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.~ <br />11 <br />Colorado River fishes, and data collected on the razorback sucker was largely <br />coincident to those studies. Localized extirpation of razorback suckers from <br />some localities, coupled with the species' continued decline in numbers and <br />distribution, has prompted some research; however, details of its life history <br />requirements, particularly in riverine environments, are still not fully <br />understood. <br />In general, a natural hydrograph with a large spring peak, a gradually <br />descending limb into early summer, and low stable flows through summer, fall, <br />and winter are thought to create the best habitat conditions for endangered <br />fishes while maintaining the integrity of the channel geomorphology. Prior to <br />construction of large main stem dams and the suppression of spring peak flows, <br />low velocity, off-channel habitats (seasonally flooded bottomlands and <br />shorelines) were commonly available throughout the Upper Basin (Tyus and Karp <br />1989; Osmundson and Kaeding 1991). The absence of these seasonally flooded <br />riverine habitats is believed to be a limiting factor in the successful <br />recruitment of razorback suckers in their native environment (Tyus and Karp <br />1989; Osmundson and Kaeding 1991). Tyus (1987) and McAda and Wydoski (1980) <br />reported springtime aggregations of razorback suckers in off-channel <br />impoundments and tributaries; such aggregations are believed to be associated <br />with reproductive activities. Tyus and Karp (1990) and Osmundson and Kaeding <br />(1991) reported off-channel habitats to be much warmer than the main stem <br />river and that razorback suckers presumably moved to these areas for feeding, <br />resting, sexual maturation, spawning, and other activities associated with <br />their reproductive cycle. While razorback suckers have never been directly <br />observed spawning in turbid riverine environments within the Upper Basin, <br />captures of ripe specimens, both males and females, have been recorded <br />(Valdez et al. 1982; McAda and Wydoski 1980; Tyus 1987; Osmundson and <br />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 <br />Rivers. 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 location). <br />Outside of the spawning season, adult razorback suckers occupy a variety of <br />shoreline and main channel habitats including low runs, shallow to deep pools, <br />backwaters, eddies, and other relatively slow velocity areas associated with <br />sand substrates (Tyus 1987; Tyus and Karp 1989; Osmundson and Kaeding 1989; <br />Valdez and Masslich 1989; Osmundson and Kaeding 1991; Tyus and Karp 1990). <br />Habitat requirements of young and juvenile razorback suckers in the wild are <br />largely unknown, particularly in native riverine environments. Life stages, <br />other than adults, have not been collected anywhere in the Upper Basin in <br />recent times. The last confirmed documentation of razorback sucker juvenile <br />in the Upper Basin was a capture in the Colorado River near Moab, Utah <br />(Taba et al. 1965). <br />The current range of the razorback sucker in the Colorado River extends <br />upstream to Rifle, Colorado. Most razorback suckers captured in the Grand <br />Valley area have been located in flooded gravel-pit ponds adjacent to the <br />river. However, Osmundson and Kaeding (1989) documented razorback sucker <br />movement in various river habitats in the Grand Valley area. Additional <br />