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106 PARENTS 3 i 5 F1 Offspring FIGURE 2.-Diagram of the di-allele crossbreeding strategy for maintaining genetic diversity and developing razorback sucker broodstock in the upper Colorado River basin. production and reintroduction of progeny into suit- able habitat to augment declining or restore self- sustaining razorback sucker populations. Managing captive populations to maximize genetic diversity preserves future options for effective management and recovery of the taxon (Ralls and Ballou 1992). Razorback sucker populations most threatened by extinction provide a management opportunity and priority for establishing AGRs or gene banks. AGRs provide a refuge of genetic material in the event a natural catastrophe destroys a majority of the wild stock. Except for Lake Mohave in the lower basin and the middle Green River in the upper basin, accurate population estimates do not exist for razorback sucker populations. Nevertheless, monitoring data suggest that the razorback sucker population in the upper Colorado River basin is small. Therefore, a di-allele crossbreeding system (Williamson and Wydoski 1994) is being used to develop AGR pop- ulations for the upper Colorado River basin (Figure 2). This crossbreeding system is particularly valu- able when the total number of adults available is small and the contribution of each individual is critical to maintaining population characteristics. A di-allele mating strategy ensures production of ev- ery genotypic combination possible among the few fish. Each fish is genetically represented in the next generation in five unique lots, thus greatly increas- ing the likelihood of preserving allelic diversity in MODDE ET AL. PARENTS A B C D E 7cI the offspring of the captive stock (Williamson and Wydoski 1994). The matrix produces 25 half-sibling family lots; the five lots along the diagonal are unique (i.e., unrelated). They provide the matrix core. The remaining family lots provide security of parental contribution through time in the event of a loss of a unique lot. The number of adults used in the matrix is arbitrary; however, any number much beyond five males and five females becomes un- wieldy and the utility of the di-allele matrix ques- tionable. After the F1 broodstock has been devel- oped from the five by five cross, additional wild fish will be used to supplement the genetic component of the original founders when available. Stocking The goal of razorback sucker stocking is to assist the establishment of naturally sustainable popula- tions while posing a minimum risk to the endemic fish population. The widespread stocking of mil- lions of razorback sucker in the lower Colorado River basin during 1981-1987 has produced disap- pointing results (Minckley et al. 1991), and the stocking of 6,400 advanced fingerling razorback suckers in the middle Green River during 1987- 1989 has failed to produce a single recapture (U.S. Fish and Wildlife Service, Colorado River Fish Project, unpublished data). However, these stock- ing efforts were initiated without habitat modifica-