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one standard deviation of the mean per freshwater species. Although a comparison with general estimates for freshwater species is crude, it provides some indication that levels of genetic variation in Virgin River chub are normal relative to other species. The Moapa River population had lower values than other sample sites for all three measures of genetic diversity. Mean allelic richness and expected heterozygosity for the Moapa River population (A = 5.34; HE = 0.56) were lower than in the other populations (A ranged from 6.11 to 6.28; HE ranged from 0.67 to 0.68). In addition, the Moapa River population (N= 66) only had six unique alleles compared to 21 unique alleles present in the Virgin River (N= 65). The lower levels of genetic variation found in chub occupying the Moapa River may indicate that historically this population was smaller than the Virgin River population. An alternative possibility is that this population has been negatively influenced by the presence of non-native species. Exotic species can impact a population of native fish by predation, competition or a combination of the two at any life history stage of the native species. Unlike the upper Virgin River, there is no barrier to prevent non-native species from invading the Moapa River. During a sampling trip in 2007, the majority of fish caught in the Moapa River were non-native species, including the reach above the Wells-siding Diversion where the highest numbers of Virgin River chub are found (Albrecht et al. 2008). The Dexter broodstock and progeny had similar levels of genetic variation (A ranged from 6.11 to 6.28; HE = 0.67) when compared to the Virgin River populations (A ranged from 6.15 to 6.22; HE ranged from 0.67 to 0.68). However, the Dexter population (N = 92) had five unique alleles compared to 10 unique alleles in the Virgin River populations (N= 65), the source of most of the captive stock. The fact that each group 18