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1044 in distant, canyon-bound river reaches. It is pre- sumed that the long-distance potamodromous mi- grations in the Green and Yampa rivers evolved as an adaptation to desert stream environments (Smith 1981; Tyus 1986). Radio-tagged fish that were tracked before, during, and after the spawn- ing season usually returned to their former spring locations in the autumn, and many tagged fish were recaptured in the same habitats in spring and autumn. These fish presumably occupied the same home ranges during winter. The paucity of large fish in the lower Green River below the spawning areas and the prevalence oflarge fish in the Yampa River above the spawning areas suggested a net downstream movement of young and a move- ment upstream of larger fish (about 400 mm TL) to augment the adult stocks. Twenty-eight Colorado squawfish tracked in my study (18.3%) were classified as sedentary during a particular year, but five of these were classified as migrants to one of the known spawning areas in other years; hence, they were mature fish that spawned less than annually. I presume that the remaining 23 were (1) mature fish that also did not migrate annually (migrated in other years), (2) fish that spawned in the location in which they resided (did not migrate), or (3) immature fish. Larger fish of adult size (>435 mm) may have been immature. Seethaler (1978) found that only 50% offemales less than 503 mm TL were mature, and Tyus and McAda (1984) found that one 510- mm fish was an immature male. Two fish in my study (385 and 421 mm) were smaller than the smallest mature fish captured (435 mm), and were therefore presumed to be immature. Some of the remaining 20 fish were also presumably imma- ture, because 3 were smaller than the average ripe male (555 mm), and 17 were smaller than the average ripe female (654 mm). Thus, nonmigra- tory behavior of Colorado squawfish was linked with non-annual spawning or sexual immaturity. Although migration of commercially valuable fishes has been intensively studied (Leggett 1977; Baker 1978, 1982; McKeown 1984), much has yet to be learned about causal mechanisms of migra- tion (Quinn and Tollman 1987; Dodson 1988), particularly for rare freshwater forms such as Col- orado squawfish. My work indicated that migra- tion of Colorado squawfish occurred each year during summer solstice. Although no analyses were performed to further evaluate effects of photope- riod on migration, this factor may be important in the reproductive cycle of the Colorado squaw- fish, as has been noted for other cyprinids (peter TYUS 1983). In addition, spawning migrations of radio- tagged Colorado squawfish were correlated with an average response time of about 28 d after spring peak flows, at water temperatures of90C or more. Although not a part of my study, other factors may also provide migratory cues, including inputs of chemical substances from snowmelt and runoff and of neutral oils from inundated shorelines (Lake 1967). These may act in concert with high river levels and increasing water temperatures to influ- ence the intrinsic mechanisms associated with Colorado squawfish spawning migrations. Homing of Colorado squawfish from various locations to the spawning reaches in Desolation- Gray and Yampa canyons is presumed to be an orientation to environmental conditions in the spawning reaches. Movements of fish to spawning sites in both up- and downstream directions are also suggestive of an olfactory orientation mech- anism, as Harden-Jones (1968) and others (re- viewed by Hasler and Scholz 1983) proposed. Some of the behaviors associated with olfactory orientation in salmonids were exhibited by Col- orado squawfish, and the presence of springs and other water inputs in the two spawning reaches may have provided olfactory cues. Tributaries such as Florence Creek and Warm Springs may provide gross cues for long-distance piloting to the spawn- ing reach, and more subtle cues unique to specific spawning sites may further guide the fish once it is in the reach. Although unknown for Colorado squawfish, more subtle cues may include several environmental components, perhaps even repro- ductive by-products from previously hatched young (Foster 1985), all of which may constitute a home-site olfactory bouquet. As also suggested by Wick et al. (1983), Colorado squawfish embry- os or larvae may imprint on odors in the areas in which they develop and later orient to these odors when they return as migrating adults. Colorado squawfish migrations were predomi- nantly downstream (76.2%). After spawning, adults returned to their upstream home range, where they presumably overwintered. When coho salmon Oncorhynchus kisutch recognize odors on which they have imprinted in water currents, they mi- grate upstream (positive rheotaxis) in response, and exhibit negative rheotaxis in the absence of this odor (Johnsen 1978). The behavior of Colo- rado squawfish that I noted is suggestive of home- site imprinting-radio-tagged fish cruised up- and downstream over wide areas, overshooting Des- olation-Gray Canyon, and Yampa Canyon spawning sites, then backtracking to them. If such .' h