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AN AUGMENTATION PLAN FOR RAZORBACK SUCKER response to the need to recover the razorback sucker, as well as the remaining three Endangered fishes of the main-stem upper Colorado River basin (Colorado squawfish Ptychocheilus lucius, hump- back chub Gila cypha, and bonytail G. elegans), a multi-agency recovery program was established among state and federal agencies and private inter- est groups in the upper Colorado River basin to accommodate water development and fish recovery simultaneously (Wydoski and Hamill 1991). The goal of this recovery program is to establish and protect self-sustaining populations and natural hab- itat of the razorback sucker (USFWS 1987). Among several components of the program are habitat re- habilitation, including minimum-flow requirements, and artificial propagation. This paper outlines a strategy to recover razorback sucker in the upper Colorado River basin by enhancing habitat neces- sary for recruitment and developing a population augmentation program that includes a genetic man- agement plan, a decision-making process to deter- mine when stocking is necessary, and the use of chemical imprinting to increase management op- tions following stocking. The initial step in the development of a recovery plan is the identification of population units of con- cern. A baseline genetic survey that included all known wild and captive populations of razorback sucker was conducted throughout its range in both upper and lower Colorado River basins. Two meth- ods of genetic analysis were chosen-mitochondrial DNA, by means of an analysis of restriction frag- ment length polymorphisms, and allozyme varia- tion, by means of starch gel protein electrophoresis. Preliminary results support provisional division of the upper Colorado River basin razorback sucker populations into Green River, Colorado River, and San Juan River stocks. In addition to genetic find- ings, behavioral and distributional data support this hypothesis. The razorback sucker occupying the Green River represents the largest population of this species occupying riverine habitat (Minckley et al. 1991). Lanigan and Tyus (1989), assuming no recruitment or tag loss over a nine-year period, estimated the middle Green River razorback sucker population to be 948 fish (95% confidence interval, 758-1,138). Using the same data, plus 4 years of additional data, Modde, Burnham, and Wick (unpublished data) estimated the population to be 524 fish (95% con- fidence interval 351-696). The latter study reported limited recruitment, an adult survival rate of 71%, and an unspecified rate of tag loss. The razorback sucker in the middle Green River 103 has been observed at two spawning sites (Figure 1), one approximately 16 km below Split Mountain Canyon (defined as the Jensen site by Tyus and Karp 1990) and the other at the mouth of the Yampa River just before it enters the Green River. During the spawning season, razorback suckers can be collected regularly at both spawning sites. Some of these fish migrate 30-100 km and show fidelity to selected spawning sites in the Green and Yampa rivers in Colorado and Utah (Holden and Stalnaker 1975; Tyus 1987; Tyus and Karp 1990). Previous investigators have speculated that homing of razor- back suckers to these sites may be related to natal imprinting (Wick et al. 1982; Tyus and Karp 1990). Suspected larval razorback suckers were col- lected below the two spawning sites by Tyus (1987) and again in 1992, 1993, and 1994 by the U.S. Fish and Wildlife Service (D. Snyder and R. Muth, Lar- val Fish Laboratory, Colorado State University, personal communication). In 1993, 700 suspected larval razorback suckers were collected between the known spawning sites and the Colorado River arm of Lake Powell (E. Wick, National Park Service, personal communication). The dates when these fish were collected and their sizes suggested they could have originated from the middle Green River, as well as other unknown spawning sites in the lower Green River. Despite the relative abundance of the razorback sucker larvae in the Green and Colorado rivers, less than 10 juvenile razorback suckers have been collected in the upper Colorado River basin during the last 10 years (near the "Min- eral Bottoms" by the Utah Division of Wildlife Resources in 1991, and on the Ouray National Wildlife Refuge in 1993). In the upper Colorado River, razorback sucker adults were commonly collected during the 1970s (Minckley et al. 1991). Despite extensive sampling, very few adult razorback suckers have been col- lected in the upper Colorado River main stem since 1986, and only seven razorback sucker adults have been collected from riverine habitats since 1988 (McAda et al. 1994). Recently a gravel pit near Grand Junction, Colorado, was found to support several hundred razorback suckers (F. Pfeifer, U.S. Fish and Wildlife Service, personal communica- tion). However, genetic analysis (A. Echelle, D. Philipp, and F. Allendorf, Recovery Program Ge- netics Advisory Panel, personal communication) personal communication) of these fish indicated that most were either the offspring of a limited number of parents that spawned in the pond during the high-flow years of 1983 and 1984 or the off- spring of hybridization between razorback sucker