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Dowling and DeMarais (1993) reported that introgressive hybridization played an important role in creating a high morphological diversity among Gila minnows in western North America. Dowling and DeMarais stated that the Colorado River Gila represent a complex of self-sustaining, genetically distinctive species that are capable of exchanging genetic material, comparable to syngameons in plants. Although the phylogenetic linkages are distinct, local introgression has clearly occurred among three Colorado River Gila in the past. The bonytail broodstock at the Dexter National Fish Hatchery, New Mexico, originated from 18 individuals that were obtained from Lake Mohave between 1976 and 1981 and were subsequently supplemented with 6 additional adults in 1988 (Minckley et al. 1989). Twenty-four second generation bonytails contained six polymorphic loci and the heterozygosity was comparable to mean values reported for other western North American cyprinids. Minckley et al. (1989) concluded that the genetic variability of the Lake Mohave broodstock was suitable for restoration stocking into appropriate natural environments. C.. Current Trends of Presumptive Stocks. Current trends of presumptive stocks of the endangered Colorado River fishes were based on the criteria of identified spawning sites, successful recruitment, relative stock status, potential for catastrophic risk, and probability for extinction (Table 2). D. Current Status of Presumptive Stocks. The current status of presumptive stocks of endangered fishes in the Upper Colorado River Basin is concisely summarized in the following summary. Razorback sucker Critical - Old individuals in current presumptive stocks are declining rapidly. The presumptive stock in the middle Green River between the confluence with the Duchesne and Yampa rivers is low but some recruitment occurs in this reach as indicated by the presence of small fish (<475 mm). However, this re- cruitment is not sufficient to maintain a self- sustaining population of this species in that river reach. Bonytail Nearly extirpated in the upper basin; only occasional individuals are captured very infrequently (i.e., years between captures). Broodstocks will have to be developed from Lake Mohave stock for restoration stocking in the upper basin. Humpback chub Apparently stable but stocks are small in isolated deep canyon areas; stocks in Black Rocks and Westwater Canyons on the Colorado River may be at catastrophic risk since a railroad tract is close to the river at this site and there is a potential of an accident such as a chemical spill. Stocks in Desolation-Gray Canyons and in the Yampa River exhibit large variation in phenotypes and may be at risk from hybridization from roundtail chubs. 13