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12 <br />Bioloev <br />Specific information on biolosical and physical habitat requirements of the razorback sucker is <br />very limited. Until very recently, fisheries research investisations throughout the Upper Basin <br />have focused on the three listed Colorado River fishes, and data collected on the razorback <br />sucker was lazgely coincident to those studies. Localized extirpation of razorback suckers from <br />some localities, coupled with the species' continued decline in numbers and distribution, has <br />prompted some reseazch; however, details of its life history requirements, particularly in riverine <br />environments, are still not fully understood. <br />In general, a natural hydrograph with a lazge spring peak, a gradually descending limb into eazly <br />summer, and low stable flows through summer, fail, and winter aze thought to create the best <br />habitat conditions for endangered fishes while maintaining the integrity of the channel <br />geomorphology. Prior to construction of lazge main stem dams and the suppression of spring <br />peak flows, low velocity, off-channel habitats (seasonally flooded bottomlands and shorelines) <br />were commonly available throughout the Upper Basin (Tyus and Karp 1989; Osmundson and <br />Kaeding 1991). The absence of these seasonally flooded riverine habitats is believed to be a <br />limiting factor in the successful recruitment of razorback suckers in their native environment <br />(Tyus and Karp 1989; Osmundson and Kaeding 1991). Tyus (1987) and McAda and Wydoski <br />(1980) reported springtime aggregations of razorback suckers in off-channel impoundments and <br />tributaries; such aggregations aze believed to be associated with reproductive activities. Tyus and <br />Karp (1990) and Osmundson and Kaeding (1991) reported off-channel habitats to be much <br />warmer than the main stem river and that razorback suckers presumably moved to these azeas for <br />feeding, resting, sexual maturation, spawning, and other activities associated with their <br />reproductive cycle. While razorback suckers have never been directly observed spawning in <br />turbid riverine environments within the Upper Basin, captures of ripe specimens, both males and <br />females, have been recorded (Valdez et al. 1982; McAda and Wydoski 1980; Tyus 1987; <br />Osmundson and Kaeding 1989; Tyus and Karp 1989; Tyus and Karp 1990; Osmundson and <br />Kaeding 1991; Platania 1990) in the Yampa, Green, Colorado, and San Juan Rivers. Sexually <br />mature razorback suckers are generally collected on the ascending limb of the hydrograph from <br />mid-April through June and aze associated with coazse gravel substrates (depending on the <br />specific location). <br />Outside of the spawning season, adult razorback suckers occupy a variety of shoreline and main <br />channel habitats including low runs, shallow to deep pools, backwaters, eddies, and other <br />relatively slow velocity azeas associated with sand substrates (Tyus 1987; Tyus and Karp 1989; <br />Osmundson and Kaeding 1989; Valdez and Masslich 1989; Osmundson and Kaeding 1991; Tyus <br />and Karp 1990). <br />Habitat requirements of young and juvenile razorback suckers in the vtiild aze not well known, <br />particulazly in native riverine environments. Prior to 1991, the last confirmed documentation of <br />razorback sucker juvenile in the Upper Basin was a capture in the Colorado River neaz Moab, <br />Utah (Tabs et al. 1960. In 1991, two eazlyjuvenile (36.6 and 39.3 mm TL) razorback suckers <br />