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Federal Register I Vol. 59, No. 54 I Monday, March 21, 1994 I Rules and Regulations and nutrient transport. and other . characteristics of the aquatic environment (Carlson and Muth 1989). The altered river conditions that have resulted now provide suitable babitats for introduced, nonnative fish. Some of these nonnative fish species have flourished in the Basin (Minckley et al. 1982; Tyus et al.1982; Carlson and Muth 1989). These physical and biological changes have i:mpacted the river environment to the extent that no completely unaltered habitat remains in the Basin for the four Colorado River endangered fish species. Razorback Sucker This species once was abundant and widely distributed in rivers of the Basin Uordan and Evermann 1896; Minckley 1973). In the Lower Basin, the razorback sucker remains in the Colorado River from the Grand Canyon to near the border with Mexica, With the exception of the relatively large stock of razorback suckers remaining in Lake Mohave (an estimated 25,000 individuals), these . populations are sinall and recruitment is virtuallynonexisteilt (Minckley.et al. 1991). The formerly large Lower Basin populations have been virtually. . extirpated from other riverine environments (Minckleyet'al. 1991). In the Upper Basin, this species remains in the lower Yampa and Green Rivers, mainstream Colorado River. end lower San]uanRiver (Tyus et al. 1982; Minckley et al. 1991; Platania et aI. 1991); however, there i.little indication of recruitment in these remnant stocks. The largest extant riverine population occurs in the upper Green River Basin. It consisted of only about 1,000 fish in 1989 (La1rigan and Tyus 1989); recent information suggests that this population may have declined to less than 500 fish (USFWS unpublished data). 'In the absence of conservation efforts, it is presumed that all wild populations in the Basin would soon be lost as old fish die without sufficient natural recruitment, Reproduction and habitat use of razorback suckers has been studied in Lower Basin reservoirs, especially in LaJ,e Mohave. Fish' reproduction has been visually observed along reservoir shorelines for many years, The fish spawn over mixed substrates that range from silt to cobble and at water temperatures ranging from 10,5 to 21. C (51 to 70. .F) (reviewed by Minckley et al. 1991). Habitat use and spawning behavior of adult razorback suckers in riverine habitats has been studied by radiotelemetry in the Green River Basin (Tyus and Karp 1990) and in the upper Colorado River (Osmundson and Kaeding 1989). Fish in the Green River Basin spawn in the spring with rising water levels and increasing temperatures. Razorback suckers move into flooded areas in early spring and begin spawning migrations to specific locations as they become reproductively active, and spawning occurs over rocky runs and gravel bars (Tyus and Karp 1990). In nonreproductive periods. adult razorback suckers occupy a variety of habitat types. including i:mpounded and riverine areas. eddies, backwaters, gravel pits, flooded bottoms, flooded mouths of tributary streams; slow runs. san.dy riffles. and others (reviewed by Minckley et al. 1991). Summer habitats used include deeper eddies, backwaters, holes, and midchannel sandbars (Osmundson and Kaeding 1989; Tyus and Karp 1990; Minckley et al. -1991), During winter, adult razorback suckers use main channel habitats that are similar to those used during other times of the year, including eddies, slow runs, riffles. and slackwaters (Osmundson and Kaeding 1989; Valdez and Masslich 1989; Tyus and Karp 1990). "Habitats used by youngl'azorbaCk suckers have not been fully described because of the low number of young fish present in the Basin. However, most studies indicate that the larvae prefer shallow, littoral zones for a few weeks after hatchll)g. then disperse to deeper water areas (reviewed by Minckleyet ai, 1991), Laboratory studies indicated that in a riverine environment, the larvae enter stream drift and are transported.. downstream (Paulin etal, 1989). Based on available data. Tyus and Karp (1989) and Osmundson and Kaeding (1989) considered that cumulative environmental i:mpacts from interactions with nonnative fisb. high winter flows, reduced high spring flows, seasonal changes in river temperatures, and lack of inundated shorelines and bottom lands are factors that potentially limit the survival, successful . reproduction, and recruitment of this species. Colorodo Squawfish This species is the only living representative of the genus Ptychocheilus endemic to the .Basin. Fossils from the Mid.Pliocene epoch (about 6 million years ago) indicate that early Ptychochellus had physical characteristics that were si:milar to modem forms. Native populations of the Colorado squawlish are now restricted to the Upper Basin in Wyoming, Colorado, Utah, and New Mexico. Colorado squawfish populations have been extirpated from the Lower Basin. 13375 - Colorado squawfish spawning has been documented in canyons in the Yampa and Green Rivers' [Tyus 1991). This reproduction is associated with declining flows in June. July, or August and average water temperatures ranging from 22 to 25 .C (72 to 77 .F) depending on annual hydrology. River mile 130 on the Colorado River. near the Colorado- Utah State line, also has been identified as a spawning site. and radio.tagged adults have moved to a specific 0.2 Ian (0,1 mil area in four different years (Osmundson and Kaeding 1989; USFWS unpublished data 1992-1993). In the mainstream Colorado River, MeAda and Kaeding (1991) stated that spawning occurs at many locations. They also suggested that Colorado squawfisl;1 spawning in the Colorado River may have been adversely impacted by construction of mainstream dams and a 48 percent reduction in peak discharge. On the San Juan River, a spawning reach has been identified between river mile 133.4 and 129.8, near the confluence of the Mancos River (Ryden and Pfeifer 1993), After spawning, adult Colorado squawfish utilize a variety of riverine habitats, including eddies. hackwaters. shorelines, and others (Tyus 1990), During winter, adult Colorado . . squawfish use backwaters, runs. pools, and eddies, but are most common'in . shallow, ice-covered shoreline areas (Osmundson and Kaeding 1989; Wick and Hawkins 1989). In spring and early summer, adult squawfish use shorelines and lowlands inundated during typical spring flooding, This natural lowland inundation is viewed as important for their general health and reproductive conditioning (Osmundson and Kaeding 1989; Tyus 1990). Use of these habitats presumably mitigates some of the effects of winter stress. and aids in providing energy reserves required for migration and spawning. Migration is an important component in the reproductive cycle of Colorado squawfish. Tyus (1990) hypothesized that migration cues, such as high spring flows, increasing river temperatures, and chemical inputs from flooded lands and springs. may be i:mportant to successful reproduction. In the Green River Basin, larval Colorado squawfish emerge from spawning substrates and enter the stream drift as young fry (Haynes et a1. 1989). The larval fish are actively or passively transported downstream for about 6 days. traveling an average distance of 160 Ian (100 mil to reach nursery areas in lower gradient reaches [Tyus and Haines 1991). rnese areas are nutrlent.rich habitats that consist of