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There have also been concerns over the impact of hybridization among razorbacks and other species of suckers, especially flannelmouths. Introgression among suckers has likely always been a relatively common phenomenon, especially when sev~,ral species are sympatric and one or more are rare (Hubbs et ai., 1942; Hubbs, 1955). Therefore, detection of low levels of introgression between razorbacks and flannelmouths (like that observed in the Lake Mohave sample) is not surprising. Concerns over hybridization between these species are not unwarranted, but introgression does not yet seem to be of unusual magnitude. Samples from the Little and upper Colorado River regions were taken from areas where razorbacks are relatively uncommon, and the incidence of hybridization was apparently higher. Therefore, as populations of native suckers decrease and/or exotic species become more abundant, the impact of hybridization (especially involving introduced species) will likely become more significant. At present, concerns over hybridization should focus on other species, especially those introduced to the Colorado River system. For example, analysis of mtDNA from a number of flannelmouths from the upper Colorado River has revealed considerable introgression with white sucker, C. commersoni (Hutchison and Dowling, unpublished data). Therefore, concerns over introgression should broaden, as introduced species (e.g., white suckers, Utah suckers [C. ardenr], longnose suckers [C. catostomus]) are most likely to have the greatest impact on dwindling populations of all native Colorado River species.- CAPTIVE POPULATIOI\S Goals of captive propagation.- Objectives for captive propagation of endangered species contrast sharply with those for fishes more traditionally under cultwe. Sport and commercial fishes maintained in hatcheries were typically common, of high demand, and had:'already been 15