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~) ...) River; 30 radio tagged razorback suckers and 656 razorback suckers marked with passive integrated transponder (PIT) tags were released in the San Juan River. There is no evidence anywhere in the Colorado River system that indicates significant recruitment to any population of razorback sucker (Tyus 1987, McCarthy and Minckley 1987, Osmundson and Kaeding 1989, Bestgen 1990, Platania 1990, Platania et a\. 1991, ), ~ ." ." . 4"~li :".1 The existing scientific literature and historic accounts by local residents strongly suggests that razorback suckers were once a viable, reproducing member of the native fish community in the San Juan River drainage. Currently, the razorback sucker is rare throughout its historic range and extremely rare in the main stem San Juan River. Specific information on biological and physical habitat requirements of the razorback sucker is very limited, Localized extirpation of razorback suckers from some localities, coupled with the species' continued decline in numbers and distribution, has prompted some research; however, details of the species' life history requirements, particularly in riverine environments, are still not fully understood, In general, a natural hydrograph with a large spring peak, a gradually descending limb into early summer, and low stable flows through summer, fall, and winter are thought to create the best habitat conditions for razorback suckers, Prior to construction of large main stem dams and the suppression of spring peak flows, low velocity, off-channel habitats Iseasonally flooded bottomlands and shorelines) were commonly available throughout the Upper Basin (Tyus and Karp 1989; Osmundson and Kaeding 1991). The absence of these seasonally flooded riverine habitats is believed to be a limiting factor in the successful recruitment oJ razorback suckers in their native environment (Tyus and Karp 1989; Osmundson and Kaeding 1991). Tyus (1987) and McAda and Wydoski (19801 reported springtime aggregations of razorback suckers in off-channel impoundments and tributaries; such aggregations are believed to be associated with reproductive activities. Tyus and. Karp (19901 and Osmundson and Kaeding (1991) reported off-channel habitats to be much warmer than the main stem river and that razorback suckers presumably moved to these areas for feeding, resting, sexual maturation, spawning, and other activities associated with their reproductive cycle. While razorback suckers have never been directly observed spawning in turbid riverine environments within the Upper Basin, captures of ripe specimens, both males and females, have been recorded (Valdez et al. 1982; McAda and Wydoski 1980; Tyus 1987; Osmundson and Kaeding 1989; Tyus and Karp 1989; Tyus and Karp 1990; Platania 1990; Osmundson and J(aeding 1991) in the Yampa, Green. Colorado, and San Juan Rivers, Sexually mature razorback suckers are generally collected on the ascending limb of the hydrograph from mid-April through June and are associated with coarse gravel substrates (depending on the specific location). Outside of the spawning season, adult .%azorback suckers occupy a variety of shoreline and main channel habitats including slow runs, shallow to deep pools. backwaters, eddies, and other relatively slow velocity areas associated with sand substrates (Tyus 1987; Tyus and Karp 1989; Osmundson and Kaeding 1989; Valdez and Masslich 1989; TytJs and Karp 1990; Osmundson and Kaeding 1991). The virtual absence of any recruitment suggests a combination of biological, physical, and/or chemical factors that may be affecting the survival and recruitment of early life stages of razorback suckers. Within the Upper Basin, recovery efforts endorsed by the "Recovery Implementation Program for Endangered Fish SpeCies in the Upper Colorado 10