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21 spring. Bulkley and Pimentel (1983) reported that razorback suckers preferred temperatures of about 22-25 °C and avoided temperatures of 8-15 °C. Razorback sucker eggs taken in the Green River experienced poor hatching at 11 °C due to fungus, but hatching was successful (80 percent or more) when incubated at 20 °C (U.S. Fish and Wildlife Service, unpubiish~d data),. Marsh (1985) noted optimal hatch in razorback sucker larvae incubated at 2Q °C, but historic water temperatures in the upper Green River at Jensen did not average 20 °C until June-July (Smith and Green 1991). Thus, required spawning and incubation temperatures of razorback sucker are not well understood and continue to be studied. Larval and Juvenile The capture and artificial spawning of ripe razorback suckers in the lower Yampa and upper Green Rivers (Severson et al. 1990) and the tentative identification of larvae in upper Green River seine collections (R.T. Muth and D. Snyder, pers. comm.) suggests that razorback suckers reproduce successfully, but there is little recruitment there or elsewhere in the Colorado River Basin (Holden 1978; McAda and Wydoski 1980; Minckley 1983; Tyus 1981; Marsh and Minckley 1989; Tyus and Karp 1990). The existing standing crops of razorback sucker in the Green River are presumably old fish (Minckley et a1., in prep.) that were spawned in the mid-1960's. The apparent lack of widespread recruitment in razorback sucker was attributed to habitat alteration, such as lower water temperatures (Marsh 1985) and predation by introduced common carp (Cvarinus ar io), green sunfish, channel catfish, flathead catfish (Pvlodictus olivaris), and other nonnative fish (Minckley 1983; Brooks et al. 1985; Tyus 1987; Marsh and Langhorst 1988; Marsh and Minckley 1989). Predation by nonnative fish is believed a serious threat to the survival and recovery efforts of razorback sucker. The absence of young fish in the Green River basin population also may be linked with the reduced availability of inundated shorelines due to curtailment of spring flooding. Naturally inundated lowlands, such as Old Charley Wash and the Stewart Lake area, that were readily accessible to razorback suckers in the spring are now less accessible due to blockage and low water ]evels following closure of Flaming Gorge Dam. Habitat requirements of this species in riverine environments 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 (McCarthy and Minckley 1987; Tyus 1987)-: Adult razorback suckers in the Green River basin are old individuals (Tyus 1987; Minckley et al., in prep.) and the low number of reproducing razorback suckers is considered limiting (Lanigan and Tyus 1989; Tyus and Karp 1990). Nonnative fish, high spring flows, warming river temperatures, and inundated shorelines and bottomlands all appear as factors that influence successful reproduction and recruitment in razorback sucker.