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V"- 51?yJ - 07 The Great Basin Naturalist PUBLISHED AT PROVO, UTAH, BY BRIGHAM YOUNG UNIVERSITY ISSN 0017-3614 VOLUME 55 30 APRIL 1995 No. 2 Great Basin Naturalist 55(2), © 1995, pp. 95-104 DIETS OF YOUNG COLORADO,SQUAWFISH AND OTHER SMALL FISH IN BACKWATERS OF THE GREEN RIVER, COLORADO AND UTAH Robert T. Muthl and Darrel E. Snyderl ABSTRACT.-We compared diet of young-of-year Colorado squawfish (Ptychocheilus Lucius), an endangered cyprinid, with diets of other fish <75 mm total length (TL) collected from backwaters of the Green River between river kilome- ters 555 and 35 during summer and autumn 1987. Species included native Rhinichthys osculus, Catostamus discobolus, and C. latipinnis, and nonnative Cyprinella lutrensis, Notropis stramineus, Pimepholes promelas, Ictalums punetatus, and Lepornis cyanellus. For each species, diet varied with size and between upper and lower river reaches but not between seasons for fish of similar size. Larval chironomids and ceratopogonids were principal foods of most fishes. Copepods and eladocerans were important in diets of P. lacius <21 mm TL and L. cyanellus <31 mm TL. Catostornus discobolus was the only species that ate moderate amounts of algae. Fish (all larvae) were in digestive tracts of only 10 P. lucius (21-73 mm TL), about 1% of P. lucius analyzed. High diet overlap occurred between some size-reach groups of P. lucius and C. lutrensis, R. osculus, C. latipinnis, 1. punctatus, and L. cyanellus. Potential for food competition between young- of-year P. lucius and other fishes in backwaters appeared greatest with the very abundant C. lutrensis. Key words: Ptyehoeheilus Lucius, Cyprinella lutrensis, nonnative fishes, young-of-year, diets, diet overlap, backwaters, Green River. Wild populations of federally endangered Colorado squawfish (Ptychocheilus Lucius) per- sist only in the upper Colorado River basin. They are most abundant in the Green and Yampa rivers of eastern Utah and northwest- ern Colorado (Tyus 1991a). Decline of this and other native fishes in the Colorado River basin has been attributed to habitat alterations caused by water development and introduc- tion and proliferation of nonnative fishes (Carlson and Muth 1989, Minckley 1991). Backwaters of the Green River below its confluence with the Yampa River are impor- tant nursery areas for young-of-year (YOY) Colorado squawfish (Nesler et al. 1988, Haines and Tyus 1990, Tyus and Haines 1991). Ichthyo- fauna of these backwaters is dominated by nonnative fishes, especially red shiner (Cypri- nella lutrensis; Tyus et al. 1982, Haines and Tyus 1990). This observation has led to a hy- pothesis that nonnative fishes adversely affect survival of young Colorado squawfish through competition or predation. Stanford (1993) sug- gested that strong food-web interactions be- tween native and nonnative fishes probably occur, but dietary relationships have not been adequately documented (Haines and Tyus 1990, Ruppert et al. 1993). Our objectives 1Larval Fish Laboratory, Department o£Fishery and Wildlife Biology, Colorado State University, Fort Collins, CO 60523. 95