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4 GENETIC VARIATION IN COLORADO SQUAWFlSH 439 TABLE 3.-Measures of genetic variability calculated for Colorado squawfish by use of the BIOSYS-I program of Swofford and Selander (1981). Standard errors are in parentheses. A locus is considered polymorphic if the frequency of the most common allele does not exceed 0.95. Mean number of Polymorphic Fish population Mean N per locus alleles per locus loci(%) Mean heterozygosity Dexter 1981 7.0 (0.0) l.l (0.1) 13.6 0.026 (0.012) Dexter 1987 30.3 (0.2) l.2 (0.1) 11.4 0.047 (0.021) Colorado River 25.8 (0.1) l.2 (0.1) 13.6 0.OS3 (0.024) Green River 25.4 (0.4) 1.1 (0.1) 9.1 O.Oll~.OI5) (3.1-5.3%). Thus, it seems that Colorado squaw- fish maintain a high level of heterozygosity throughout their natural range. Two loci (GPl-2* and EST-i*) deviated signif- icantly (P < 0.01) from expected Hardy-Weinbefl proportions in the Colorado River samples due to a heterozygote deficiency (Table 2). In the Green River population, EST-i'" deviation was signifi- cant at the P = 0.05 level, as revealed by the pool- ing procedure applied to loci with more than two alleles (Swofford and Selander 1981). Such devia- tions may suggest reduced gene flow between lo- calities and thus bring into question the definition of individuals in a river system as a single popu- lation. Nei's (1978) unbiased genetic identity (1) was greater than 0.99 for all pairwise comparisons. The cop he netic correlation coefficient was 0.896, an indication of a good fit of the data to the con- structed phenogram. Estimates of similarity among the four popu- lations of Colorado squawfish were similar to those observed among geographic populations of other cyprinid species. A vise and Ayala (1976) found little divergence (I = 0.99) between populations of Sacramento squawfish separated by compara- ble river distances. However, the hatchery stocks of Colorado squawfish are at most two generations removed from the initial wild specimens used as breeding stock. Significant effects of isolation and inbreeding, such as loss of variation and rare al- leles in the hatchery fish, were not evident in our study. Greater divergence may be seen after sev- eral more generations of breeding in these captive populations unless such variability is preserved by carefully designed breeding programs. We were able to resolve five polymorphic loci (GPl-i*, EST-i*, TPl-]*, TPl-2*, PEPS*) from fift tissue. These genetic markers are ideal for monitoring allele frequency changes in breeding stock of Colorado squawfish. This nonlethal sam- pling method has the potential to aid fishery man- agers in breeding and restocking programs involv- ing endangered species once informative genetic markers have been identified. Our data suggested that significant dri ft and bot- tleneck effects, as reflected by allozyme frequen- cies and average heterozygosities, have not oc- curred in the captive populations of Colorado squawfish. From these data, there is no reason to suspect significant changes in allele frequencies at loci that affect overall individual fitness. Had we detected significant drift effects by use ofallozym- ic markers, there might be reason to expect that selective changes under hatchery conditions may be lowering overall fitness (Allendorf and Ryman 1987). In summary, hatchery populations of Co 1- orado squawfish appear to be representative of the two wild populations sampled. Acknowledgments This research was supported by the Denver Re- gional Office of Endangered Species, U.S. Fish and Wildlife Service (USFWS). We thank Holt Wil- liamson of USFWS, Region 2, for transporting specimens. Lisa Clepper, Lela Limmer, and Rene Schmidt provided laboratory assistance. Mike Dixon and Brian Crother provided helpful com- ments on the manuscript. References Allendorf, F. W., and N. Ryman. 1987. Genetic man- agement of hatchery stocks. Pages 141-159 in N. Ryman and F. Utter, editors. Population genetics and fishery management. University of Washington Press, Seattle. Aquadro, C. E, and J. C. Avi5e. 1982. An assessment of "hidden" heterogeneity within electromorph5 at three enzyme loci in deer mice. Genetics 102: 269- 284. Avise, J. c., and F. J. Ayala. 1976. Genetic differen- tiation in specio5e versus depauperate phylads: evi- dence from the California minnows. Evolution 30: 46-58. Behnke, R. J., and D. E. Benson. 1980. Endangered and threatened fishes of the upper Colorado River