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398 The Southwestern Naturalist vol. 38, no. 4 opened, and the contents identified under a stereo microscope. A total of 466 fish was examined, including 184 red shiner (36-79 mm TL), 47 sand shiner (30-65),42 fathead minnow (32-60), 176 redside shiner (36-77), and 17 channel catfish (51-144). Fish larvae were found in 15% of the red shiner, always in the foregut region; no larvae were de- tected in digestive tracts of other species. The temporal distribution of fish larvae in red shiner was as follows (parentheses show number of red shiner examined, number with fish larvae, and number of fish larvae per digestive tract): 30 June, 1600-1800 h (35, 13, 1-6); 2 July, 1200-1400 h (20, 3, 2), 1600-1800 h (19, 2, 1-9), 2000- 2200 h (11, 7,1-7); 14 July, 1200-1400 h (6,1, 1),1600-1800 h (17, 2,1-3). One to 17 red shiner (X = 5.1) were selected for examination from collections made during the remaining sampling times. Of the 58 fish larvae found, most were too digested for species identification or accurate length measurements, but all were cypriniforms (mostly catostomids) and probably S 15 mm TL. Seven fish larvae, 11-13 mm TL, were identified as bluehead sucker, five from red shiner collected at 1600-1800 h on 30 June and one each from red shiner collected at 1200-1400 hand 2000- 2200 h on 2 July. Insects, including adult or immature caddisflies, mayflies, beetles, and water striders, were the principal food items found in digestive tracts of all fishes except fathead min- now, which contained mostly algae and organic debris. Larval fishes were present in embayments dur- ing each sampling period throughout our study. Those collected and identified included roundtail chub, speckled dace, and bluehead sucker. How- ever, fish larvae were found only in red shiner collected during daylight or dusk on 30 June and 2 and 14 July, and their frequency of occurrence was 37% on 30 June, 20% on 2 July, and 8% on 14 July. Minckley (1973) reported that red shin- er may be piscivorous, but we only found one published account of this species eating fish lar- vae. Jennings and Saiki (1990) reported that of 79 digestive tracts from red shiner collected in the San Joaquin River drainage, only one con- tained cypriniform larvae. The high incidence of fish larvae ingested by red shiner suggests that this species may be an important predator on native fishes in the Col- orado River system. Red ,>iner is one of the most abundant and widespread nonnative fish species in the system. Because red shiner occupy nursery habitats used by young of native fishes, including Colorado squawfish and razorback sucker, pre- dation by red shiner may have a significant effect on survival of larval fish. Also, red shiner has been described as an opportunistic drift feeder (Sublette, 1975), and therefore may prey on drift- ing fish larvae. One possible explanation for fail- ure of previous diet studies to produce evidence of such predator-prey interactions in backwaters is that preferred invertebrate forage was suffi- ciently abundant to reduce the degree of piscivory below detectable limits. Because water levels in shoreline embayments fluctuate greatly with small changes in river discharge (resulting in periods of dewatering or flushing and substrate scouring during flooding), abundance of invertebrate for- age in this type of nursery habitat may be low, especially in spring-early summer when changes in river discharge are greatest and densities of insects in low-velocity habitats are inherently lower (L. W. Crist and S. D. Hiebert, U.S. Bu- reau of Reclamation, pers. comm.). In laboratory tests, presence of alternative invertebrate prey re- duced the rate of predation on Colorado squaw- fish and razorback sucker larvae by nonnative fishes (R. T. Muth, pers. obser.). Early larvae of Colorado squawfish were cap- tured in the main channel of the Yampa River at the upstream end of our study area during 7- 30 July 1991 (K. R. Bestgen, Colorado State University, pers. comm.), but none was collected in embayments we sampled. Our study began at least a few weeks after the suspected spawning season of razorback sucker in the upper Colorado River system (April-June; Tyus and Karp, 1989). Young razorback sucker produced at the lower Yampa spawning site may have been transported beyond our study area or were sufficiently large (e.g., >20 mm TL; R. T. Muth, pers. obser.) to avoid predation by smaller nonnative fishes. However, if predation by red shiner on fish larvae is in part a function of availability of alternative prey, and if abundance of preferred invertebrate forage is lower during spring and early summer in at least some nursery habitats (e.g., shoreline embayments), early larvae of razorback sucker may be especially vulnerable. Effective assess- ment of the extent and potential impacts of pre- dation by nonnative fishes on larvae of native fishes req uires better investigation of spatio-tem- poral patterns in distribution and abundance of fish larvae and invertebrate forage and the sea- ~ liiI .'