High Diet Overlap between Native Small-Bodied Fishes and Nonnative Fathead Minnow in the Colorado River, Grand Canyon, Arizona

Abstract

River regulation may mediate the interactions among native and nonnative species, potentially favoring nonnative species and contributing to the decline of native populations. We examined food resource use and diet overlap among small-bodied fishes in the Grand Canyon section of the Colorado River as a first step in evaluating potential resource competition. We compared the diets of the predominant small-bodied fishes (native Speckled Dace Rhinichthys osculus, juvenile Flannelmouth Sucker Catostomus latipinnis, and juvenile Bluehead Sucker C. discobolus, and nonnative Fathead Minnow Pimephales promelas) across seasons at four sites downstream of Glen Canyon Dam using nonmetric multidimensional scaling and Schoener's similarity index. The diets of these fishes included diatoms, amorphous detritus, aquatic invertebrates (especially simuliid and chironomid larvae), terrestrial invertebrates, and terrestrial vegetation. Diets varied with season and were affected by high turbidity. Fish consumed more amorphous detritus and terrestrial vegetation during the summer monsoon season (July–September), when turbidity was higher. The diets of all species overlapped, but there was large variation in the degree of overlap. The diets of juvenile suckers and Fathead Minnows were most similar, while Speckled Dace had relatively distinct diets. The differences took the form of higher proportions of diatoms and amorphous detritus in the diets of Bluehead Suckers and Fathead Minnows and higher proportions of simuliids and chironomids in those of Speckled Dace. If food resources are or become limiting, diet overlap suggests that competition may occur among native and nonnative species, which could have implications for the population dynamics of these fishes and for the management of the Colorado River ecosystem in Grand Canyon.

Received September 4, 2013; accepted February 11, 2014

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ACKNOWLEDGMENTS

We thank many people for laboratory and field assistance, especially A. Adams, A. Aubeneau, K. Behn, N. Bender, A. Copp, K. Donner, J. Kampman, H. Kelly, D. Kincaid, and M. Schroer. We thank Carol Fritzinger, Mike Yard, Brian Dierker, and Humphrey's Summit support staff for the excellent logistical support and river knowledge they provided. This work was funded by the Bureau of Reclamation's Glen Canyon Dam Adaptive Management Program through U.S. Geological Survey cooperative agreement 05WRAG005 to R.O.H. and E.R.M. and through a fellowship to S.E.Z.S. from the Loyola University of Chicago Graduate School. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.