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homing behavior, and the fish possess the biological "equipment" necessary for responding to those cues. The chemical cues could govern the direction of fish movements by eliciting positive (upstream) or negative (downstream) responses, for ~ example. Chemical inputs, such as natural organic matter, from tributaries, seeps, or flooded lands may provide gross cues for locating a spawning reach, and more subtle cues, such as reproductive byproducts from previously hatched young, may guide the fish to more specific locations within the spawning reach (reviewed by Tyus 1990). Spawning probably occurs within a very small part of the range inhabited by each population of the Colorado pikeminnow. The location of each spawning site could be established unequivocally if spawning activity were observed directly. However, direct observation is unlikely because mature adults are rare and the rivers they inhabit are usually so turbid that the substrate is obscured. Consequently, most evidence supporting the case for spawning at a particular site will be circumstantial. Deductions ~ can be based on the movements and reproductive status of adults, as well as the geographic distribution of larvae. Because newly-hatched larvae have virtually no ability to swim against river currents, their presence establishes a downstream limit on the area within which spawning must have occurred. Captures of larvae can be used to delineate a "suspected" spawning area, according to USFWS criteria (USFVNS 1987). ~ The boundary is imprecise insofar as the larvae can drift far from the spawning area in just a few hours after hatching. The movements of mature adults at the time of spawning also can provide circumstantial evidence for the location of spawning sites. Radiotracking of adults has ~ shown convergence of individuals on a particular river reach (e.g., Tyus 1985, 1990). Tracking data can define a "suspected" spawning area, but the boundaries of the river reach are likely to be overly broad because it may be very difficult to distinguish staging areas from the actual sites of egg deposition. . Collection of adults in "ripe" condition indicates that a spawning area is in close proximity, although adults can still move some distance when in ripe condition (cf. Tyus et al. 1987). The presence of ripe males is sufficient for defining a "suspected" spawning area (USFWS 1987). Females in "running ripe" condition are close enough to the time of egg laying that they are unlikely to move far. Thus, the USFWS (1987) has set the presence of running ripe females as the only acceptable criterion for ~ "confirming" the location of a spawning site. The best documentation of spawning sites has come from the Green River system. The reproductive ecology of Colorado pikeminnow in the Green River basin was studied intensively over a 10-year period (1980-90), in which studies encompassed many facets ~ of reproduction including the timing of pre- and past-spawning movements, the extent and duration of spawning, and habitat use (Haynes et al. 1984, Nester et al. 1988, Tyus 1990, Tyus and Haines 1991). Thousands of larvae and juveniles were captured, the movements of 150 radio-tagged adults were monkored, and 233 fish were captured in breeding condition. In the Yampa River, evidence supporting a suspected spawning 13