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Colorado pikeminnow have been reported to begin spawning 4-6 weeks after the flows peak in the Yampa River by Nesler et al. (1988). A temperature threshold of 20-22 °C has been suggested as optimal for spawning by Hamman, (1981) and others, however Haynes et al. (1984) estimated spawning to have begun at temperatures as low as 18°C in 1979. Nesler et al. (1988) suggested that temperature plays a secondary role in the spawning requirements of Colorado pikeminnow in the Yampa River. They proposed that temporary, sharp increases in flows occurring after the peak flow (secondary flow spikes) were likely a spawning cue for Colorado pikeminnow, in association with other environmental variables such as temperature threshold in the Yampa River. In this study, spawning activity was estimated to commence 1 to 4 weeks post-peak. The onset of spawning occurred at flows ranging from 232 to 1062 m3/s occurring later with higher peak flows (Table 5, and Figure 8). The magnitude of the peak flow was significantly correlated with the date of the onset of spawning (rz = 0.96, p=.01) The correlation between later onset of spawning and high average flow in June was slightly stronger, with rz of 0.98 (p=.005) (Table 5). Peak discharge was not correlated to the duration (days) of spawning activity, or to the number of days pikeminnow larvae were actually collected. However, peak discharge was significantly correlated (p< 0.05) to the duration of the drift, from the first to last day larvae were collected (Westwater: rz = -0.94; Moab: rz = -0.99). In each year of this study, spawning activity appeared to be associated with secondary flow spikes on the descending limb of the hydrograph after peak discharge had occurred (Figure 8). However, the number of days between the flow spike and the onset of spawning was quite variable, and sometimes spawning was estimated to have occurred slightly before a flow spike. An association does appear to exist between flow spikes and the onset of spawning, however the relationship could have been obscured by the overestimation of larval ages during this study. The magnitude and duration of the peak also influenced temperatures, both discrete and accumulated. Peak discharge was strongly correlated to the date the river reached 18°C (rz = 0.89, p=.04), with the river reaching 18°C at a later date during high water years in the five years of study (Table 5). Degree day accumulations (DD) through the end of each month were lower in the high water years. The rz values for the Pearson's correlations between peak flow and the degree day accumulations were significant to the p < 0.05 level for July and August. The values for each month were: June, rz = -0.71; July, rz = -0.93 (p=0.02); and August, rz = -0.96 (p~.01). The onset of spawning occurred in a narrow range of degree day accumulations (1600 to 1868) for all years except 1995, when spawning did not occur unti12210 DD had been reached. Because the onset of spawning was several weeks later than in the other years of the study, the temperature accumulation was greater despite lower average temperatures. Degree day accumulation was strongly correlated with the onset of spawning (rz = +0.94, p = 0.02), and to the date the river reached 18°C (rz = +0.97, p = 0.007) (Table 5) Each year, spawning commenced soon after river temperatures reached 17 or 18°C, except in 1992, when the river reached 18°C eleven days before spawning was estimated to begin for the Moab site (Figure 9). In 1995, the river did not reach 18°C until July 17, and spawning did not occur until July 11 at the Moab site, and July 30 at the Westwater site. The dates of the onset of spawning and water temperature of 18°C were significantly correlated, with an rz of 0.98 (p=0.003). Both the date of 18°C and the onset of spawning were correlated with average June flows (Figure 10). A step-wise regression using the average June flow and date of 18°C with onset of spawning resulted in an adjusted R2 of 0.988. However, the co-linearity of flows and 9