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INTRODUCTION The Endangered Species Act (ESA) of 1973 provided new policy and funding to Federal and State agencies involved in protection and recovery of endangered animals and plants. However, little was known of the ecological requirements of many endangered species, and wildlife management concepts were more oriented toward sport hunting and fishing. In the American Southwest, progress toward recovery of endangered Colorado River fishes has been constrained by a paucity of information on their life history requirements, their rarity, and the harshness of the environments in which they live. In addition, current habitat use information may only reflect marginal use because the species have been in decline for many decades. Thus, caution must be used when interpreting current habitat use data. Under the ESA, recovery must be accomplished within an ecosystem context. It is imperative that ecological aspects prevail in endangered species management, and an understanding of biotic and abiotic factors limiting the distribution and abundance of the target species be well understood and incorporated into any management attempt. Because remaining populations of endangered species are fully protected under the ESA, it is incumbent upon management agencies to demonstrate that proposed management or recovery actions will not further jeopardize the existence of a listed species. This paper presents a recommended course of action for the study of chemoreception and imprinting in Colorado squawfish and razorback sucker in the upper Colorado River basin. We address the potential role of these mechanisms in the life cycle of these fishes, and the application of this knowledge to recovery plans. Study objectives include the following: To determine the mechanism by which adult Colorado squawfish and razorback sucker locate suitable spawning areas, and to evaluate the role of olfaction in habitat selection. To identify the roles of imprinting, learning, and genetic control on the reproductive cycles of Colorado squawfish and razorback sucker. To explore means by which new populations of Colorado squawfish and razorback sucker may be established using innate behavioral mechanisms. BACKGROUND: Minnows and suckers belong to the Suborder Cyprinoidei. The fish family Cyprinidae, or minnows, consists of about 275 genera and over 1600 species and is the largest and most diverse in the world. The closely allied suckers, family Catostomidae, consists of 12 genera and 58 species. Spawning migrations occur in the Cyprinoidei and some of these are well-known in various parts of the world. In North America, potamodromous migrations of catostomids to specific spawning streams or reaches have been reported for many years (e.g., Dence et al. 1940, Dence 1948, Raney and Webster 1942). Similar migrations have also been reported in Asia for various minnows (Nikolskii 1961, Breder and Rosen 1966). Several experiments have demonstrated that some minnows exhibit an exquisite sense of smell and can detect minute quantities of dissolved substances (Pfeiffer 1963, Kleerekoper 1969, Smith 1975). The use of chemical 10