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I I I I approached 650 m3s/s (- 23,000 cfs; Wick and Cluer 1998). At lower streamflows, the backwater effect does not occur and the channel becomes narrower so that higher water velocities can scour sand from the cobble substrate to make it suitable for razorback sucker spawning. Wick and Cluer (1998) suggested the low catch rate of razorback sucker larvae in light traps from the middle Green River during 1995 reported by Muth et al. (1998) may have been caused by excessive sedimentation in the cobble and gravel substrate at Razorback Bar that resulted in poor reproductive success. Although razorback suckers spawn in rivers on the ascending limb of the hydrograph and can spawn in floodplain ponds or isolated lagoons that may not have gravel or rubble substrate, these observations demonstrate the magnitude and timing of releases from Flaming Gorge Dam are crucial to Razorback Bar, the primary razorback spawning site in the middle Green River. Therefore, the magnitude and timing of regulated water releases from Flaming Gorge Dam are important to ensure that (1) the substrate at Razorback Bar is clean and suitable for razorback sucker spawning, and (2) flows will allow inundation of floodplains so that larval razorbacks at swimup have access to productive floodplains during their critical period. Lower releases from Flaming Gorge Dam are needed to maintain suitable conditions on "Razorback Bar" for spawning but higher releases are needed shortly afterward to inundate floodplains and provide access for swimup razorback larvae to productive nursery areas. I I I I IV. ROLE OF PREDATION AND COMPETITION IN SURVIVAL OF LARVAL ENDANGERED FISHES I Stanford et al. (1996) reported that one of three general principles of stream ecology related to streamflow regulation is that "native biodiversity decreases and nonnative species proliferate". Predation and competition by nonnative fishes have been demonstrated to have an adverse impact on native fishes in various North American waters (Moyle et al. 1986). These two factors are also believed to be significant in the decline of the endangered Colorado River fishes. The responses of both native (including endangered) and nonnative fishes will have to be monitored closely with habitat enhancement of floodplains in the Upper Colorado River Basin. I I Minckley et al. (1991) considered predation by nonnative fishes to be the single-most important factor in recruitment failure of razorback sucker in Lake Mohave in the Lower Colorado River Basin. Jonez and Sumner (1954) reported that carp (Cvorinus caroio) fed on the eggs of razorback suckers in Lake Mohave and young bass (Micropterus salmoides) and sunfish (Lepomis sp.) fed on razorback larvae. Researchers also believe that predation by nonnative fishes is an important factor in the Upper Basin (Tyus and Saunders 1996). Reference to predation by nonnative fishes is most often related to fish species that are known to be piscivorous such as members of the Families Centrarchidae, Ictaluridae, and Esocidae. However, the most numerous fishes in backwaters of the Upper Basin are nonnative minnows that may be more important predators than more widely recognized piscivorous species. I I I I Although predation on endangered fishes in the Upper Basin occurs by nonnative, piscivorous fishes in the Family Centrarchidae such as the green sunfish (Leoomis cvanellus), smallmouth bass (Microoterus dolomieui), largemouth bass, the Family Ictaluridae primarily in the channel catfish (Ictalurus ounctatus), and one member of the Family Esocidae, the northern pike (Esox lucius), several nonnative minnows (particularly, fathead minnows, Pimeohales oromelas and red shiners, Cvorinella lutrensis) may be even more important predators on the endangered Colorado River fishes and may also be important competitors .with the endangered fishes. I I I A. Predation. predation on larval fish significantly decreases their survival (Leggett 1986). Although razorback sucker gametes are viable and larvae are produced through successful natural reproduction in the Upper Basin (Muth et al. 1998), the swimup larvae are highly vulnerable to 23 I