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natural recruitment. The evaluation of these sites as nursery areas for razorback suckers may require stocking naturally spawned larvae into protected, predator-free sites, an activity recommended by Dowling and Minckley (1993) to augment the razorback stock in Lake Mohave. 2. Guidelines for Maintaining Genetic Diversity in Wild Larval Fish to be Stocked in Protected Environments. Larval fish should be collected from or near a known spawning site of a suspected historic nursery area. Temporal sampling of larvae should be made throughout the natural spawning period (Tyus and Karp 1991). For example, collections could be made weekly using light traps during this period. An attempt should be made to collect equal numbers of larvae during each collection, although that is not very likely to occur. Standardized techniques should be used during all collections so that mortality of larvae during capture and handling remains similar among collections. Ideally, larvae from each collection should be marked. However, batch-marking techniques for at least 10 collections are not available at the present time. 3. Genetic Characterization of Fish Produced in Protected Environments. If wild fish reared in protected habitats are genetically similar to the wild spawning stock, they can be reintroduced near the spawning site. The number can not exceed 10 per cent of the spawning population. All released fish must be identifiable by a unique PIT tag so they can be tracked during future studies. If only a few larvae survive in the protected environment or the genetic characterization indicates that the surviving fish originated from only a few parents, the fish should not be stocked. The disposition of such fish will follow the guidelines in Section VI.B. D. Hybridization. Hybridization has important management and evolutionary consequences. Hybridization increases when allopatric populations are placed into sympatry through human activities and when sympatric populations 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 Colorado River Basin. The pallid sturgeon may be hybridizing with the shovelnose sturgeon in the Mississippi River. Dowling and DeMarais (1993) reported that hybridization played an important role in creating a high morphological diversity among Gila minnows in western North America. Dowling and DeMarais stated that the Colorado River Gila represent a complex of self-sustaining, genetically distinctive species that are capable of exchanging genetic material. Although the phylogenetic linkages are distinct, local introgression has clearly occurred among three Colorado River Gila in the past. 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 and candidate fishes should prevent increased 13