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166 VALDEZ AND MUTH (McCarthy and Minckley 1987). Adults are slightly compressed laterally with a bony, predorsal keel behind the occiput. The keel is formed by the growth and fusion of interneural bones. Size and shape of these bones are diagnostic characteristics for young of the species (Snyder and Muth 2004). Scales are well developed with 68-87 in the lat- eralline. Razorback sucker hybridizes with native flannelmouth sucker and bluehead sucker (Hubbs and Miller 1953; Suttkus et al. 1976; Maddux et al. 1987; Douglas and Marsh 1998), as well as with nonnative white sucker C. commersonii (McAda and Wydoski 1980; Buth et al. 1987). Razorback sucker was listed as federally endangered in 1991 (56 FR 54957) with critical habitat designated in 1994 (59 FR 13374). A recovery plan was com- pleted in 1998 (U.S. Fish and Wildlife Service 1998a), and amended and supplemented with re- covery goals in 2002 (U.S. Fish and Wildlife Ser- vice 2002d). Razorback sucker was historically common to abundant in most warm regions of the Colorado River system during the 19th and 20th centuries (Jordan and Evermann 1896; Minckley et al. 1991). In the upper basin, the species was com- mon in the Green and upper Colorado rivers and in some warm tributaries, including the White, Duchesne, Little Snake, Yampa, and Gunnison riv- ers (Burdick 1995; Holden 1999), and possibly as far up the San Juan River as the Animas River (Jor- dan 1891; Minckley et al. 1991; Holden 2000). Razorback sucker declined throughout the 20th century, and the species now exists naturally in only a few locations. Natural reproduction has occurred with little recruitment over the past 40-50 years, and wild populations are composed primarily of old, senescent adults (Bestgen 1990; Bestgen et al. 2002). Reproduction has been documented in the Green River with collection oflarvae (Muth et al. 1998; Chart et al. 1999). Small numbers ofjuve- niles and young adults provide evidence of some recruitment attributed to unusually high spring flows during 1983-1986 that provided critical floodplain nurseries (Modde et al. 1996). Numbers of wild razorback sucker captured in the upper basin have decreased dramatically since 1974. Razorback sucker are found in small numbers in the middle Green River and in lower reaches of the Yampa, Duchesne, White, and San Rafael riv- ers (Tyus 1987; Figure 5). The middle Green River population was estimated at 1,000 wild adults in 1985 (Lanigan and Tyus 1989) and 524 in 1995 (Modde et al. 1996), and data from 1998-1999 suggest that about 100 wild adults remained at that time (Bestgen et al. 2002). The wild population is considered extirpated from the Gunnison River, where only two fish were reported in 1976 (Burdick and Bonar 1997). Only 52 individuals, all old adults, were captured during a 3-year study (1979-1981) in 465 km of the upper Colorado River from Hite, Utah, to Rifle, Colorado (Valdez et al. 1982b); and only 12 were captured in the Grand Valley, Colorado, during 1984-1989 (Osmundson and Kaeding 1989). Wild razorback sucker in the San Juan River are limited to two fish captured in 1976 in a riverside pond near Bluff, Utah, and one fish captured in the river in 1988, also near Bluff (Ryden 2000). Large numbers were anecdotally reported in a drained pond near Bluff in 1976, but identification was not verified. Wild razorback sucker were not found in a 7 -year study of the Sanjuan River (1991-1997; Holden 1999). Adult razorback sucker in the upper basin oc- cupy low-velocity pools, tuns, and slackwaters in al- luvial reaches and less frequently in canyon-bound areas (Tyus 1987; Lanigan and T yus 1989; Tyus and Karp 1990; Bestgen 1990). Adults summer in deep eddies, slow tuns, and backwaters with silt or sand substrate, depths of 0.6-3.4 m, and velocities of 0.3- 0.4 m/s (Valdez et al. 1982b; Tyus 1987; Tyus et al. 1987; Osmundson and Kaeding 1989; Tyus and Karp 1990; Osmundson et al. 1995). During win- ter, adults use depths of 0.6-2.16 m in slow runs, pools, eddies, and slack water (Osmundson and Kaeding 1989; Valdez and Masslich 1989). Razor- back sucker is a moderately migratory, potadromous species that moves to and from spawning and over- wintering areas (Tyus and Karp 1990; Modde and Wick 1997; Modde and Irving 1998). Greatest movements are associated with spawning in spring and may account for historic reports oflarge concen- trations of adults (Jordan 1891; Hubbs and Miller 1953; Sigler and Miller 1963; McAda and Wydoski 1980). Adults are relatively sedentary outside of