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t why the fish were listed as endangered and the Program to recover these fish species. Stocking of hatchery- raised fish for both restoration and augmentation purposes will be used to establish or facilitate increases in fish populations in the following circumstances: (1) where populations no longer exist; (2) where populations are low in abundance and subject to inbreeding risks or extirpation; (3) where adult habitat is suitable but unoccupied; (4) where natural recolonizarion rates or recruitment is absent or.impaired; and (5) where limiting factors need to be evaluated. Stocking is not appropriate, however, to sustain fish numbers indefinitely without adaptive management in response to the alleviation of the factors keeping the fish from sustaining themselves. III. CRITERIA FOR CHARACTERIZATION OF ENDANGERED FISH STOCKS Individuals of natural animal populations are razely distributed randomly and their distributions may range from essentially continuous to highly clumped (Bazker 1989). The term "Evolutionary Significant Unit" has been applied to natural, biological groupings of animals that exhibit patterns of diversity associated with evolutionary history, breeding isolation, unique adaptation, and production (Dizon et al. 1992). The relationship between geographic distribution and genetic diversity varies considerably among animal populations. Avise (1989) proposed four categories of this relationship: (1) great genetic divergence and strong geographic partitioning, (2) great genetic divergence accompanied by weak geographic partitioning, (3) geographically separated assemblages characterized by little genetic differentiation, and (4) extensive gene interchange without subdivision by geographic harriers. The first three categories contain animals that have evolved independently in response to environmental variables and can be considered a stock or Evolutionary Significant Unit that maybe potentially valuable to the recovery of the species. The fourth category results when there is little or no reproductive isolation and considerable intermingling on the breeding grounds that results in a panmictic population (Dizon et al. 1992). Panmictic refers to random mating within a breeding population of animals (i.e., comprises a single stock). The Program philosophy, mission, and goals regazding genetic management of the endangered fishes are based on the recognition that stocks (i.e., Evolutionary Significant Units) or local self-sustaining populations must be considered the operational unit in the recovery effort. For example, such endangered fish stocks may contain unique genetic attributes that might have allowed these endemic Colorado River fish species to evolve and occupy habitats in the Upper Colorado River Basin that are in the northernmost part of their historic ranges (Kapuscinski and 3acobson 1987). Criteria for Identification and Characterization of Endangered Fish Stocks. A stock is defined as a randomly breeding group of individuals that has spatial, temporal, or behavioral integrity distinct from other randomly breeding groups of that same species (Kutkuhn 1981). This definition of stock applies to multiple stocks within an individual river as well as single stock comprising fish from various rivers (Philipp et al. 1993). All available information (i.e., distribution and abundance, behavior, migration, phenotypic information, and genotypic information; Dizon et al. 1992, Ihssen et al. 1981) were considered to identify and characterize endangered fish stocks in the upper basin. Although consistency in this process is difficult, the approach used to determine the geographical, spawning site and movement information and, when genetic and rnorphornetric data was available separate, the stocks based on all the available information. The criteria used to identify and chazacterize endangered fish stocks in the Upper Colorado River Basin (Wydoski 1994) were (see Appendix 2 for the use of these criteria to each of the species and stocks): 1. Identified Spawning Sites/Early Life History Habitats/Known Movements. All major lazge tributaries in the Upper Colorado River Basin have been sampled to determine the distribution and abundance of endemic and introduced fishes (Bestgen 1990; McAda et al. 1993; Miller et al. 1982; U.S. Fish and Wildlife Service 1990a, 1990b, 1991). Spawning sites were determined through various studies by the collection of ripe fish from specific river reaches in different years and, for some species, by the collection of larval fishes downstream from suspected spawning sites. Some Stable fish stocks sustain a constant frequency of genotypes from one generation to another (Hazdy-Weinberg equilibrium Booke 1981).