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17 <br />reproductive activities. Tyus and Karp (1990) and Osmundson and <br />Kaeding (1991) reported off-channel habitats to be much warmer than <br />the main stem river and that razorback suckers presumably moved to <br />these areas for feeding, resting, sexual maturation, spawning, and <br />other activities associated with their reproductive cycle. Prior to <br />construction of large main stem dams and the suppression of spring <br />peak flows, low velocity, off-channel habitats (seasonally flooded <br />bottomlands and shorelines) were commonly available throughout the <br />Upper Basin (Tyus and Karp 1989; Osmundson and Kaeding 1991). Dams <br />changed riverine ecosystems into lakes by impounding water, which <br />eliminated these off-channel habitats in reservoirs. Reduction in <br />spring peak flows eliminates or reduces the frequency of inundation of <br />off-channel habitats. The absence of these seasonally flooded <br />riverine habitats is believed to be a limiting factor in the <br />successful recruitment of razorback suckers in their native <br />environment (Tyus and Karp 1989; Osmundaon and Kaeding 1991). Wydoski <br />and Wick (1998) identified starvation of larval razorback suckers due <br />to low zooplankton densities in the main channel and loss of <br />floodplain habitats which provide adequate zooplankton densities for <br />larval food as one of the most important factors limiting recruitment. <br />While razorback suckers have never been directly observed spawning in <br />turbid riverine environments within the Upper Basin, captures of ripe <br />specimens, both males and females, have been recorded (Valdez et al. <br />1982; McAda and Wydoski 1980; Tyus 1987; Osmundson and Kaeding 1989; <br />Tyus and Karp 1989; Tyus and Karp 1990; Osmundson and Kaeding 1991; <br />Platania 1990) in the Yampa, Green, Colorado, and San Juan Rivers. <br />Sexually mature razorback suckers are generally collected on the <br />ascending limb of the hydrograph from mid-April through June and are <br />associated with coarse gravel substrates (depending on the specific <br />location). <br />Outside of the spawning season, adult razorback suckers occupy a <br />variety of shoreline and main channel habitats including slow runs, <br />shallow to deep pools, backwaters, eddies, and other relatively slow <br />velocity areas associated with sand substrates (Tyus 1987; Tyus and <br />Karp 1989; Oamundson and Kaeding 1969; Valdez and Masslich 1989; <br />Osmundson and Kaeding 1991; Tyus and Karp 1990). <br />Habitat requirements of young and juvenile razorback suckers in the <br />wild are not well known, particularly in native riverine environments. <br />Prior to 1991, the last confirmed documentation of a razorback sucker <br />juvenile in the Upper Basin was a capture in the Colorado River near <br />Moab, Utah (Tabs et al. 1965). In 1991, two early juvenile (36.6 and <br />39.3 mm TL) razorback suckers were collected in the lower Green River <br />near Hell Roaring Canyon (GUtermuth et al. 1994). Juvenile razorback <br />suckers have been collected in recent years from Old Charley Wash, a <br />wetland adjacent to the Green River (Modde 1996). Between 1992 and <br />1995 larval razorback suckers were collected in the middle and lower <br />Green River and within the Colorado River inflow to Lake Powell (Muth <br />