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1 INTRODUCTION Endangered razorback sucker Xyrauchen texanus was once widespread and abundant 1 throughout the Colorado River Basin but is now rare (Minckley 1983, Bestgen 1990, Minckley et al. 1991). Concentrations occur in lakes Mohave and Mead, Arizona and Nevada, and in the middle Green River, Utah but elsewhere occur only as scattered individuals (Minckley 1983, Tyus 1987, Bestgen 1990, Minckley et al. 1991, Modde et al. 1996, Holden et al. 2001•). Wild fish in Lake Mohave have declined in abundance to about 4,000 individuals in 2001 (Minckley et al. 1991, Marsh 1994, G. Mueller pers. comm.). Abundance of adult Green River razorback suckers was estimated at about 300 to 950 during the 1980 to 1992 period (Lanigan and Tyus 1989, Modde et al. 1996), but the present status of that population is unknown. Wild populations of razorback suckers are dominated by large, old individuals, and recruitment rates everywhere are thought low ornon-existent (Minckley 1983, Minckley et al. 1991, Tyus 1987, Modde et al. 1996). Decline of razorback suckers has been attributed to alterations of physical habitat and negative effects of introduced fishes. Mainstem dams alter flow patterns, water temperature, and sediment loads and also serve as barriers to upstream fish movement (Carlson and Muth 1989). In the upper Colorado River Basin, flow reduction due to storage of spring runoff, and effects of channelization and levee placement, reduce frequency and duration of flood plain inundation. A decrease in warm, food-rich flood plain areas, which are likely important as rearing and resting habitat for early and life adult stages ofspring-spawning razorback suckers, may limit recruitment (Modde et al. 1996). Predation on early life stages of razorback suckers, combined with slow growth, is also thought a primary factor limiting recruitment (Minckley et al. 1991). 1