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Migration and Spawninp_. The initiation of spawning migration is an important event in the reproductive cycle of the Colorado pikeminnow. Because most home ranges are upstream of spawning areas, migrations usually begin in the downstream direction, but ~ upstream migrations also occur (Tyus et al. 1987, Tyus 1990, Irving and Modde 1995). In fact, the longest migration on record occurred when a fish tagged in Lake Powell moved 318 km upstream in about one month to join an aggregation of other fish in the UCR. Based on radio tracking data, fish in the Green River begin spawning migrations about June 21 (range: May 23'~ to July 22"x) and fish in the Yampa River migrate about June 15 (range: May 27"' to July 13"' ). The timing of migrations in the UCR and the San Juan River is similar to that reported for the Green River, occurring from late June through early August (Archer et al. 1985, Ryden and Ahlm 1996). Spawning migrations begin just after peak runoff and are initiated earlier in low water years than in high water years (Tyus and Karp 1989). The time between the date of ~ peak runoff and the initiation of migration is negatively con-elated with the date of peak flow; in the Green and Yampa rivers, there were statistically significant correlations in which earlier dates of peak flows were associated with longer time intervals before the migration occurred (Tyus 1990). The potential importance of flow is highlighted by the recent hypothesis that flow spikes from spring rainstorms may influence ovulation and ~ spawning (Nester et al. 1988). Temperature may also influence the timing of migration because temperatures must be at least 9°C (average 14°C) before migration occurs. The annual pattem of spawning migrations of Colorado pikeminnow in the Green River basin demonstrates a clear capacity for homing to particular sites (Tyus 1985, 1990). ~ The precise mechanism by which adults locate these sites is not understood fully, but circumstantial evidence supports natal imprinting (Wick et al. 1983, Tyus 1990). Electron microscopy has revealed that larvae possess a functional olfactory mechanism (R. Muth, personal communication). Not only are the receptors present, but a burst of hormones in the early larval stage suggests that imprinting is occurring (Scholz et al. i 1993). Furthermore, recent studies with adult fish that have been exposed to hormones have shown an extremely acute sense of smell (A. Scholz, personal communication). The pattem of movement of adults in the Green River basin is too complex to be explained as a response to gradients in environmental variables or to odors from conspecifics. Individual fish may travel long distances to spawning grounds, and fish ~ with adjacent home ranges may go to different spawning reaches that are separated by many miles. Each fish appears to use only one spawning area (Wick et al. 1983; Tyus 1985, 1990; Irving and Modde 1995), but fish migrating to one location may pass through another one. Moreover, before fish begin these migrations, they are dispersed widely in the Green, White, and Yampa rivers, and may be upstream or downstream of ~ their destinations. It is possible that Colorado pikeminnow migrate to and spawn in locations where they were hatched (i.e., homing to natal areas), however this has never been proven. Orientation to olfactory cues provides an explanation consistent with the observed 12