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The exquisite sense of smell in minnows has been the subject of various experiments, and minute quantities of dissolved substances can be detected by them (Pfeiffer 1963, Kleerekoper 1969, Smith 1985). The use of chemical stimuli to orient to rearing areas has been extensively evaluated in salmonids and clupeids, but various minnow and sucker species also use olfaction to orient to natal streams (reviewed by McKeown 1984„ Smith 1985). As an example, Werner (1979) found that migrating white suckers were impaired in their ability to detect their home stream when their Hares were plugged; thus demonstrating a sensory basis. for homing in the fish. Colorado squawfish and razorback sucker are presumably endangered, in part, because of complex life histories that are poorly suited to regulated environments. Recovery of both species requires a better understanding of their reproductive ecology. Considering that neither fish is secure, it is assumed that more intensive management/recovery options must be developed. Existing information about Colorado squawfish life history suggests some options. So little is known about the razorback sucker, that more basic information must be obtained before management tools can be developed. Because environmental conditions vary so greatly between upper basin rivers, it is assumed that recovery programs for both species must be determined and implemented on a site-specific basis. The following provides some review of the basic components of the life history of both species as a foundation for development of this work plan. Migration Much is written in the popular (e.g., Hay 1959) and scientific (reviewed by McKeown 1984, Smith 1985) literature about the migrations of fishes. Anadromous migrations of salmon, striped bass, and shad from the sea to spawning grounds in freshwater are well-known. Other fish migrations also occur, including catadromous movements of adults from freshwater to marine environments (e.g., eels); oceanadromous movements in the seas (e.g., herring and tuna); and potamodromous movements in freshwater (e.g., sturgeons, suckers, and minnows). Although migrations in freshwater include species that exhibit relatively short-distance movements, some euroasian species undertake long-distance movements of over 1,000 km. Although migrations of commercially-valuable fishes has been intensively studied (reviewed by McKeown 1984), much has yet to be learned about causal mechanisms (Dodson 1988, Quinn and Tollman 1987), and very little is known about rare freshwater forms such as Colorado squawfish and razorback sucker. Spawning migrations of Colorado squawfish and razorback sucker have been reported for over 100 years (Jordan 1889), but. environmental factors associated with migration are not well understood. It is assumed that, as in other species, environmental cues influence intrinsic biological mechanisms that result in spawning.Migrations of Colorado squawfish to spawning locations, presumably an adaptation to the fluctuating environment in which it evolved (Smith 1981, Tyus 1986), has been documented (Wick et al. 1983; Tyus and McAda 1984; Tyus 1985, in press). However, stimuli that cause the fish to migrate at the proper time and arrive at distant spawning locations have not been identified (Tyus and Karp 1989). In addition, return of young squawfish to natal areas as reproducing adults is suspected, but not documented. Less is 3