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<br /> <br />InHially, trends in drift densities closely followed trends in water <br />temperature. Mean weekly densHies increased from 0.0056 fish per cubic <br />meter in the second week of sampl ing to a peak of 0.082 fish in the fourth <br />week as river temperatures rose from 16 to 200C and discharge decreased from <br />about 525 to 230 cms (Figure 7). The continued warming and decreased <br />discharge of the river in the fifth week of sampl ing, however, did not <br />result in increased drift. mean weekly densHy at that time dropped to only <br />0.011 fish as the river temperature peaked between 20.5 and 21.00C. <br /> <br />No drifting fish were observed the seventh week of sampling (August 17- <br />23), suggesting that drift activHy was completed. however, a second pulse <br />of drifting fish was observed in late August and early September as the mean <br />weekly densHy rose to 0.047 fish per cubic meter. This pulse cannot be <br />explained as a function of river temperature or discharge, since these <br />parameters were relatively stable during this period (Figure 7). This <br />second pulse probably reflects the history of spawning activity in the area. <br /> <br />~ <br />~ <br />.. <br />.. >, -;:- <br />:0 <br />.. .. .. <br />.. -;;; .. <br />X- c:: 1 <br />.. <br />I 0'" <br />.. . <br />... .... <br />.. ~ 0 <br />.. ... c:: <br />.. c ~ <br />;; <br />20 0.07 <br /> 0.06 <br />15 0.05 <br /> 0.04 <br />10 <br /> 0.03 <br /> 0.02 <br />5 <br /> 0.01 <br />0 0 <br /> <br />- <br />.. <br />.... <br />... <br />.!. <br /> <br />.. <br />... <br />.. <br />~ <br />u <br />.. <br />C; <br /> <br />900 <br /> <br /> <br />. <br />\ OlsCNrge <br /> <br />\ / . <br />\ .--/ <br />\ ,- <br />y--..' <br />./... <br />" .......... <br />.' ....... <br />\ <br /> <br />...... W.ter TetI~perlture <br /> <br />..... '" .....- <br />el -e- - -., <br />... . <br />'e-e--_e"" '. <br />, <br />'. <br /> <br />750 <br /> <br />600 <br /> <br />~oenstty <br /> <br /> <br />450 <br /> <br />. <br />\ <br />\ <br />~- <br />-.... <br /> <br />300 <br /> <br />150 <br /> <br />o <br /> <br />SEPTEHIlEa <br /> <br />JULY <br /> <br />FIGURE 7. Mean weekly densities of drifting larval fish compared to water <br />temperatures and discharge at the Palisade Site on the upper <br />Colorado River, 1984. <br /> <br />574 <br />