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Imprinting It is presumed that Colorado squawfish and razorback sucker return to natal areas by use of olfactory cues (i.e., memory of specific home-site odor bouquet). If the young fish are imprinted, it must occur at an early ontogenetic stage as in pink salmon and other fishes which emigrate immediately after emergence (Hasler and Scholz 1983). Although it is of interest to know when imprinting occurs (i.e., egg stage, swim-up, or as the young fish migrate downstream from spawning areas), some problems can be avoided if the fish can be imprinted to a synthetic chemical. If artificial imprinting is successful, then maintenance of release sites would be necessary to provide release of the imprinting chemical in perpetuity. Therefore, it would also be important to learn if reproductive by- products in spawning areas could function to attract young in the absence of imprinting odors. H3: Colorado squawfish and razorback sucker are imprinted to a specific home- site odor bouquet (HSOB) on spawning areas, and use this induced memory to return to those areas as spawning adults. Task: Determine if fish learn a HSOB, subsequently use these as a cue to locate spawning sites. Methods: Capture local spawners from two areas, fertilize eggs from those sites, split egg lots into two from each area and place one-half of each lot in baskets of media and return to parental spawning sites. After 1-2 weeks remove these to hatchery, rear them to about 300 mm (about 1 year before the males are expected to mature) and mark them with coded wire or PIT tags. Of these, stock all but 100 fish in areas upstream and downstream of the hatching sites. Of the remainder, radiotag 50 fish from each location and release them upstream (25) and downstream (25) of spawning areas in the summer or fall after they first reach sexual maturity. Monitor these fish and determine their behavior during the next spawning season. Task: Determine if razorback suckers reared at the Ouray hatchery facility are imprinted to hatchery discharge water. Methods: Place traps in hatchery discharge pipes and attempt to attract fish that were reared at Ouray and released into the Green River 3 to 5 years previously. H4: Colorado squawfish and razorback sucker can be artificially-imprinted to a suitable synthetic substance. Task: Test synthetic substances for imprinting effectiveness. Methods: Obtain newly-fertilized eggs from incubation and early larval development, rear at a re-introduction site. Three groups would alcohol and a control group that would be hat, reintroduction site would be carefully chosen spawning and rearing area, and the fish would a hatchery, mark them during to sub-adult size, and release be marked: morpholine, phenethyl :hed in hatchery water. The for its adequacy as a future be attracted as spawning adults 15