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Larval Diet, Growth, and Survival Larval razorback suckers consume most of their yolk and begin exogenous feeding on planktonic or benthic organisms by 10-11 mm TL (Minckley and Gustafson 1982; Marsh and Langhorst 1988; Papoulias and Minckley 1990; Snyder and Muth 1990; USFWS 1997), We found that the diet of razorback sucker larvae 11-18 mm TL caught by light traps in nursery habitats of the Green River consisted mainly of small chironomid larvae, supplemented by zooplankton (mostly small cladocerans and rotifers) and algae (e,g., diatoms), particularly in fish smaller than 14 mm TL. Our description of diet might be confounded because light traps also attract and collect free-swimming invertebrates (Ervin and Haines 1972; Mueller et al. 1993), and we do not know the length of time razorback sucker larvae were in the light traps before retrieval or the evacuation rates of digestive tracts of larval razorback suckers. However, most of the invertebrates captured by the light traps were corixids or larger cladocerans and usually in low numbers. Further, similar to our observ~tions, hatchery-produced razorback sucker larvae recaptured by seines 1 week after stocking in a backwater ofthe Salt River, Arizona, had consumed primarily larval chironomids (Bestgen 1990). This dietary pattern likely indicates opportunistic feeding because chironomids are among the more common benthic invertebrates in quiet-water soft-sediment riverine habitats ofthe Colorado River basin (Ward et al. 1986; Grabowski and Hiebert 1989; Muth and Snyder 1995; Wolz and Shiozawa 1995). In contrast, Marsh and Langhorst (1988) reported that larval razorback suckers less than 21 mm TL from a shoreline section of Lake Mohave or an adjacent, isolated backwater without nonnative fishes ate primarily rotifers, cladocerans, or copepods. However, the diet of larvae in the backwater was comparatively more diverse and included larval chironomids and trichopterans, The digestive tracts of 33% of all specimens (41 out of 124) from Lake Mohave and 63 % of all specimens (47 out of75) from the backwater contained food. Similar to the isolated backwater, we found food in the digestive tracts of 67% of 480 larval razorback suckers; 59% of 379 specimens 11-13 mm TL, which averaged 35-45% full, and 100% of all specimens 14-18 mm TL, which averaged 51-65% full. Poor growth and survival of razorback sucker larvae due to low food abundance has been postulated as contributing to the low or nonexistent recruitment in populations (Minckley 1983; 20 I I I I I I I I I I I I I I I I I I I