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<br />Green River system, especially if population <br />augmentation is attempted. <br /> <br />Razorback Sucker <br /> <br />Adult razorback suckers in the Green River basin are <br />old individuals (Tyus 1987; Lanigan and Tyus 1989; U.S. <br />Fish and Wildlife Service, unpublished data), and the <br />small number of reproducing razorback suckers is <br />considered limiting. Razorback suckers spawn on the <br />ascending limb of the hydrograph in the Green River <br />basin; therefore, the interrelation of high spring flows, <br />warming temperatures, and other factors are important <br />for successful reproduction in this species. <br />The apparent lack of widespread recruitment in this <br />species has been attributed to habitat alteration, such as <br />lower water temperatures (Marsh 1985) and predation <br />by introduced common carp (Cyprinlls carpio), green <br />sunfish, and other nonnative fishes (Minckley 1983; <br />Tyus 1987; Marsh and Langhorst 1988; Marsh and <br />Minckley 1989). Brooks et al. (1985) documented <br />significant predation of stocked larval and <br />fingerling-size razorback suckers by channel catfish and <br />flathead catfish (Pylodictis olivaris). Predation by <br />nonnative fishes is believed to be a serious threat to the <br />survival of razorback suckers and is a consideration in <br />recovery efforts for this species. The absence of young <br />fish in the Green River basin population may also be <br />linked with the reduced availability of inundated <br />shorelines due to curtailment of spring flooding <br />following closure of Flaming Gorge Dam. The <br />introduction of young fish (from sex products taken <br />from wild fish captured on nearby Green River <br />spawning grounds and reared at Ouray National <br />Wildlife Refuge) may ultimately result in natural <br />recruitment if the number of spawning adults - and <br />consequently the number of young - is increased. <br /> <br />Conclusions and Recommendations <br />The Yampa and Green rivers constitute the most <br />important riverine system for the maintenance and <br />recovery of rare Colorado River fishes. Flows of the <br />Yampa River are singularly important for providing a <br />natural shape to the hydrograph of the mainstream <br />Green River and thereby mitigating possible adverse <br />effects of flow regulation on the native fish fauna. Flows <br />ofthe Yampa River, particularly spring runoff, may also <br />enhance usable rare fish habitat by inhibiting the <br />invasion and proliferation of introduced fishes that <br />evolved in more mesic environments. As indicated in <br />previous discussions, the Colorado squawfish and <br />razorback sucker depend on habitats in the Yampa and <br />Green rivers for fulfillment of various life history <br />requirements. Therefore, these two river basins must be <br />considered as a single ecosystem when determining the <br />needs of indigenous rare fishes. <br /> <br />The Green River basin supports the largest numbers <br />of Colorado squawfish (Tyus 1989) and razorback <br />suckers (Lanigan and Tyus 1989) in native riverine <br />habitats. The humpback chub is self-sustaining in the <br />Yampa River and represents one of few remaining <br />extant populations of this species. The persistence of <br />native fishes in the Yampa River indicates that habitat <br />conditions are suitable for their survival, despite the <br />proliferation of many nonnative fishes. Population <br />augmentation and study of razorback suckers and <br />bonytail chubs in the upper Green River are proceeding <br />and new information will hopefully aid in their recovery. <br />Flow needs of the rare fishes in the Yampa River are <br />determined by many factors, including time of year, life <br />history stage, and associated species. Reproductive <br />activities of the Colorado squawfish, razorback sucker, <br />and humpback chub in the Yampa River are closely <br />associated with spring runoff (Fig. 15). Alteration of this <br />hydrologic event may affect initiation of Colorado <br />squawfish migration and spawning of Colorado <br />squawfish, humpback chubs, razorback suckers, and <br />other native fishes. Maintenance of low, stable flows in <br />late summer and fall is necessary for growth and survival <br />of young Colorado squawfish and presumably young of <br />the other rare native fishes (Fig. 15). In addition, stable <br />flows through ice breakup are important to overwinter <br />survival of young and adults. Abrupt fluctuations in <br />water level from late summer to spring could strand <br />Colorado squawfish (larvae and adults) and presumably <br />other native fishes. The relations shown in Fig. 15 <br />indicate that the natural flow events characterizing the <br /> <br />'" <br /> <br />-. - - - - -~~:~. - - _. -- <br /> <br />.c_'.-.ld__ <br /> <br />'50 <br /> <br />.c.s___s. <br />i'!8:_s <br /> <br />'00 <br /> <br />~ <br />'5250 <br />~2.00 <br />5150 <br />g <br /> <br />'" <br /> <br /> <br />.__....__~_-.N__ <br /> <br />__c:s.-!I!-.!l;:.w.__ <br /> <br />so <br /> <br />S~~NG <br /> <br />"..... <br /> <br />AUT\,IMN <br /> <br />~"" <br /> <br />SEASON <br /> <br />Fig. 15. Conceptual relation between Yampa River annual <br />distribution hydrograph (1922-S7 average) and timing of <br />some life history events of Colorado squawfish, rawrback <br />sucker, and humpback chub. (Spring, 21 March-21 June; <br />summer, 22 June-22 September; autumn, 23 <br />September-21 December; winter, 22 December-20 March. <br />CS = Colorado squawfIsh, HB = humpback chub, RZ = <br />razorback sucker, M = migration, S ;:;;:: spawning, N ;:;;:: <br />nursery, W = winter.) <br /> <br />22 <br />