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of only about 1,000 fish (Lanigan and Tyus 1989). Recent information suggests that this population may have declined further. In the absence of conservation efforts, it is presumed that wild populations will be lost as old fish die and are not replaced. Reproduction and habitat use of razorback suckers has been studied in lower basin reservoirs, especially in Lake Mohave. Fish reproduction has been visually observed in reservoir shorelines for many years. The fish spawn over mixed .substrates that range from silt to cobble, and at water temperatures ranging from 50 to 700P (reviewed by Minckley et aI. 1991), Habitat use and spawning behavior of adult razorback suckers in riverine habitats have been studied by radiotelemetry in the Green River Basin (Tyus and Karp 1990) and in the upper Colorado River (Osmundson and Kaeding 1989). The fish in the Green River Basin spawned in the spring with rising water levels and increasing temperatures. The fish moved into flooded areas in early spring, and they made spawning migrations to specific locations as they became reproductively active. Spawning occurred over rocky runs and gravel bars. In nonreproductive periods, adult razorback suckers occupy a variety of habitat types. These include impounded and riverine areas and habitats represented by: eddies, backwaters, gravel pits, flooded bottoms and the flooded mouths of tributary streams, slow runs, sandy riffles, and others (reviewed by Minckley et aI. 1991). Summer habitat use included deeper eddies, backwaters, holes, and rnidchannel sandbars (Osmundson and Kaeding 1989; Tyus and Karp 1990; Minckley et aI. 1991). Habitats used by young razorback suckers have not been fully evaluated because of the low number of young fish present in the river system. However, most studies indicate that the larvae prefer shallow, littoral zones for a few weeks after hatching, then they disperse to deeper water areas (reviewed by Minckley et al, 1991), Laboratory studies indicated that, in a riverine environment, the larvae enter stream drift and are transported downstream (Paulin et al. 19.89). During winter, adult razorback suckers utilize 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). Although habitat use of razorback suckers has been studied for years, the habitat preferences and factors limiting their abundance in native riverine habitats are not well known because of the scarcity of extant populations (Minckley 1983; Lanigan and Tyus 1989) and the absence of younger life history stages (Minckley et al. 1991). However, based on available data, Tyus and Karp (1989) and Osmundson and Kaeding (1989) considered interactions with nonnative fishes, impacts to low winter flows, high spring flows, seasonal changes in river temperatures, and inundated shorelines and bottomlands as factors that potentially limit the survival, successful reproduction, and recruitment of this species. 14