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t endangered Colorado River fishes are known to migrate to spawning areas but return to a home area during the remainder of the year. 2. Fidelity to Spawning Sites. Distinct spawning sites and strong fidelity to those sites is a primary factor for dete~~ning a population. Fidelity to spawning sites is extremely important to maintenance of genetically separate populations when no physical barriers exist between populations. 3. Geo~ranhic Distribution and Abundance. The initial criteria used to separate endangered fish stocks was distribution during the spawning season and abundance of each species in the upper basin. Some stocks are geographically separated from others and do not appear to interact with other stocks. Geographical levels were subjectively assigned based on the knowledge of the overall movement/dispersal of a stock to/from a spawning site and habitat types within river reaches. Depending on the species, some populations have very broad geographic distributions, while others are specific to a canyon or river reach. Dams fragment the habitat in the upper basin and prevent free movement of the endangered fishes. Hence, some of the geographic separation among stocks is due to altered habitats. Some interchange has been documented for several razorback sucker, humpback chub, and Colorado pikeminnow stocks. In most cases, only a few fish were known to move between different spawning seasons, but a relative few fish have been observed. A small amount of interchange among stocks is natural during the evolutionary process. However, even limited interaction of different stocks during the spawning season can result in genetic homogenization. Stocks with Limited interaction will be classified separately until adequate information is available to justify combining them. This conservative classification is intended to prevent or inimi~e the risk of losing unique genetic attributes that may be important in the recovery effort. 4. Molec 1Geneticl Identification and Characterization of Stocks. The genetic diversity of endangered fish stocks in the Upper Colorado River Basin has been characterized, in some cases, by companng the distribution of genetic diversity through appropnate techmques such as protein electrophoresis for protein analysis and restriction endonuclease analysis for mtDNA. Protein electrophoresis is an indirect method of genetic analysis because it focuses on phenotypes that imply the genotypes. The method allows detection of the presence or absence of allozymes and relative frequencies and allelic differences among groups of animals. Allozymes provide a phenotypic expression of alleles. Restriction fragment length polymorphism (KELP) analysis of mitochondrial deoxyribonucleic acid (mtDNA) allows a more direct genetic characterization of stocks or populations. A combination of the two methods provides a powerful tool for genetic identification and chazacterization. Quantification of within and among population variation is necessary to 1) define stock structure and 2) define the level of within population genetic variation within captive stocks to mimic that found in the wild population (Allendorf and Phelps 1987; Allendorf and Ryman 1987; Gauldie 1981; Hynes et al. 1981). Unlike many phenotypic traits, molecular traits aze not subject to environmental induction (effects). Verification of distribution patterns is extremely important to ensure that differences have a genetic basis and are not due to other factors such as environmental effects, development stage offish that are sampled, or treatment of samples (Kapuscinslci and Jacobson 1987). In addition, eazly detection of inadvertent introgression from stocking will allow changes in stocking procedures that could preserve the genetic integrity of remaining wild endangered fish stocks (Hynes et al. 1981). 5. Morphometric/Merisric Identification and Characterization of Stocks. Morphological analyses aze used to identify and characterize fish stocks in some cases. i~ 1