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SPAWNING CUES FOR COLORADO SQUAWFISH This is particularly relevant because the spawning ecology of Colorado squawfish presents a cypri- nid reproductive strategy that incorporates salmo- nid attributes. In salmonids, ovulation occurs after ovarian development is completed, and is analogous to spontaneous ovulation. Spawning migrations of salmonids have evolved as a repro- ductive tactic to synchronize gonadal maturation, ovulation, and egg deposition with suitable tem- peratures and flows, proper substrate quality, absence of predators, and so forth. In contrast, ovulation and spawning by cyprinids occurs rap- idly in response to specific external factors rele- vant to reproductive success (Stacey 1984). Sta- cey et al. (1979) demonstrated that the presence of aquatic vegetation was an effective stimulus for ovulation and spawning by goldfish Carassius auratus, even at suboptimal water temperature. In the absence of vegetation, goldfish spawned only when the preferred temperature threshold was reached. Colorado squawfish reproduction displays a mixture of these attributes, placing the species somewhere in the middle of the ecological continuum between cyprinids and salmonids. How does the flow cue hypothesis fit with the reproductive strategy of the Colorado squawfish? Based on the accumulated works and ideas of numerous researchers in the Colorado River ba- sin, the following scenario may be developed. In the spring, adult Colorado squawfish respond to- nne or more specific cues, such as increasing flow and warm water temperatures in backwater habi- tats, and begin gonad maturation and migratory movements. From radiotracking studies, Wick et al. (1983) hypothesized that Colorado squawfish using backwater habitat in the spring were stimu- lated by the warmer water, which accelerated gonad maturation and triggered migration. The spring runoff flow and temperature pattern may accelerate maturation and migration processes. As adult fish continue their spawning migration, other cues such as olfactory stimuli from water quality changes and groundwater seepage may aid the fish in locating the appropriate migration route and spawning ground. Observations of the move- ments of downstream-migrating Colorado squaw- fish around the primary spawning ground in Yampa Canyon by Wick et al. (1983) and chazac- terization of this spawning area as one of sand- stone-limestone seeps by Tyus (1985), led to an adaptation of Harden-Jones's (1981) groundwater seepage hypothesis, whereby migrating Colorado squawfish may locate the spawning ground by olfactory detection of water sources on which 77 they had imprinted. Arriving at the spawning ground, the fish first aggregate in-quiet pools, then spawn in shallower, faster current over cobble- gravel substrate (Tyus et al. 1987). The observed transition between these behaviors and habitat types is abrupt. In the lower Yampa River, a spike in the baseline flow is the cue for ovulation and spawning by the aggregated spawners. In the event a flow spike of appropriate magnitude does not occur, an ultimate increase in water tempera- ture past the 20-22°C threshold may stimulate spontaneous ovulation and spawning. Historically, rivers of the Colorado River basin have had great seasonal fluctuations in discharge and associated variations in temperature, turbid- ity, dissolved solids, sediment transport, and a1- locthonous material (Wick et al. 1983; Tyus et al. 1987; Carlson and Muth in press). The reproduc- tive migrations and larval drift displayed by Col- orado squawfish in the Green-Yampa subbasin have evolved in this rigorous environment (Wick et al. 1983). These long-range, potamodromous migrations by both adult and larval fish are matched by few other North American cyprinid species (Tyus 1986; Tyus et al. 1987), but the adaptive importance of migration and larval drift in the reproductive ecology of fishes has been well established. Spawning migrations optimize repro- ductive success through the aggregation of suit- able mates under favorable spawning conditions and the placement of newly hatched young in an environment relatively free of predators (Ni- kolsky 1963; Northcote 1967; McKeown 1984). Downstream displacement of young fish, either through passive drift or active migration, is con- sidered adaptive because it carries the progeny to productive feeding areas, enhancing growth and thereby further limiting the vulnerability of the young fish to predation (Nikolsky 1963; McKeown 1984). Stream drift takes larval Colo- rado squawfish spawned in the lower Yampa River into the more abundant backwaters of the Green River in late summer, when wazmer water temperatures enhance food production and fish growth in these nursery habitats (Tyus and McAda 1984; Haynes et al. 1985; Tyus 1986). Tyus (1986) suggested that spawning by Colorado squawfish is timed to occur at the end of the flood season to exploit river transport as a means to disperse progeny and to coincide with the forma- tion of the productive nursery backwaters down- stream from spawning areas. It is plausible to suggest that the flow spike cue, in a background of decreasing flow and increasing water tempera-