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KNOWN HABITAT REQUIREMENTS The bonytail chub and the razorback sucker have been isolated in the Colorado River drainage for millions of years, along with a number of other minnows and suckers. During this time, these large,somewhat bizarre- looking fishes became specialized for life in the main river channels. Based on morphology and collecting records, both fish appear to be attuned to a river environment characterized by enormous fluctuation in flows, variable water quality, and shifting substrates. The razorback sucker possesses a flat- tened breast and depressed head, a sharp, bony nuchal keel, and grows to 15 pounds, which are thought to be adaptations for navigating the often tor- rential flows of the Colorado River during spawning migrations and floods (Hubbs and Miller 1953). Sigler and Miller (1963) stated that the razorback sucker was usually found in water from four to eight feet deep over a bottom of sand, mud, or rock. The water may be muddy or clear, the current usually strong, and vegetation sparse or lacking. -Minckley (1973) reported that adults tend to remain in eddies and backwaters, lateral to the strongest currents, and were often concentrated behind obstructions or deep holes near cut banks or fallen trees. Razorback suckers probably rest in the quieter parts of rivers, such as backwaters and large pools, between spawning periods and as juveniles (Minckley and Deacon 1968). Spawning takes place when water temperatures in- crease to 580 to 680F, probably in June in Colorado (Behnke 1973). The young move to the backwater areas where water temperatures are 70oto 750F, noticeably warmer than the adjacent rivers (Sigler and Miller 1963). Razorback suckers possess numerous, fuzzy gill rakers which suggest a filter feeding type of nutrition. Examination of preserved specimens re- vealed a sheet of minute organisms between the gill rakers and pharyngeal pad - 2 -