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Me9e and Vrijenhoek which can be interpreted as the equivalent of one mi- grant between populations per generation. The effective migration rate is much higher along the 10 km portion of Salt Creek inhabited by G bovinu~ being roughly equivalent to an exchange of 17 to 18 individuals be- tween colonies per generation. Values for G pecosensu and C elegans, respectively, are three and two individ- uals per generation. These estimates, however, are sensitive to the under- lying model of population structure (Slatkin 1985). Al- though accurate in a true island situation, they probably underestimate gene flow in a hierazchical situation, and more realistic models of population structure are needed. Even if such models were available, though, it is likely that each dendritic system would have idiosyn- cratic properties that would require simulation to more accurately estimate gene flow. Thus, no single analytical approach would pertain in every case. Nevertheless, a useful rule of thumb is that an exchange of one individ- ual between populations per generation within a river system would maintain most of the allelic diversity, but would not swamp the opportunity for local adaptation (Allendorf 1983). Management of species such as these Cyprinodon should be aimed at preserving the integrity of each spe- cies while maintaining its genetic variability. Movement of fish of the same species among colonies within a river (except, perhaps, for G tularos~ which has the highest D«) would probably pose no specific problems since this appears to occur naturally. However, precautions should be taken to avoid mixing between species. If mixed, it is likely that all four of these Cyprinodon spe- cies could hybridize (see Turner & Liu 1977; Cokendol- pher 1980); introduced Cyprinodon variegates have already hybridized with native C. pecosensu (Echelle, Echelle, & Edds 1987). Into the Future-How Do We Proceed with Management of Southwestern Fishes? The coming decades will present tremendous chal- lenges to the manager of dwindling aquatic resources in the Southwest. Continued population growth in the re- gion will further drain limited natural resources, the spread of exotics will further influence local biota, and the status of native species will certainly continue to decline. However, careful management, based on sound genetic and ecological studies, can slow or reverse these negative trends. One of the more powerful tools in the effort to maintain healthy, diverse populations is the application of genetic principles to management. We particulazly emphasize the need to incorporate experimental studies of population genetics and fitness Conservation Geaeacs of Desert Fishes 165 into management of endangered fishes to better predict the survival and long-term success of remnant popula- tions. Such reseazch would directly benefit the tazget species, and the principles could be applied co manage- ment of other taxa as well. Desert fishes provide oppor- tunities that do not exist with most endangered species. Populations of whooping cranes, cheetahs, or condors cannot be manipulated to determine optimal genetic structure. However, such studies are feasible with many endangered fishes. We thus recommend pursuit of at least three azeas of experimentation that would benefit management of threatened and endangered fishes in the Southwest. 1. Test the relationship betu+een heterozygosity and individual fitness Studies with a variety of organisms have resulted in a "heterorygosity concensus," the be- lief that overall fitness is enhanced by genetic diversity (Frankel & Soule 1981; Mitton & Grant 1984). How- ever, we know little of actual mechanisms of heterory- gote superiority, and we do not know whether all species or populations are affected similarly by heterorygosity. Experimental studies of relationships between genetic diversity and growth rates. survivor- slu~ fecundity, developmental stability, and competi- tive ability aze needed to determine the importance of maintaining populations with high heterorygosity. Such stud esi u e currently underway in our laboratories with poeciliid fishes, and early results indicate superior fit- ness of more heterozyttous individuals in some cases: we encourage exploration of such questions in other groups. 2. Experimentally mix genetic stocks Although we strongly discourage mixing stocks among major genetic subdivisions, we encourage stocking experiments that combine populations from within the same subdivision. Such experiments would permit evaluation of the ef- fects of mild outcrossing and may prove to be valuable in restocking efforts. For example, in P. occidentalis from Group I in Arizona, Cienega Creek and Monkey Spring fish aze both homorygous for all loci examined (Appendix). Preliminary laboratory results indicate that Monkey Spring fish may grow slower than individuals from the more polymorphic Sharp Spring population (Vrijenhoek & Sadowski, unpublished data): Because they are fixed for alternative alleles at two loci, crosses of Cienega Creek (CC) and Monkey Spring (MS) stocks should increase heterorygosity for these and many other loci. Tests of relative fitness of the CC/MS "hybrids," the parental stocks, and the Sharp Spring stock would indicate how outaossing and concomitant increase in heterorygosity affects fitness. If such exper- imental crosses prove to be more robust than single- locality fish, their use in stocking new field sites should be considered. Existing„natural localities should always remain genetically pure, however. Conservation Biology Volume 2, No. 3, June 1988