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<br />ABSTRACT <br /> <br />We compared gut contents of young-of-year Colorado squawfish Ptvchocheilus <br />lucius. an endangered cyprinid, with that of other fish < 75 mm total length (TL) collected <br />from backwaters of the Green River, river kilometers 555 to 35 in Colorado and Utah, during <br />summer and fall 1987. Species included native Rhinichthvs osculus, Catostomus <br />discobolus, and C. latipinnis and nonnative Cvprinella lutrensis, Notropis stramineus, <br />Pimephales promelas, Ictalurus punctatus, and Lepomis cvanellus. For each species, diet <br />varied with size interval and between upper and lower river reaches but not between <br />seasons for fish of similar size. Immature dipterans, particularly chironomid larvae, were the <br />principal foods of most fishes. Cladocerans and especially cope pods were particularly <br />important in diets of .1:. lucius < 21 mm TL and 1. cvanellus < 31 mm TL. Catostomus <br />discobolus was the only species to eat moderate amounts of algae. Fish (all larvae) were <br />found only in digestive tracts of 10 P. lucius (21-73 mm TL), representing about 1 % of the <br />P. lucius analyzed. Degree of diet overlap between .1:. lucius and other small fish was <br />influenced mainly by zooplankton and especially immature dipterans. High overlap occurred <br />in some comparisons with C. lutrensis, R. osculus. C. latipinnis, b. cyanellus, and <br />particularly 1. punctatus. Potential for food competition between young-of-year ,e. lucius and <br />other fishes in backwaters appeared greatest with the very abundant C. lutrensis. <br /> <br />Key Words: Ptvchocheilus lucius, nonnative fishes, young-of-year, diets, diet overlap, <br />backwaters, Green River <br /> <br />2 <br />