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Threats to the Species <br />The primary threats to razorback sucker are stream flow regulation and habitat <br />modification; competition with and predation by nonnative fishes; and pesticides and <br />pollutants (USFWS 2002b). The existing habitat, altered by these threats, has been <br />modified to the extent that it impairs essential behavior patterns, such as breeding, <br />feeding, and sheltering. The threats to razorback sucker are essentially the same threats <br />identified for Colorado pikeminnow. <br />Life History <br />McAda and Wydoski (1980) and Tyus (1987) reported springtime aggregations of <br />razorback suckers in off-channel habitats and tributaries; such aggregations are believed <br />to be associated with reproductive activities. Tyus and Karp (1990) and Osmundson and <br />Kaeding (1991) reported off-channel habitats to be much warmer than the mainstem river <br />and that razorback suckers presumably moved to these. areas for feeding, resting, sexual <br />maturation, spawning, and other activities associated with their reproductive cycle. Prior <br />to construction of large mainstem dams and the suppression of spring peak flows, low <br />velocity, off-channel habitats (seasonally flooded bottomlands and shorelines) were <br />commonly available throughout the Upper Basin (Tyus and Karp 1989; Osmundson and <br />Kaeding 1991). Dams changed riverine ecosystems into lakes by impounding water, <br />which eliminated these off-channel habitats in reservoirs. Reduction in spring peak flows <br />eliminates or reduces the frequency of inundation of off-channel habitats. The absence of <br />these seasonally flooded riverine habitats is believed to be a limiting factor in the <br />successful recruitment of razorback suckers in their native environment (Tyus and Karp <br />1989; Osmundson and Kaeding 1991). Wydoski and Wick (1998) identified starvation of <br />larval razorback suckers due to low zooplankton densities in the main channel and loss of <br />floodplain habitats which provide adequate zooplankton densities for larval food as one <br />of the most important factors limiting recruitment. <br />While razorback suckers have never been directly observed spawning in turbid riverine <br />environments within the Upper Basin, captures of ripe specimens (in spawning <br />condition), both males and females, have been recorded (Valdez et al. 1982a; McAda and <br />Wydoski 1980; Tyus 1987; Osmundson and Kaeding 1989; Tyus and Karp 1989; Tyus <br />and Karp 1990; .Osmundson and Kaeding 1991; Platania 1990) in the Yampa, Green, <br />Colorado, and San Juan Rivers. Sexually mature razorback suckers are generally <br />collected on the 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 shoreline <br />and main channel habitats including slow runs, shallow to deep pools, backwaters, <br />eddies, and other relatively slow velocity areas associated with sand substrates (Tyus <br />1987; Tyus and Karp 1989; Osmundson and Kaeding 1989; Valdez and Masslich 1989; <br />Osmundson and Kaeding 1991; Tyus and Karp 1990). <br />Habitat requirements of young and juvenile razorback suckers in the wild are not well <br />known, particularly in native riverine environments. Prior to 1991, the last confirmed <br />documentation of a razorback sucker juvenile in the Upper Basin was a capture in the <br />Colorado River near Moab, Utah (Taba et al. 1965). In 1991, two early juvenile (36.6 <br />18