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INTRODUCTION The bonytail (Gila elegans) is thought to be functionally extirpated from the upper Colorado River basin (UCRB). Listed as Federally endangered in 1980, their decline has been attributed to loss of habitat, alteration of flow regimes, hybridization with other Gila species and negative interactions with nonnative fishes (Vanicek and Krammer 1969; USFWS 1990; Chart and Cranney 1993; Quartarone 1993). Of the four large river fishes listed as endangered in the Colorado River Bas~ bonytail may be the most rare. Since their listing in 1980, there have only been a few captures of adult bonytail in the upper basin and no documented recruitment (Valdez 1990; Kaeding et al. 1986; Tyus et aI. 1987). Since the mid-1980's, efforts to improve riverine function and habitability for native fishes have been undertaken. Examples of these efforts include alteration of dam operations to include seasonal variability in flow, reconnection of floodplains, and attempts to reduce nonnative fish numbers. Although these efforts could have positive effects on endemic fishes and their recruitment, bonytail do not have the necessary population base to reestablish themselves. Therefore, reintroduction of hatchery-reared fish is seen as a necessary step for the recovery of a wild bonytail population (Valdez and Clemmer 1982). The Upper Colorado River Endangered Fish Recovery Program (UCRRP) required that a stocking plan be approved before hatchery-reared fish could be stocked. Lentsch et al. (1996) produced the Bonytail Reintroduction Plan for the upper Colorado River to outline the reestablishment ofbonytail in the upper Colorado River basin. This plan provided a 6-year guideline for stocking and monitoring hatchery-reared juvenile bonytail, and tracking radio-tagged adult bonytail. Laboratory experiments were developed to examine predator interactions and conditioning of hatchery-reared juveniles to maximize post-stocking survival (Alder and Crowl 1995; Crowl and Rivera 2000). The results from these experiments and each year's monitoring were to be assessed and incorporated into the following year's stocking and rearing methods. In 1998, the interim management objective (IMO) for the UCRB bonytail stock was defined (Lentsch et al. 1998). The IMO specifies the population size required to establish long-term persistence of a self-sustaining population. The broodstock being used to reintroduce bonytail into the UCRB has been established from a single relict population found in Lake Mohave (Minkley et al. 1989). It was determined that reestablished upper basin populations would constitute one stock for management purposes (Czapla 1999). The IMO level for the entire UCRB stock was set at 4,118 (:f: 817) bonytail, which assumed migration between all stocked populations. In 1998 and 1999, the Five-Year Stocking Plan for Endangered Colorado River Fish Species in Colorado (Nesler 1998) and the State of Utah Stocking Plan for Endangered Fish Species of the upper Colorado River basin (Hudson et al. 1999) were completed. These plans updated the overall stocking objectives and IMO levels for the upper basin. Monitoring and radio-tracking ofbonytail and empirical data from humpback chub (Gila cypha) populations suggested that little migration was likely to occur between stocked populations. Therefore, fish released at the three stocking locations in the Green, Colorado, and Yampa rivers were delineated as separate populations in terms of persistence. For this reason, the original IMO level was applied to each population to maintain adequate numbers for long-term persistence. 1