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F1 1 1 11 1 mark larvae from at least 10 collections are not available at the present time. Genetic Characterization of Endangered Fish Produced in Protected Environments. When the razorback suckers reach a total length between 250-300 mm in total length, the fish will be captured and PIT-tagged. Non-lethal muscle plugs will be obtained from the fish for genetic characterization (using allele frequencies, genotypic proportions, etc.) and compared to similar information from the wild stock. ' If genetic characterization of the wild fish reared in protected habitats is similar to the wild spawning stock, a sample of not to exceed 10% of the numbers in the wild spawning stock can be released near the spawning site to augment the wild stock. All released fish ' must be identifiable by a unique PIT tag when recaptured during future studies. ' If only a few larvae survive in the protected environment or the genetic characterization indicates that the surviving fish in the protected environment originated from only a few parents, the fish should not be stocked into the riverine environment to prevent ' genetic swamping of the wild stock. The disposition of such fish will follow the guidelines in Section VI.B. D. Introgressive Hybridization. Introgressive hybridization has important management and evolutionary consequences and increases when allopatric species or stocks are placed into sympatry through human activities and ' when sympatric species or stocks inhabit altered environments (Buth et al. 1987). No hybridization has been documented in the Colorado squawfish. However, hybridization has been documented for razorback suckers and the endangered Gila in the upper basin. ' The frequency of introgressive hybridization of razorback suckers with flannelmouth suckers (Catostomus latipinnis) is low (0 - 5% toward ' flannelmouth suckers; Buth et al. 1987) but may increase if the riverine environment in the upper basin is altered further, forcing the two species to spawn in the same areas. 1 Dowling and DeMarais (1993) reported that introgressive hybridization played an important role in creating a high morphological diversity among Gila minnows in western North America. Dowling and DeMarais stated that t the Colorado River Gila represent a complex of self-sustaining, genetically distinctive species that are capable of exchanging genetic material, comparable to syngameons in plants. Although the phylogenetic ' linkages are distinct, local introgression has clearly occurred among three Colorado River Gila in the past. Introgressive hybridization is apparently low at present but could ' increase in Gila if the habitat is altered further or in the razorback sucker if nonnative suckers expand their ranges in the upper basin. Preservation of critical habitat for the endangered fishes should prevent 15