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. ,, known about the razorback sucker, but migration to spawning areas and repeated use of the same sites have been reported (Tyus and Karp, in press). Early studies of fish migration were directed at understanding migratory patterns and factors affecting the control of fish migration. One outstanding contribution was the olfactory hypothesis for salmon homing proposed 'by Hasler and Wisby (1951). They suggested that young salmon imprint to the odo'~,r of their natal tributary, store this information in a long-term olfactor memory, and later use this memory to relocate the stream during spawning migrations. In the following 40 years, many other workers have tested this hypothesis and further refined it. It is now recognized that fish imp int to a "home-site olfactory bouquet" which consists of environmental. odors perhaps including both a geologic and species-specific component (Hasler and Scholz 1983,. Foster 1985). However, our knowledge of the adaptive significance of migrations and reproductive cycles is poor for all but a few, commercially" important species. Migration patterns of adult Colorado squawfish are similar year to year. These movements are presumably an orientation to environmental condit'ons in the spawning reaches, and movements of fishes in up- and downstream d'rections is suggestive of an olfactory orientation mechanism (Harden-Jones 196, Hasler and Scholz 1983). Some behaviors associated with olfactory orientatio in salmonids were exhibited by Colorado squawfish, and the presence of spring-fed tributaries and other water inputs in spawning reaches may provide pi oting cues. Tributary streams may provide gross cues for long-distance orie tation, while more subtle cues, unique to specific sites, may be used for egg deposition (Tyus, In press). Reproductive by-products from previously hatched young (Foster 1985) may also be included in a home-site olfactory bouquet. Colorado squawfish larvae may imprint such odors in the areas in which they develop, and later recognize these odors as migrating adults. Recaptured and radiotagged adult Colorado squawfish have demonstrated a fidelity (repeated use) to the same spawning areas (Wick et al. 1983,Tyus 1985, in preparation), however, it has yet to be shown that the fish return to natal areas for spawning. The similarity of their behavior to that of many other fishes makes it probable that they do so, and this could lead to reproductive isolation and separate genetic stocks. It is noted that the maintenance of discreet stocks can be developed through spawning site imprinting and homing (Horrall 1981), and thus, specific migration routes and positive or negative rheotaxis in homing orientation suggests different genetic stocks (reviewed by Smith 1985). Some genetic interchange between stocks may occur from fish that "stray" from one area to another, and could be important for successful evolution of the species (Baker 1982; Leggett 1984). It is therefore important that the genetic identity of separate stocks of Colorado squawfish be identified and protected. Knowledge of the reproductive ecology of the razorback sucker is poorly known, principally because successful recruitment is lacking throughout the Colorado River Basin (Lanigan and Tyus 1989, Marsh and Minckley 1989). However, razorback sucker migrations have been documented, and there is compelling evidence that homing behavior occurs as in the white sucker. The possibility of at least two separate spawning stocks of the razorback sucker 4