 

and describes how much energy is 

available for consumption by fish. When 

conditions are favorable for invertebrates, 

their biomass and growth rates increase, 

and production estimates integrate both 

of these changes into a single metric. 

Because we wanted to resolve the degree 

to which fish populations in the Colorado 

River were food limited, we estimated 

consumption of invertebrates by fish in 

the same measurement units—biomass 

consumed. Thus, we could directly 

compare estimates of invertebrate 

production with estimates of invertebrate 

As one of the most carefully managed river systems in the world, the aquatic ecosystem of the Colorado 

River in the Grand Canyon has been heavily influenced by Glen Canyon Dam and the decades of 

controlled release of water for power generation. Photo by Robert O. Hall, Jr., used with permission.
rainbow trout and native humpback chub 

(Gila cypha) were well described prior 

to our studies, native suckers (Family 

Catostomidae) dominate downstream 

reaches in Grand Canyon, and those food 

webs had not been described. Additionally, 

early food web studies focused on the 

use of simple metrics of food availability 

such as density or biomass of algae and 

invertebrates, and those metrics could not 

be directly compared with estimates of fish 

consumption. Thus, the food web analyses 

we conducted represented an essential next 

step in Colorado River research. 

Recent Major Findings 

In Glen Canyon, the diversity of 

animals is low, nonnative species dominate 

the community, the food web structure 

is simple, and transfers of energy from 

the base of the web (algae) to the top of 

the web (rainbow trout) are extremely 

inefficient. Microscopic algae (diatoms) 

produced in Glen Canyon fuel more than 

Paria River

RM30

Rainbow trout

Midge and blackfly

Flannelmouth sucker

RM63

RM127

RM165

RM225

225 165 127 30 0

RM0

Lake

Mead

Lake

Powell

37°

36°

114° 113° 112° 111°

Diamond

Creek

Havasu

Creek

Little

Colorado

River

Kanab

Creek

Colorado River

Glen Canyon Dam Glen Canyon Dam

63

0 10 20 30

0 25 50 KILOMETERS

MILES

Fish production (g AFDM m-2 yr-1) Fish

production

River mile

Invertebrate

production

Invertebrate production (g AFDM m-2 yr-1) 

RM225 RM165 RM127 RM63 RM30

0

0.1

0.2

0.3

0.4

0.5

Sampling sites

Increasing

turbidity with 

downstream flow Fish

Invertebrate

RM0

0

5

10

15

20

25

30

35

Rainbow trout

Flannelmouth 

sucker

Humpback chub

Other

Mudsnails

Scuds

Sludgeworms

Earthworms

Blackflies

Midges

Caddisflies 

Other

Fish

Invertebrate

Nevada Utah

Arizona

70 percent of invertebrate production there, 

but none of the invertebrates consume the 

large beds of algae that dominate riverbottom habitats (Wellard Kelly and others, 

2013); thus, there is a large surplus and 

net export of filamentous algae from this 

segment of the river. Although invertebrate 

production in Glen Canyon is high, the most 

productive species are not important food 

items for rainbow trout, which constitute 

95 percent of total fish production in this 

segment of the river (fig. 1). Nonnative 

New Zealand mudsnails (Potamopyrgus 

antipodarum) constituted nearly half 

of the available invertebrate production 

prior to the 2008 controlled flood, and 

this species of invertebrate cannot be 

effectively digested by rainbow trout 

(Cross and others, 2010; Cross and others, 

2011). Blackflies (Family Simuliidae) and 

midges (Family Chironomidae) supported 

more than half of rainbow trout production 

in Glen Canyon. Although these insects 

represent only 1 to 10 percent of total 

invertebrate production, they are among 

the few prey species in Glen Canyon 

readily available to rainbow trout. The 

2008 controlled flood caused a 60 percent 

decline in overall invertebrate production 

that was driven by a large reduction in the 

production of mudsnails (Cross and others, 

2011). However, the production of midges 

and blackflies increased by 30 and 200 

percent, respectively, in the year following 

the controlled flood, and these insects 

supported a 200 percent increase in rainbow 

trout production (Cross and others, 2011). 

Algae production decreases from 

Glen to Marble Canyon and is even lower 

in Grand Canyon (Hall and others, 2010), 

because the higher suspended sediment 

load reduces light availability. Invertebrate 

production also decreases downstream 

from Glen Canyon, because the snails and 

scuds that dominate production in Glen 

Canyon are rare downstream (Cross and 

others, in press). Midges and blackflies 

dominate invertebrate production in 

Marble and Grand Canyons (fig. 1), and 

cobble bars are “hotspots” for midge 

Figure 1. Species 

contributing to fish and 

invertebrate production change 

with distance downstream 

from Glen Canyon Dam, as 

shown by the pie charts for six 

sampling sites in Glen, Marble, 

and Grand Canyons. Total 

production (shown in the graph) 

also changes with distance 

downstream and is related to 

the sediment load (turbidity) 

of the water. Sediment load 

increases seasonally due to 

flood-derived inputs from key 

tributaries including the Paria 

River, Little Colorado River, and 

Kanab Creek. Error bars on 

graph represent the 95-percent 

confidence interval of the 

average production estimate; 

where bars are not shown, 

errors are smaller than size of 

data point. AFDM stands for 

“ash-free dry mass,” a measure 

of the biological production 

exclusive of inorganic material 

such as bones and shells; it is 

given in units of grams AFDM 

per square meter per year. RM, 

river mile. 
and blackfly production. Production of 

these two insects in cobble bars is 2–10 

times higher than in other habitat types. 

Humpback chub are abundant immediately 

downstream from the Little Colorado River, 

constituting around 20 percent of total 

fish production; however, flannelmouth 

suckers (Catostomus latipinnis), which 

are omnivores and consume invertebrates, 

algae, and detritus (Zahn-Seegert, 2010; 

Donner, 2011), represent 95 percent of fish 

production at reaches farther downstream 

(fig. 1). 

Food web analyses also indicate that 

native fish production and distribution 

in Grand Canyon may be limited by the 

availability of high-quality invertebrate 

prey such as midges and blackflies because 

native fishes in Grand Canyon consume 

nearly all of the available midge and 

blackfly production annually. However, 

total fish production does not appreciably 

change with distance downstream from 

Glen Canyon Dam (fig. 1), because native 

fishes also make use of noninvertebrate 

foods such as algae and detritus (Cross and 

others, in press). 

The food webs in Grand Canyon 

are more complex relative to Glen 

Canyon because each species in the web 

is involved in more feeding connections 

(fig. 2). Based on theory and studies from 

other ecosystems, all of these attributes—

complexity, incorporation of detritus into 

the web, and dominance by omnivores—

indicate that Grand Canyon food webs 

should be more stable and less susceptible 

to large changes following perturbations 

of the flow regime than is the case in Glen 

Canyon; in support of this hypothesis, 

Grand Canyon food webs were much 

less affected by the 2008 controlled flood 

relative to the food web in Glen Canyon 

(Cross and others, in press; Rosi-Marshall 

and others, 2010). 

Management Implications 

These food-web studies help inform 

management of the Colorado River 

ecosystem by identifying the likely 

causes of fish-population response to 

adaptive management experimentation. 

In the absence of controlled floods, the 

invertebrate assemblage in Glen Canyon 

would likely be dominated by nonnative 

New Zealand mudsnails that do not 

sustain the rainbow trout fishery. Our 

studies indicate that future controlled 

floods should favor insects such as midges 

and blackflies that are important prey for 

rainbow trout (Cross and others, 2011). 

The direct effects of controlled floods 

released from Glen Canyon Dam on 

food webs and invertebrate populations 

further downstream in Marble and Grand 

Canyons appear minimal; however, floods 

may indirectly affect food webs in Marble 

Canyon, because the large cohorts of 

rainbow trout produced in Glen Canyon 

migrate downstream and prey upon 

humpback chub and other native fish. 

Low invertebrate production in the 

Colorado River may be responsible for 

several undesirable food web properties. 

The low availability of invertebrate prey 

likely limits the growth of large rainbow 

trout. Low invertebrate production may 

also limit the abundance and distribution 

of humpback chub and may be responsible 

for the dominance of flannelmouth 

suckers in the native fish assemblage, 

because flannelmouth suckers can make 

efficient use of low-quality food items 

(algae and detritus) that are not accessible 

to humpback chub. Low invertebrate 

production in Grand Canyon may also 

limit the success of efforts to reintroduce 

native fishes that have been extirpated 

from Grand Canyon, such as the razorback 

sucker (Xyrauchen texanus) and Colorado 

pikeminnow (Ptychocheilus lucius). 

An investment portfolio of stocks 

is a useful metaphor for the food web; 

an ideal investment portfolio is one that 

contains a diversity of stocks that can 

grow the account over variable economic 

conditions. Similarly, riverine food webs 

tend to be most efficient and productive 

when there is a variety of invertebrate prey 

resources available such that fish growth 

and production can occur over variable 

Figure 2. Food web diagrams depicting consumption by invertebrates and fish in the Colorado River through Glen Canyon and Grand Canyon. 

Line widths correspond to the magnitude of consumption. Food web complexity, or the number of connections in a web, increases with distance 

from Glen Canyon Dam for two main reasons: (1) there are more species of fish in downstream assemblages (see top row of interactions in each 

panel) and (2) invertebrates are consuming both algae-based and detritus-based resources at downstream sites (bottom row of interactions in 

each panel). Because biomass consumption by fishes was much lower in magnitude than by invertebrates, consumption by fishes in Glen Canyon 

was multiplied by 50, and consumption by fish in Grand Canyon was multiplied by 5, for purposes of visualization. RM, river mile.

algae-based detritus-based

algae-based detritus-based

RM30

RM0

RM60

RM125

37°

36°

112°

Paria

River

Little Colorado

River Colorado River

Glen Canyon

Dam

Glen Canyon

Dam

0 5 10 15

0 25 KILOMETERS

MILES

Energy Flows

fish

invertebrate

organic matter
environmental conditions. Because the 

portfolio of invertebrate “stocks” in the 

Colorado River is extremely limited relative 

to unregulated rivers, the overall production 

of the Colorado River’s investment 

portfolio of invertebrates is also limited, as 

is the production of the fish populations that 

depend on these invertebrates. 

Next Steps 

A current goal of the Glen Canyon 

Dam Adaptive Management Program 

(GCDAMP) is to manage food webs to 

support the biodiversity of native fish 

species, while limiting the negative effects 

of nonnative sport fish like rainbow trout. 

One of the benefits of describing food webs 

in detail is that key species interactions 

are identified; managers can use this 

information to alter the food web to achieve 

these management goals. 

Fish eat most of the midge and 

blackfly biomass that is produced annually 

throughout Glen, Marble, and Grand 

Canyons. Thus, management actions that 

alter the production of these aquatic insects, 

such as controlled flood releases, could lead 

to changes in fish production; however, 

controlled flood releases only appear to 

increase midge, blackfly, and by extension, 

fish production in Glen Canyon. If increased 

native fish production is desired in Marble 

and Grand Canyons, other management 

actions could be considered. For example, 

hydroelectric power generation from Glen 

Canyon Dam causes large daily changes to 

the Colorado River’s discharge and lowers 

algae production relative to more stable 

discharges (Robert Hall, Jr., and others, 

unpub. data, 2013). Thus, stabilizing the 

discharge regime could lead to increased 

algae production at downstream sites, 

which may in turn have positive effects on 

invertebrate and fish production. 

The GCDAMP could also attempt 

to increase native fish production by 

increasing the number of species in 

the invertebrate assemblage, which is 

analogous to increasing the number of 

stocks in the investment portfolio. One 

option that may help achieve this goal is 

manipulation of the river’s seasonal water 

temperature regime. Aquatic insects are 

very sensitive to temperature; for example, 

egg hatching and emergence of adult 

insects is often triggered by low and high 

temperature extremes, respectively. The 

water temperatures of the Colorado River 

downstream from Glen Canyon Dam 

lack strong seasonal variability, which 

may prevent successful colonization by 

native insects that historically lived in this 

river. River temperatures may be different 

in the future because of installation of a 

selective withdrawal structure that allows 

warmer surface water from Lake Powell 

to be released, experimental flow regimes 

that both maximize summer warming 

and winter cooling, or climate-driven 

changes in the temperature of water 

released from Lake Powell. Changes in 

the river’s seasonal temperature regime 

may allow other regional invertebrate 

species to colonize the Colorado River. For 

example, many species of mayflies (Order 

Ephemeroptera) and caddisflies (Order 

Trichoptera) are abundant and productive 

in tributaries, but are conspicuously absent 

from Glen, Marble, and Grand Canyon. 

Concerns exist, however, that warmer 

water in Grand Canyon could inadvertently 

benefit nonnative warm-water fishes that 

may compete with or eat native fishes. 

If direct temperature manipulation is not 

an option, the GCDAMP could consider 

actively introducing native species of 

invertebrates that persist in other parts of 

the Colorado River that also have altered 

seasonal temperature regimes. 

Regardless of the future direction of 

adaptive management experimentation in 

the Colorado River downstream from Glen 

Canyon Dam, food web investigations 

like those described here will help inform 

these experiments by identifying the causal 

pathways underlying changes in fish 

populations. 

Donner, K.C., 2011, Trophic basis of production 

of fishes in the Colorado River, Grand Canyon—

an assessment of potential competition for food: 

Pocatello, Idaho State University, M.S. thesis.

Hall, R.O., Jr., Kennedy, T.A., Rosi Marshall, E.J., 

Cross, W.F., Wellard, H.A., and Baxter, C.F., 

2010, Aquatic production and carbon flow in 

the Colorado River, in Melis, T.S., Hamill, J.F., 

Bennett, G.E., Coggins, L.G., Jr., Grams, P.E., 

Kennedy, T.A., Kubly, D.M., and Ralston, B.E., 

eds., Proceedings of the Colorado River Basin 

Science and Resource Management Symposium, 

November 18–20, 2008, Scottsdale, Arizona: 

U.S. Geological Survey Scientific Investigations 

Report 2010-5135, 105–112 p., http://pubs.usgs.

gov/sir/2010/5135/.

Wellard Kelly, H.A., Rosi-Marshall, E.J., 

Kennedy, T.A., Hall, R.O., Cross, W.F., and 

Baxter, C.V., 2013, Macroinvertebrate diets 

reflect longitudinal and seasonal changes in 

food availability downstream of a large dam: 

Freshwater Science, v.32, no. 2, p. 397–410,

http://dx.doi.org/10.1899/12-088.1.

Rosi-Marshall, E.J., Kennedy, T.A., Kincaid, 

D.W., Cross, W.F., Kelly, H.A.W., Behn. K.A., 

White, T., Hall, R.O., Jr., and Baxter, C.V., 

2010, Short-term effects of the 2008 highflow experiment on macroinvertebrates in the 

Colorado River below Glen Canyon Dam, 

Arizona: U.S. Geological Survey Open-File 

Report 2010–1031, 28 p., http://pubs.usgs.gov/

of/2010/1031/.

Zahn-Seegert, S.E., 2010, Diet overlap and 

competition among native and non-native smallbodied fishes in the Colorado River, Grand 

Canyon, Arizona: Chicago, Loyola University, 

M.S. thesis, 70 p.

Theodore A. Kennedy1, Wyatt F. Cross2, 

Robert O. Hall, Jr.3

, Colden V. Baxter4, 

and Emma J. Rosi-Marshall5

1

U.S. Geological Survey

2

Montana State University

3

University of Wyoming

4

Idaho State University

5

Cary Institute of Ecosystem Studies

Edited by Claire M. Landowski

Graphic design by Jeanne S. DiLeo

References Cited

Cross, W.F., Baxter, C.V., Donner, K.C., RosiMarshall, E.J., Kennedy, T.A., Hall, R.O., 

Jr., Wellard-Kelly, H.A., and Rogers, R.S., 

2011, Ecosystem ecology meets adaptive 

management—food web response to a 

controlled flood on the Colorado River, Glen 

Canyon: Ecological Applications, v. 21, no. 6, 

p. 2016–2033, http://www.esajournals.org/doi/

abs/10.1890/10-1719.1.

Cross, W.F., Baxter, C.V., Rosi-Marshall, E.J., Hall, 

R.O., Jr., Kennedy, T.A., Donner, K.C., Wellard 

Kelly, H.A., Seegert, S.E.Z., Behn, K., and Yard, 

M.D., in press, Foodweb dynamics in a large river 

discontinuum: Ecological Monographs (online), 

http://dx.doi.org/10.1890/12-1727.1.

Cross, W.F., Rosi-Marshall, E.J., Behn, K.E., 

Kennedy, T.A., Hall, R.O., Fuller, A.E., and 

Baxter, C.V., 2010, Invasion and production of 

New Zealand mud snails in the Colorado River, 

Glen Canyon: Biological Invasions, v. 12, 

no.9, p. 3033–3043, doi: 10.1007/s10530-010-

9694-y, http://www.springerlink.com/content/

bv834031865h2077/.

For more information, contact:

U.S. Geological Survey

Southwest Biological Science Center,

Grand Canyon Monitoring and Research 

Center, Flagstaff Arizona

http://www.gcmrc.gov

This Fact Sheet and any updates to it are 

available online at:

http://pubs.usgs.gov/fs/2013/3039