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<br />. <br /> <br />. <br /> <br />. <br /> <br />Lt. Colonel Dorothy F. Klasse <br /> <br />14 <br /> <br />The existing scientific literature and historic accounts by local residents <br />strongly suggests that razorback suckers were once a viable. reproducing <br />member of the native fish community in the San Juan River drainage. <br />Currently. the razorback sucker is rare throughout its historic range and <br />extremely rare in the main stem San Juan River. <br /> <br />Rioloav <br /> <br />Specific information on biological and physical habitat requirements of the <br />razorback sucker is very limited. Localized extirpation of razorback suckers <br />from some localities. coupled with the species' continued decline in numbers <br />and distribution. has prompted some research: however. details of its life <br />history requirements. particularly in riverine environments. are still not <br />fully understood. <br /> <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 razorback <br />suckers. Prior to construction of large main stem dams and the suppression of <br />spring peak flows. low velocity. off-channel habitats (seasonally flooded <br />bottomlands and shorelines) were commonly available throughout the Upper Basin <br />(Tyus and Karp 1989: Osmundson and Ka~ding 1991). The absence of these <br />seasonally flooded riverine habitats is believed to be a limiting factor in <br />the successful recruitment of razorbac~ suckers in their native environment <br />(Tyus and Karp 1989: Osmundson and Kaeding 1991). Tyus (1987) and McAda and <br />Wydoski (1980) reported springtime aggregations of razorback suckers in <br />off-channel impoundments and tributaries: such aggregations are believed to be <br />associated with reproductive activities. Tyus and Karp (1990) and Osmundson <br />and Kaeding (1991) reported off-channel habitats to be much warmer than the <br />main st=m river and that razorback suc~ers presumably moved to these areas for <br />f=~~ing. resting. sexual maturation. spawning. and other activiti=o associated <br />with their reproductive cycle. While razorback suckers have never been <br />directly observed spawning in turbid riverine environments within the Upper <br />Basin. captures of ripe specimens. both males and females. have been recorded <br />(Valdez et al. 1982: McAda and Wydoski 1980; Tyus 19B7: Osmundson and Kaeding <br />1989: Tyus and Karp 1989: Tyus and Karp 1990; Platania 1990: Osmundson and <br />Kaeding 1991) in the Yampa. Green. Colorado. and San Juan Rivers. Sexually <br />mature razorback suckers are generally collected on the ascending limb of the <br />hydrograph from mid-Apcjl through June and are associated with coarse gravel <br />substrates (depending on the specific location). <br /> <br />Outside of the spawning season. adult razorback suckers occupy a variety of <br />shoreline and main channel habitats including slow runs. shallow to deep <br /> <br />, \ <br />