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Razorback sucker populations have declined markedly in the last 50 years. At one time it was thought that the razorback sucker was "holding its own" and perhaps even expanding in large impoundments (Wallis 1951; Miller 1961), but most of these populations have almost entirely disappeared. Extant populations consist primarily of old fish believed to be nearing their maximum life expectancy (Minckley et al. 1991). There are no historic population estimates for this fish, but it is presumed that extant populations represent a 90% decline in historic range and abundance. General Description Adult razorback suckers may grow to a total length of about 1 m and a weight of 5-6 kg. However, most specimens of the fish are smaller. The largest fish presently occur in the warmer climate of the lower Colorado River, and sizes of 470-740 mm are reported in Lake Mojave, Arizona (Minckley 1983). In the upper Colorado River basin, sizes of 405 to 597 mm were reported (Tyus and Karp 1990). Females are generally larger than males of the same age. Females ranged in size from 470 to 740 mm and males ranged in size from 370 to 640mm in the lower basin (Minckley 1983). In the upper basin, females captured on spawning grounds averaged 547mm and males averaged 507 mm (Tyus and Karp 1990). The razorback sucker shares many characters with other catostomid, but is distinguished from all other catostomid by a pronounced bony keel that grows from the dorsal surface of its back (i.e., behind the occiput). Formed mostly from enlarged neural and interneural bones, this relatively thin, sharp-edged keel is the basis for its common name. The species also is distinguished by well-developed, elongated filaments on its gill rakers, an adaptation for feeding on zooplankton. Characteristics shared with other members of its family include an intermediate number of moderately compressed pharyngeal teeth arranged in comb-like fashion, which is presumably an adaption to benthic feeding (Eastman 1977; Sublette et al. 1990). Its bony dorsal keel, heavily ossified caudal skeleton, and thickened and foreshortened caudal rays are thought to be adaptations to the strong river~currents in which the fish lives (La Rivers 1962, Eastman 1980, Moyle and Cech 1996). The razorback sucker has an elongated head with a flattened dorsal surface and awell- developed fontanelle. The moderate-sized ventral mouth has a cleft lower lip, with lateral margins continuous and rounded. Meristics include: dorsal fin rays usually 14-15 (13-16), anal fin rays 7, total vertebrae 45-50, lateral line scales 68-87, and gill rakers 44-50 on the first arch. Pharyngeal teeth are 64-74 per arch and occur in a single row (Minckley 1973, Sublette et al. 1990). Razorback sucker coloration ranges from dark brown to olivaceous dorsally and yellow to white ventrally, but color and morphology differ due to a sexual dimorphism that is especially obvious during reproductive seasons. Breeding males are dark colored dorsally and bright yellow to orange on lateral and ventral surfaces. Pelvic and anal fins are generally longer in males than in females, whereas the reverse is true for the 3