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based on criteria associated with operational costs and recovery potential. Razorback sucker Razorback sucker work would include an evaluation of existing stocks, but more emphasis would be placed on artificial imprinting as a stop-gap measure. Emphasis would be placed on stock evaluation and use of hatchery facilities to a greater extent than in the Colorado squawfish. The Ouray experimental hatchery facility in Utah has been given a high priority for razorback sucker work because of its proximity to a large population of spawning adults and potential recovery areas, staff experience with propagation of the fish, and reduced costs associated with using an existing facility. Releases of imprinted fish into the Green River would ostensibly provide the-best existing habitat to insure high r~urns. However, the importance of this work dictates that more than one site be used if possible, and results compared. This study is..restricted to the Upper Colorado River Basin, and it is noted that state regulations may not allow work on the endangered fishes outside the basin. WORK ITEMS: Genetic Assessment -~ Recent studies indicate that some Colorado squawfish and razorback suckers in the Green and Yampa rivers return to the same spawning sites for spawning (Wick et al 1983, 198b; Tyus 1985, in press; Tyus and Karp, In press). If return to these specific spawning sites is a general reproductive phenomenon for these species (i.e., the fish return to natal spawning areas as adults and spawn in the reach(a) or exact site(b) at which they were hatched), then genetic differentiation among disjunct spawning populations or stocks is suspected. The existence of genetically-distinct populations of Colorado _ squawfish and razorback suckers should be evaluated for these different spawning areas could be evaluated by collecting and analyzing genetics data In addition, there may be a genetic basis for sub-stocks within a spawning area; in squawfish, the different rheotaxes (positive and negative) noted in migrating fish suggest the existence of these sub-stocks. ,These different movement patterns must also be considered in testing the following hypotheses: Hl:Breeding populations of Colorado squawfish and razorback suckers are genetically differentiated. Tasks: Determine if Colorado squawfish and razorback sucker are ~y~T 4.--•--~•~ reproductively isolated between (a) reaches or (b) spawning sites: Methods: Identify spawning sites in the upper Colorado River Basin. At each site during the spawning season, capture fish in breeding condition and mark them with a uniquely-numbered tag. Take superficial somatic tissues and gametes from each fish for genetic analyses. If possible, obtain tissues and sex products of fish known to use reaches(a) and sites(b) for spawning, or of fish emerging from such areas. Perform analyses (i.e. allozymes and mtDNA) to