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I I I I I I I I I I I I I I I I I I I BIOLOGICAL OPINION Status of the Species The razorback sucker is the only representative of the genus Xyrauchen and was described from specimens taken from the "Colorado and New Rivers" (Abbott 1861) and Gila River (Kirsch 1889) in Arizona. This native sucker is distinguished from all others by the sharp edged, bony keel that rises abruptly behind the head. The body is robust with a short and deep caudal peduncle (Bestgen 1990). The razorback sucker may reach lengths of one meter and weigh five to six kg (Minckley 1973). Adult fish in Lake Mohave reached about half this maximum size and weight (Minckley 1983). Razorback suckers are long-lived, reaching the age of at least the mid-40's (McCarthy and Minckley 1987). Life History Life history information for the razorback sucker was recently summarized in the status review for the species (Bestgen 1990), in Battle Against Extinction: Native Fish Management in the American West (Minckley and Deacon 1991), and in the biological support document for critical habitat designation (USFWS 1993a). The life history information presented in this BO is primarily taken from these sources. The razorback sucker was once abundant in the Colorado River and its major tributaries throughou t the Basin, occupying 3,500 miles of river in the United States and Mexico (USFWS 1993a). Records from the late 1800's and early 1900's indicated the species was abundant in the lower Colorado and Gila River drainages (Kirsch 1889, Gilbert and Scofield 1898, Minckley 1983, Bestgen 1990). Adult razorback suckers utilize most of the available riverine habitats, although there may be an avoidance of whitewater type habitats. Main channel habitats used tend to be low velocity ones such as pools, eddies, nearshore runs, and channels associated with sand or gravel bars (summarized in Bestgen 1990). Backwaters, oxbows, and sloughs were well-used habitat areas adjacent to the main channel; flooded bottomlands are important in the spring and early summer (summarized in Bestgen 1990). Razorback suckers may be somewhat sedentary, however considerable movement over a year has been noted in several studies (USFWS 1993a). Spawning migrations have been observed or inferred in several locales (Jordan 1891, Minckley 1973, Osmundson and Kaeding 1989, Bestgen 1990, Tyus and Karp 1990). Spawning takes place in the late winter to early summer depending upon local water temperatu res. Various studies have presented a range of water temperatures at which spawning occurs. In general, temperatures between 100 to 200 C are appropriate (summarized in Bestgen 1990). Spawning areas include gravel bars or rocky runs in the main channel (Tyus and Karp 1990), and flooded bottomlands (Osmundson and Kaeding 1989). There is an increased use of higher velocity waters in the spring, although this is countered by the movements into the warmer, shallower backwaters and inundated bottomlands in early summer (McAda and Wydoski 1980, Tyus and Karp 1989, Osmundson and Kaeding 1989). 41