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100 GREAT BASIN NATURALIST [Volume 55 (many identified as Cyclops) represented most zooplankton found in digestive tracts. Identified genera of rotifers included Brachionus, Cephalodella, Keratella, Lecane, Monostyla, Polyarthra, and Trichocerca. Proportion of zooplankton in diets of all fishes tended to decrease with increasing fish length. Bluehead sucker was the only species that ate moderate amounts of algae (10-30% of food volume); other fishes consumed minor amounts. Algae consisted mostly of six diatom genera (Cymbella, Fragilaria, Gyrosigma, Navicula, Surirella, and Synedra), one desmid genus (Closterium), and, to a lesser extent, Pediastrum (a colonial green alga). Most diges- tive tracts contained debris that accounted for moderate or large proportions of gut contents (>30%) in all fishes except speckled dace and green sunfish. It was over 80% of gut content in fathead minnow and bluehead sucker. Debris consisted of fibrous particles of vascular plant tissue usually mixed with large amounts of clay particles and sand grains, suggesting bot- tom feeding. Seeds (many identified as tama- risk [Tamarix gallica]) were eaten by all fishes, especially red shiner <31 mm TL. Two observations were unique to Colorado squawfish. Fish larvae were found in digestive tracts of 10 Colorado squawfish (about 1% of total examined); 1 was 21 mm TL, 8 were 36-48 mm TL, and 1 was 73 mm TL (probably a yearling). No fish were detected in digestive tracts of other species. Of the 18 fish larvae found, most were too digested for species identification or accurate length measurement, but all were cypriniforms (mostly cyprinids) and probably <- 10 mm TL. Six fish larvae (6-9 mm TL) were identified as red shiner, and one (about 8 mm TL) as fathead minnow. Interest- ingly, the smallest Colorado squawfish had four prey fish (all red shiner), whereas only one or two fish were found in digestive tracts of the others. Gut contents of six Colorado squawfish, 36-48 mm TL, and the 73-mm-TL specimen were exclusively fish; those for the remaining specimens were 70-80% fish. Digestive tracts of six Colorado squawfish contained 2-6 cestode parasites (probably Proteocephalus ptychocheilus; Flagg 1982); cestodes were not found in guts of other fish- es. Colorado squawfish infested with cestodes were larger than 27 mm TL and were collect- ed from both river reaches in autumn. Diet Overlap Degree of diet overlap between YOY Colo- rado squawfish and other fishes was influenced mainly by zooplankton and especially imma- ture dipterans (Table 2). Within each reach, diet overlap for all length intervals of Colorado squawfish generally decreased as lengths of other species increased. Degree of diet overlap among fish of similar size was generally greater in the lower than upper reach. Overlap values were <0.60 (range = 0.10-0.59) for most comparisons; generally, values were lowest for comparisons with fat- head minnow and bluehead sucker (range = 0.10-0.44). Biologically important overlap (values >0.60) occurred only between Colorado squawfish >10 mm TL and some size-reach groups of native speckled dace and flannelmouth sucker and nonnative red shiner, green sunfish, and especially channel catfish. These higher overlap values were primarily attributed to high proportions of larval chi- ronomids in diets and, secondarily, especially for diet overlap with green sunfish >40 mm TL (upper reach) and 21-30 mm TL (lower reach), to proportions of zooplankton. Degree of diet overlap was greatest with channel cat- fish and green sunfish. DISCUSSION Comparisons among food-habits investiga- tions are difficult because of differences in study design, location, and season. However, our observations on diets of native and nonna- tive fishes in backwaters of the Green River generally agree with results of prior studies in the upper Colorado River basin (e.g., Vanicek and Kramer 1969, Jacobi and Jacobi 1982, McAda and Tyus 1984) and reported food habits of the nonnative species within their native ranges (e.g., Carlander 1969, 1977, Pflieger 1975, Harlan et al. 1987). Larger YOY or yearling red shiner, sand shiner, speckled dace, flannelmouth sucker, channel catfish, and green sunfish eat mainly immature aquatic insects. Diets of larger YOY or yearling fathead minnow and bluehead sucker consist mostly of algae and organic debris. Diet of YOY Colo- rado squawfish consists primarily of zooplank- ton and immature insects (especially chirono- mid larvae) and occasionally includes fish. Reported size at which wild Colorado squawfish shift to a more piscivorous diet