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Fact Sheet 2013–3039

Native and Nonnative Fish Populations of the Colorado River are Food Limited—Evidence from New Food Web Analyses

By Theodore A. Kennedy, Wyatt F. Cross, Robert O. Hall, Jr., Colden V. Baxter, and Emma J. Rosi-Marshall

Thumbnail of and link to report PDF (2.7 MB)Summary

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.

First posted June 28, 2013

For additional information contact:
GCMRC Staff, Southwest Biological Science Center
U.S. Geological Survey
Grand Canyon Monitoring and Research Center
2255 N. Gemini Drive
Flagstaff, AZ 86001

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Suggested citation:

Kennedy, T.A., Cross, W.F., Hall, R.O., Jr., Baxter, C.V., and Rosi-Marshall, E.J., 2013, Native and nonnative fish populations of the Colorado River are food limited—evidence from new food web analyses: U.S. Geological Survey Fact Sheet 2013–3039, 4 p.,

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