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allowed adaptation to their environment in the Upper Colorado River Basin. If the endangered fish stock or population from a specific river reach has been extirpated or is extremely low, then the founders should be obtained from a stock that would be the nearest neighbor from an evolutionary (not necessarily geographic) standpoint. Fish from nearest neighbor stock should exhibit life history patterns similar to the extirpated population and be taken from an environment that has the greatest ecological similarity to the environment where the species is to be restored. If restoration cannot be done by natural reproduction, small numbers of wild founders should be collected from the wild stock to minimize demographic impacts on the genetic resources, life history patterns, and natural recruitment of the wild stock. F. -Estimating the Number of Fish Required to Produce Broodstocks. The number of endangered fish needed at the end of each growing season to produce broodstocks is dependent upon the attrition (i.e., mortality) rates of the fish at the end of each growing season. Conservative attrition rates were estimated by consensus among members of the Propagation Subcommittee and Biology Committee. These attrition rates will be refined from empirical data from captive propagation of the fish in different facilities. Estimates of razorback suckers or Colorado squawfish needed at the end of each growing season until the fish mature and can be maintained as broodstocks in a primary and a backup refuge are provided in Figure 2. To be certain that about 30 adults (preferably equal sexes) from each family lot are available as broodstocks in primary and backup refuges, the numbers of fish required at the end of each growing season is estimated to be: 1 - 500 fish; 2 - 250 fish; 3 - 200 fish; 4 - 160 fish; 5 - 152 fish; and 6 - 144 fish. The bonytail and humpback chub are expected to mature at the end of the fourth growing season. About 200 fish per family lot would be needed at the end of the first growing season to produce 60 adults based on present estimated attrition rates. The production of 300 fish per family lot at the end of the first growing season would ensure that 60 Gila adults survive until the end of the fourth growing season. Obviously, more endangered fish will be produced than are needed for broodstock development. Disposition of fish will follow the guidelines provided in Section VI of this document. G. Important Considerations in Ca tive Pro a ation of Endangered Fishes. Equal sex ratios among breeding adults (Kapuscinski et al. 1993; Lacava and Hughes 1989; Tave 1984), equal numbers of offspring among family lots from pedigreed matings (Allendorf 1993; Kapuscinski et al. 1993), extended intervals between generations, and avoidance of selection are important in maintaining large effective population sizes and genetic diversity of broodstocks in captive breeding programs (Frankham et al. 1992; Kapuscinski et al. 1993). Ignoring these factors has resulted in rapid loss of genetic diversity in broodstocks under captive rearing conditions (Allendorf 1993; Kapuscinski et al. 1993). Equal numbers of fish from pedigreed family lots at the time of release will provide genetic diversity that will increase effective population size and reduce the potential for inbreeding while enhancing the potential for adaptation and survival after stocking (Allendorf 1993; Kapuscinski et al. 1993). Captive rearing of endangered fish results in a gain in total offspring but with a tradeoff in which there may be a simultaneous reduction in the effective population size (Ryman and Laikre 1991). A reduction in 22 11 C 1