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October 1986 400 GREEN 300 '< 0:: W (I) 200 ~ :::> z 100 TYUS: COLORADO SQUAWFISH 659 WHITE YAMPA Y Y J Fig, 3, Catch of Colorado squawfish from the Green, White, and Yampa rivers (Tyus et al. 1982, Miller et aI., White River, 1982; YampaRiver, 1982. Y=youngofyear, J=juveniles, A=adult. strategy, and adults have been documented as homing to desirable spawning sites (Tyus 1985). Figure 2 illustrates the spectacular spawning migrations to the Yampa River spawning site in 1983 and 1984. Migrations of young are not so easily documented, but downstream transport of larvae have been noted by Haynes et al. (1984) and Tyus and McAda (1984). A net long-term movement of juveniles must occur to populate adult areas upstream, probably in the late young-adult stage, is indicated by collection data (Tyus et al. 1982). Figure 3 illustrates that, in the mainstem Green River, young P. Lucius are relatively abundant and juveniles common; however, in the major tributaries (White and Yampa rivers) where adults predominate, ju- veniles are rare and young absent during most of the year. Habitat selection appears to be the driving force for migration. Hence, adults move up to 200 km to spawn in white-water canyons. Af- ter hatching, young larvae can drift down- stream and occupy warm shallow habitats where rapid growth is possible. These move- ments also aid in reducing intraspecific preda- tion since the adults and young tend to con- centrate in different river sections. Recent studies (Archer et al. 1985) also show that during flood periods adult P. lucius move out of the river banks and occupy flooded bot- toms, where they presumably feed on terres- trial wildlife such as small mammals (Beckman 1952). Potamodromous migrations of cyprinid fishes are not well documented for North American forms, at least not for migrations of 100 km or more. Such migrations are not un- common in flood plain rivers in other parts of the world (Welcomme 1979). Ptychocheilus lucius appears to take advantage of river trans- port at the end of the flood period for the dispersal of young from the spawning grounds downstream into productive nursery habitat (Tyus and McAda 1984). This behavior resem- bles some South American freshwater species in this regard, and it has been noted that in Africa potamodromy may protect the young from predation and secure dispersal over the river basin (Welcomme 1979). Reproductive Adaptations The spawning of P. Lucius occurs in middle to late summer under a decreasing flow regi- men. This is unusual among most stream fishes, which spawn in the spring and early summer with rising water levels. As with other potamodromous riverine species, tim- ing of reproduction is very important, and studies of spawning P. lucius (Archer et al. 1984) indicate the fish apparently times its spawning to coincide with the descending limb of the hydrograph, a time when down- stream transport of young would distribute