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and Wydoski 1980, Tyus 1987, Tyus and Karp 1990, USFWS unpublished records on file in Vemal, UT). ~ The capture of larval razorback suckers in upper and lower reaches of the Green River (seine collections; R. T. Muth and D. Snyder, personal communication) suggests that razorback suckers spawn successfully in the Green River basin. However, the razorback suckers in the Green River are primarily old fish (Minckley, pers. comm.); very little annual recruitment has been documented since the 1960s (Lanigan and Tyus ~ 1989, Modde et al. 1996). The timing coincides roughly with the closure of Flaming Gorge Reservoir. Although operation of the reservoir reduced flooding and decreased availability of inundated shorelines and overbank habitat, it is not clear that this alone is responsible for the absence of recruitment to the razorback population. The question remains open, because present operation of the reservoir sti{I permits inundation of ~ bottomlands that should be sufficient for some recruitment. Perhaps operation of the reservoir leads to more stable habitat conditions that favor nonnatives. In the UCR, similar flooded areas that were once used by razorback suckers in the spring have been lost, including areas that reportedly supported razorback suckers in spawning condition (Archer et al. 1985). Reproducing populations of razorback sucker persisted in the UCR above Debeque Canyon for more than 60 years after the construction of ~ dams blocked fish movement downstream, but habitat destruction has been cited as the cause for the demise of this population (Vllestwater Engineering 1996). Integration of Life Histoy. Although the life history of the razorback sucker has been documented in less detail than that of the Colorado pikeminnow, there is still a large ~ body of information derived from years of research. The following narrative seeks to describe the general life cycle in a cohesive manner, patterned after the comparable section for the Colorado pikeminnow. As before, this overview simplifies by intentionally ignoring details that distract from a coherent view. ~ Adult razorback suckers spend most of the baseflow period (Sep-Apr) in low velocity habitats (e.g., backwaters, eddies, etc.) of the main channel. They remain active even in cold water, but movements are local. In the spring, when flows increase during runoff, the adults begin spawning migrations. Movements appear to be guided by olfactory cues, which lead each fish to the spawning area where natal imprinting ~ occurred. Main channel temperatures are 14-15 °C at this time. The fish move into off- channel staging areas (backwaters, oxbows, flooded bottomlands), where warmer temperatures (17.5-21 °C) probably facilitate the final maturation of gametes. Females remain ripe for an extended period of time (perhaps weeks}, and they move into the main channel and deposit eggs in flowing water over coarse (gravel and cobble) substrate. The rest of this narrative becomes speculative, because there is little documentation of life history. Hatching occurs during, or slightly before, peak runoff. Historically, larvae would have access to flooded bottomlands and probably spent a few weeks there. 21