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SPAWNING CUES FOR COLORADO SQUAWFISH flow patterns between the gages and the spawning area is that the beginning of the -flow spike oc- curred simultaneously at the two gage sites, sug- gesting that rainstorms affected the region and not a more limited upstream locale. Sampling Artifact or Spawning Cue? The frequency distribution of estimated spawn- ing dates was a function of number of larvae sampled. The question arises as to whether peaks observed in these frequency distributions in 1983- 1986 were sampling artifacts caused by a flushing effect in the spawning grounds due to the flow spike or a result of drift nets sampling more water (and thus more larvae) during rising flow even though catch per unit of water volume sampled remained the same. The answers are no to both, because lengths, not numbers, of captured larvae were used to estimate spawning dates, that is, when the eggs were deposited in the gravel. The bimodal distribution of Colorado squawfish larvae captured in 1985 (Nesler 1986) supports this argu- ment. The length range of Colorado squawfish larvae responsible for the July 23 spawning peak was similar to that of larvae responsible for the June 25 spawning peak (8.0-9.5 mm versus 7.9- 9.4 mm total length, respectively). This indicates that both groups consisted of protolarvae and that the later spawning peak was not an artifact caused by older, larger larvae being flushed out with the second flow spike. Hamman (1981) observed that Colorado squawfish eggs are adhesive and that hatched larvae do not leave the interstitial spaces of the cobble substrate for 3-4 d following hatching. These observations suggest that the eggs are fairly well protected from flow variations and that Col- orado squawfish larvae would emerge from the gravel some time after the flow spike occurrence. Larvae were captured at lower flow levels well after the flow spike event occurred. Also, larvae were captured by drift nets in the nearshore zone of the river cross section. The characteristic low flow of this zone is unaffected by the amount of water moving downstream. The nearshore zone, and hence the drift-net sampling site, merely changed its relative position in the channel with little increase in current velocity or volume of water sampled. The critical point is that estimated spawning dates derived from total lengths of the captured larvae indicate the larvae were deposited in the gravel as eggs during a flow spike event. The spawning periods estimated from collec- tions of larval Colorado squawfish have been 7s corroborated by radiotracking data on adults and collections of ripe adults on the spawning ground. Estimates of spawning dates made from larval collections, radiotracking, and collection of ripe adults were generally in agreement during 1981- 1985 (Tyus et al. 1987). Estimations of optimal spawning periods for 1981-1985 were presented by Tyus et al. (1987). Their estimates and ours agree closely except for 1984; their estimated optimal spawning period was July 20-August 10 in 1984, but we estimated the end of peak spawning activity at July 18. The optimal spawning period estimated by Tyus et al. (1987) for 1984 was based on data from five radiotracked fish, the collection of 16 ripe adults (only two of which were females), and 58 larvae collected in the Green River below the confluence of the Yampa River in late August- early September. Our estimate resulted from 365 larval Colorado squawfish collected from July 17 through August 8. Tyus et al. did observe ripe Colorado squawfish adults on the primary spawn- ing reach in the Yampa River at least as early as July 15. We believe that substantial spawning activity occurred prior to this date but was not detected from radiotracking observations or adult fish sampling efforts by Tyus et al. (1987). In reference to flow conditions at the cessation of peak spawning activity, remarkably similar flows occurred in 1983, 1985, and 1986, whereas the 1984 flow seemed notably higher. Another unaccountable variable was the overall number of adult Colorado squawfish participating in the spawning run from year to year. Most of the available adult Colorado squawfish might have participated during the peak of spawning in 1984, leaving relatively feyv adults to spawn afterwards even though flows were still above 55-65 m3/s. The cessation and resumption of spawning with fluctuating flows in 1985 supported the hypothesis that major spawning activity ceased at flows be- low this threshold. Flow Spikes as Spawning Cues Our approach to the observed relationship be- tween Colorado squawfish spawning and seasonal flow regimes in the Yampa River is quite simplistic in scope relative to the complex interaction of external and internal cues that may initiate and regulate gonadal development, migratory behav- ior, spawning, and egg deposition. The relation- ship between internal and external rhythms is subtle and the timing of a species' reproductive cycle is a compromise involving many environ- mental considerations (Bye 1984). Environmental