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with brook trout. Cutthroat trout have hybridized with rainbow trout throughout their range. Dowling and DeMarais (1993) reported that hybridization played an important role in creating a high morphological diversity among Gila sp. in western North America. They stated that the Colorado River Gila so. 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 S. in the past. Hybridization is apparently low at present but could increase in Gila sR. if the habitat is altered further or in the razorback sucker if nonnative sucker expand ranges in the upper basin. Preservation of habitat for listed and candidate fishes should prevent increased hybridization (Maddux et al. 1993) Wild stocks should be monitored and actions taken to reduce hybridization. Hybridization between bull trout and brook trout typically results in infertile offspring (Leary et. al. 1983). However, brook trout mature at a younger age and have a higher reproductive rate, life history differences that tend to work in concert with hybridization to favor brook trout and result in the elimination of bull trout. • V. GENETICS MANAGEMENT OF CAPTIVE STOCKS A. Determining the Need for Captive Propagation and Reintroduction Captive propagation may be necessary if a fish stock does not have sufficient recruitment to maintain a self-sustaining population. (Williams et al. 1988). Captive-reared fish can be used for (1) genetic refugia and broodstock development, (2) research and development, (3) information and education, and (4) augmentation or restoration (Wydoski 1994). Captive-reared fish are needed for research to conduct laboratory and field experiments related to ecological requirements, habitat use, interactions with nonnative fishes, response to contaminants, olfaction and chemoreception studies, fish passage, taxonomy, anatomy, physiology, performance, and fitness. Fish specimens are also needed for use in public education programs. Augmentation may be required to provide a "jump-start" mechanism to supplement fish in habitats where wild populations are depleted and restoration stocking may be necessary in river reaches where stocks have been extirpated. B. Genetic Riczkc dccnrin+nA w;+1, wr:rv .?a: ., v1 VY 4?.4 1 V11 Planning for restoration and recovery will be accomplished following a systematic approach to minimize potential genetic risks (See Box 3). A genetic risk is broadly defined as the sum of critical uncertainties associated with any management action or inaction that may change genetic • 13