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_ 1 T ~ INTRODUCTION: With passage of the Endangered Species Act of 1973 (ESA), Federal and State agencies were provided with policy direction and funding to protect and recover remaining populations of endangered animals and plant. However, little was known about the life requirements of most endangered species, and wildlife management concepts have been largely oriented toward sport hunting and fishing. In case of the endangered Colorado River fishes, study of these animals was made more difficult by their rarity and the harshness of the environments in which they live. In addition, some Colorado River fishes lost most of their habi s to water resources development before the ESA was passed (Minckley 1973, Carlson and Carlson 1982), making information obtained from the remaining, but ostensibly declining populations, difficult to interpret. 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 un~er 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 evaluate the mechanism by which adult Colorado squawfish and razorback sucker locate suitable spawning areas, and to determine the role of olfaction in habitat se]ection. 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 215 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 1940, Dence 1948, Raney and Webster 1942). Similar migrations have also been reported in Asia for various minnows (Nikolskii 1961, Breder and Rosen 1966). 2