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954 OSMUNDSON ET AL. bolic demands require more food, but appropriate food for adults is scarce. Evidently the motivation for upstream dispersal is strong given that these movements are against the current and away from a more preferred ther- mal regime. Stuntz and Magnuson (1976) reported that preferred temperatures of bluegills Lepomis macrochirus decreased in laboratory studies as food ration decreased, and they suggested that bluegills use temperature selection as a method of decreasing weight loss during periods when daily rations are low. We suggest here that this same phenomenon, acting in concert with searches for greater food availability, motivate upstream dis- persal in Colorado squawfish and result in the lon- gitudinal structuring of size distributions observed in the Colorado River. Factors other than changing food requirements may induce long-range displacements by adult Colorado squawfish; these include predator avoid- ance, reproduction, and physical habitat prefer- ences. Of these, predator avoidance is probably least important given the rarity of predators ca- pable of preying on adult Colorado squawfish. Be- cause seasonal movements related to spawning were excluded from our analyses, displacements from one spring period to the next were probably not the direct result of reproductive behavior; that is, Colorado squawfish generally return to non- spawning home ranges by late summer or fall (Tyus 1990; McAda and Kaeding 1991). Searches for areas containing preferred meso- or microhab- itats may contribute to observed movements and distribution, particularly within strata. However, because habitat and food are so tightly interrelated, it is difficult to separate selection for food from selection for habitat types that allow efficient for- aging (Magnuson et al. 1979). It seems reasonable to assume that in an environment with minimal intraspecific competition and predation risk, po- sitioning of adults, both within and among strata, would be primarily driven by growth maximiza- tion (see Hughes 1998), and growth is largely de- pendent on the interaction of temperature and food availability (Weatherley 1972). Achieving maxi- mum growth potential enhances the ability of the individual to survive and reproduce successfully. In many fish species, large females produce more and larger eggs, thereby enhancing larval survival (Monteleone and Houde 1990; Brandt and Kirsch 1993). Adults probably select reaches containing the best combination of resources, and the upper reach, particularly strata 6 and 7 (the Grand Valley) where adults were most concentrated, may rep- resent the best balance between suitable temper- atures and food availability. At some distance up- stream, annual thermal units should decline to the point where plentiful forage can no longer provide adequate compensation, resulting in reduced growth. This probably occurs within the reach im- mediately upstream of the Grand Valley where an- nual thermal units are low. At Rulison, mean daily temperatures never reached 20°C during 3 of the 5 years studied. Black and Bulkley (1985b) found that growth of yearling Colorado squawfish held at 20°C and fed unlimited food was only 54% that of growth at the optimum temperature of 25°C. This reach, blocked by diversion structures since the turn of the century, may represent the fringe of the former range of this species in the Colorado River. Seasonal timing of and stimulus for movement could not be discerned in this study by comparing consecutive capture events. However, the available evidence allows us to offer four hypotheses. First, movements may be motivated by hunger with up- stream exploration for food being more rewarding than downstream exploration. Second, an innate physiological mechanism may prompt upstream movement; however, the lack of dispersal move- ments in many fish and the downstream movement of some are counter to a species-wide directional disposition. Third, movements may be motivated by the urge to spawn, and better feeding areas are discovered in the process. Movement primarily by sexually mature individuals supports the latter hy- pothesis, but sizable numbers of adult fish are found in upper White and Yampa river reaches far upstream of their spawning areas (Tyus 1990), sug- gesting that upstream feeding areas were not lo- cated while en route to a spawning site. A fourth hypothesis, combining the hunger and spawning hypotheses above, may be the most plau- sible given all lines of circumstantial evidence. Gradients in food resources may be discovered by young adults during their initial spawning migra- tions, and these learned gradients are then pursued after the spawning period is completed, resulting in further upstream explorations by young adults. If this latter hypothesis has merit, we might predict that such displacements take place during and im- mediately following the spawning period. The extent to which distribution and dispersal patterns observed in this study reflect historic con- ditions is unknown. Intraspecific competition for- merly might have played a role in structuring riv- erwide size distributions; now such competition in l