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~ bottomland habitats along the Green River but rarely reached that density in backwaters, and never reached it in the river (Tables 1 and 2; Cooper and Severn 1994 a,b,c,d,e; Grabowski and Hiebert 1989; Mabey and Schiozawa 1993). The dec 1 i ne in razorback suckers and the near ext i rpat i on of the bonyta i 1 in the upper basin is associated with the lack of survival during the early life stages. Starvation during the larval critical period of the endangered fishes is likely one factor that causes' low or the complete 1 ack of recru itment. Flooded bottomland habitats produce the highest densities of zooplankton that would meet the food requirement during their early life stages for razorback suckers in the Upper Colorado Ri ver Bas in. Zooplankton produced in productive off-channel habitats such as flooded bottomlands also provide food of the proper quantity and size at the right time needed for larval survival of endangered fishes that occupy main channel habitats (i.e., backwaters) such as the Colorado squawfish and humpback chub. Off-channel habitats are important in supplying production of zooplankton to riverine habitats either by escapement from the floodplain habitats or by displacement into the river as high streamflows subside (Welcomme 1985). X. ROLE OF PREDATION AND COMPETITION TO THE SURVIVAL OF LARVAL ENDANGERED FISHES Potential Role of Predation on Survival of Fish Larvae. Predation on larval fish is a significant factor that decreases their survival (Leggett 1986). Minckley et al. (1991) and Mueller (1995) consider predation by nonnative fishes as the single-most factor in recruitment failure of razorback sucker in Lake Mohave. Although razorback sucker gametes are vi abl e and successful natural spawn i ng occurs on rubble substrate that is kept clean through the action of wind and waves along the shoreline of Lake Mohave, survival of larvae only occurs in habitats where nonnative fish predators are absent or rare such as isolated coves (Minckley et al. 1991). However, in the absence of predaceous fi sh, odonate nymphs exploded in these coves and the insect predators replaced fish predators. Mueller et al. (1993) reported that, in laboratory experiments, odonate nymphs were very effective in capturing and consuming razorback sucker larvae. Nonnative red shiners (Cyorinella lutrensis) and fathead minnows (Pimeohales oromelas) are often the most abundant fish species in backwaters of the Upper Colorado River Basin rivers. For example, Cranney (1994) reported that red shiners constituted 66.5% and fathead minnows 31.9% of 3,599 fish collected in 13 seine hauls in the Lower Duchesne River, a tributary to the Middle Green River. Only one native fish (a bluehead sucker, Catostomus discobolus) was captured during that sampling effort. Red shiners and fathead minnows constituted 90.4% of 149,489 fish collected between 1986 and 1994 in the first seine haul from primary backwaters of the Colorado and Green rivers during the Interagency Standardized Monitoring Program (McAda et al. 1994a, 1994b, 1995). Adult red shiners have been documented to be predators on fish larvae in the Yampa and Green rivers (Ruppert et al. 1993). Fathead minnows have also been documented to be predators on catastomid larvae (Dunsmoor 1993). Therefore, it is reasonable to assume that nonnative minnows such as the red shiner and fathead minnow are important predators on razorback sucker larvae. The razorback sucker spawns on the ascending limb of the hydrograph and their larvae drift downstream during May and June when zooplankton numbers are low in backwaters in the turbid waters of upper basin rivers. Fish larvae would be highly susceptible to 10