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"Reasons for decline of most native fishes in the Colorado River Basin have been <br />attributed to habitat loss due to construction of mainstream dams and subsequent <br />interruption or alteration of natural flow and physio-chemical regimes, inundation of river <br />reaches by reservoirs, channelization, water quality degradation, introduction of <br />nonnative fish species and resulting competitive interactions or predation, and other man- <br />induced disturbances (Miller 1961, Joseph et al. 1977, Behnke and Benson 1983, Carlson <br />and Muth 1989, Tyus and Karp 1989). These factors are almost certainly not mutually <br />exclusive, therefore it is often difficult to determine exact cause and effect relationships." <br />Extremely limited recruitment suggests a combination of biological, physical, and/or chemical <br />factors that may be affecting the survival and recruitment of early life stages of razorback <br />suckers. Within the Upper Basin, recovery efforts include the capture and removal of razorback <br />suckers from all known locations for genetic analyses and development of discrete brood stocks <br />if necessary. These measures have been undertaken to develop refugia populations of the <br />razorback sucker from the same genetic parentage as their wild counterparts such that, if these <br />fish are genetically unique by subbasin or individual population, then separate stocks will be <br />available for future augmentation. Such augmentation may be a necessary step to prevent the <br />extinction of razorback such ers in the Upper Basin. <br />Life History <br />McAda and Wydoski (1980) and Tyus (1987) reported springtime aggregations of razorback <br />suckers in off-channel habitats and tributaries; such aggregations are believed to be associated <br />with reproductive activities. Tyus and Karp (1990) and Osmundson and Kaeding (1991) <br />reported off-channel habitats to be much warmer than the main stem river and that razorback <br />suckers presumably moved to these areas for feeding, resting, sexual maturation, spawning, and <br />other activities associated with their reproductive cycle. Prior to construction of large main stem <br />dams and the suppression of spring peak flows, low velocity, off-channel habitats (seasonally <br />flooded bottomlands and shorelines) were commonly available throughout the Upper Basin <br />(Tyus and Karp 1989; Osmundson and Kaeding 1991). Large main stern dams changed riverine <br />ecosystems into lakes by impounding water, which eliminated these off-channel habitats within <br />the inundated areas created by the reservoirs. Reduction in spring peak flows eliminates or <br />reduces the frequency of inundation of off-channel habitats. The absence of these seasonally <br />flooded riverine habitats is believed to be a limiting factor in the successful recruitment of <br />razorback suckers in their native environment (Tyus and Karp 1989; Osmundson and Kaeding <br />1991). Wydoski and Wick (1998) identified starvation of larval razorback suckers due to low <br />zooplankton densities in the main channel and loss of floodplain habitats which provide adequate <br />zooplankton densities for larval food as one 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, both males and females, have <br />been recorded (Valdez et al. 1982; McAda and Wydoski 1980; Tyus 1987; Osmuundson and <br />Kaeding 1989; Tyus and Karp 1989; Tyus and Karp 1990; Osmundson and Kaeding 1991; <br />Platania 1990, Ryden 2000b) 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 are associated with coarse gravel substrates. <br />13