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AN AUGMENTATION PLAN FOR RAZORBACK SUCKER large river systems, adults have survived well in both lacustrine (Wallis 1951; Marsh and Langhorst 1988) and riverine (Tyus 1987) habitats. However, be- cause few immature fish have been captured, little is known of their habitat needs. Most suspected larval razorback suckers have been collected in slack-wa- ter habitats in both the Green and Colorado rivers (R. Muth and D. Snyder, personal communication). Tyus and Karp (1990) observed that razorback sucker spawns during increasing and peak spring flows suggested that flooded bottomlands are nec- essary for recruitment. Modde, Burnham, and Wick (unpublished data) reported limited recruitment of the razorback sucker in the middle Green River and found a significant positive correlation between the number of small (<475 mm total length) razorback suckers collected during 1980 and 1992 and spring flow magnitude 5 years prior to their collection. Discovery in 1991 of several hundred razorback suckers of the same apparent age in Etter Pond (F. Pfeifer, personal communication), which is adjacent to the Colorado River and had been isolated since the 1984 flood, suggests these flooded bottomlands adjacent to the main channel are important to re- cruitment. Larval razorback suckers stocked into isolated coves in Lake Mohave (Mueller et al. 1993) and a gravel pit adjacent to the Colorado River (Osmundson and Kaeding 1989) demonstrated rapid growth and relatively high survival. Rearing of razorback suckers in hatchery ponds at the Dexter National Fish Hatchery and Technology Center support this hypothesis. Given the limited informa- tion available, it appears recruitment of razorback sucker is likely associated with high-flow events, most notably with the availability of flooded bot- tomlands. Recovery Plan Habitat Enhancement Floodplain enhancement in the upper Colorado River basin is a necessary component to any aug- mentation plan if self-sustaining populations of ra- zorback sucker are desired. Wetlands needed for razorback sucker recruitment can be provided by removing barriers to historic bottomlands and by providing sufficient flow to inundate bottomlands in a manner that approximates the natural hydro- graph. Because the peaks and duration of high flows have decreased following construction of main-stem impoundments, habitat and water flow manipula- tions will be necessary to regain connectivity of wetlands at the lower flows. Management (e.g., draining and water elevation control) of flooded 105 bottomlands will be needed during the early phases of the recovery process to reduce the abundance of nonnative fish predators and to capture and mark razorback suckers produced in these nursery sites. Marking of fish produced in rehabilitated bottom- lands is imperative to monitor the importance of these sites to recovery. Specific bottomland man- agement strategies that will maximize recruitment have yet to be developed. In isolated coves in Lake Mohave, Mueller et at. (1993) have successfully reared razorback suckers to nearly adult size (>400 mm total length) in a single growing season. In the upper Colorado River basin, two growing seasons may be required before vulnerability of razorback suckers to predators is reduced substantially. Flooded bottomland management strategies that will be effective in the upper Colorado River are in the process of being developed and will be deter- mined by available floodplain resources. Experi- mental evaluation of flooded bottomland manage- ment is currently underway at Ouray National Wildlife Refuge, Ouray, Utah. Captive Propagation Strategy The goal of the captive propagation program for razorback sucker in the upper Colorado River basin is to avoid imminent extinction and preserve genetic diversity of the species, particularly the heritable component associated with fitness, adaptation, and long-term survival. The intent is to rely on inherent genetic characteristics that will increase the proba- bility that fish reintroduced into suitable habitat will survive, reproduce, and recruit over successive gen- erations, ultimately resulting in a naturally sustain- ing population. Genetic conservation units (GCU) are local riv- erine stocks defined by spawning location, move- ment and distribution, and genetic information, in that order (Simpson 1961; Mayr 1977; Utter 1981; Dizon et al. 1992; Ruggiero et al. 1994). Dowling and Minckley (1993) suggested, based on haplotype similarity, the razorback sucker in the entire Colo- rado River drainage may be panmictic. In view of repeated homing and fidelity of razorback sucker to spawning sites previously determined from a tag- ging and telemetry study (Tyus 1987), we have taken a conservative approach of identifying each major tributary as a unique GCU until conclusively proven otherwise. The first captive propagation objective is devel- opment of artificial genetic refuges (AGRs), pro- duction broodstocks, and progeny of presumed stocks. The second captive propagation objective is