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Green River system, especially if population augmentation is attempted. Razorback Sucker Adult razorback suckers in the Green River basin are old individuals (Tyus 1987; Lanigan and Tyus 1989; U.S. Fish and Wildlife Service, unpublished data), and the small number of reproducing razorback suckers is considered limiting. Razorback suckers spawn on the ascending limb of the hydrograph in the Green River basin; therefore, the interrelation of high spring flows, warming temperatures, and other factors are important for successful reproduction in this species. The apparent lack of widespread recruitment in this species has been attributed to habitat alteration, such as lower water temperatures (Marsh 1985) and predation by introduced common carp (Cyprinus carpio), green sunfish, and other nonnative fishes (Minckley 1983; Tyus 1987; Marsh and Langhorst 1988; Marsh and Minckley 1989). Brooks et al. (1985) documented significant predation of stocked larval and fingerling-size razorback suckers by channel catfish and flathead catfish (Pylodictis olivaris). Predation by nonnative fishes is believed to be a serious threat to the survival of razorback suckers and is a consideration in recovery efforts for this species. The absence of young fish in the Green River basin population may also be linked with the reduced availability of inundated shorelines due to curtailment of spring flooding following closure of Flaming Gorge Dam. The introduction of young fish (from sex products taken from wild fish captured on nearby Green River spawning grounds and reared at Ouray National Wildlife Refuge) may ultimately result in natural recruitment if the number of spawning adults-and consequently the number of young-is increased. Conclusions and Recommendations The Yampa and Green rivers constitute the most important riverine system for the maintenance and recovery of rare Colorado River fishes. Flows of the Yampa River are singularly important for providing a natural shape to Che hydrograph of the mainstream Green River and Chereby mitigating possible adverse effects of flow regulation on the native fish fauna. Flows of the Yampa River, particularly spring runoff, may also enhance usable rare fish habitat by inhibiting the invasion and proliferation of introduced fishes that evolved in more mesic environments. As indicated in previous discussions, the Colorado squawfish and razorback sucker depend on habitats in the Yampa and Green rivers for fulfillment of various life history requirements. Therefore, these two river basins must be considered as a single ecosystem when determining the needs of indigenous rare fishes. The Green River basin supports the largest numbers of Colorado squawfish (Tyus 1989) and razorback suckers (Lanigan and Tyus 1989] in native riverine habitats. The humpback chub is self-sustaining in the Yampa River and represents one of few remaining extant populations of this species. The persistence of native fishes in the Yampa River indicates that habitat conditions are suitable for their survival, despite the proliferation of many nonnative fishes. Population augmentation and study of razorback suckers and bonytail chubs in the upper Green River are proceeding and new information will hopefully aid in their recovery. Flow needs of the rare fishes in the Yampa River are determined by many factors, including time of year, life history stage, and associated species. Reproductive activities of the Colorado squawfish, razorback sucker, and humpback chub in the Yampa River are closely associated with spring runoff (Fig. 15). Alteration of this hydrologic event may affect initiation of Colorado squawfish migration and spawning of Colorado squawfish, humpback chubs, razorback suckers, and other native fishes. Maintenance of low, stable flows in late summer and fall is necessary for growth and survival of young Colorado squawfish and presumably young of the other rare native fishes (Fig. 15j. In addition, stable flows through ice breakup are important ko overwinter survival of young and adults. Abrupt fluctuations in water level from late summer to spring could strand Colorado squawfish (larvae and adults) and presumably other native fishes. The relations shown in Fig, 15 indicate that the natural flow events characterizing the aoo 380 300 ~ zso 200 a (=j 150 s 100 SO Fig. 15. Conceptual relation between Yampa River annual distribution hydrograph (1922-87 average) and timing of some life history events of Colorado squawfish, razorback sucker, and humpback chub. (Spring, 21 March-21 June; summer, 22 June-22 September; autumn, 23 September-21 December; winter, 22 December-2D March. CS =Colorado squawfish, HB =humpback chub, RZ = razorback sucker, M = migration, S = spawning, N = nursery, W = winter.] 22 army °~"~"~ SfASON ~ vim. ..