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ends in water Ish per cubic in the fourth lec rea sed from nd dec rea sed !ver, did not "Opped to onl y ,O.C. Diel Dri ft Ig (August 17- second pul se ~r as the mean I se cannot be since these re 7). This , in the area. Drift nets were set over a 24-hour period to evaluate diel patterns of drifting fishes. This sampl ing was conducted during the week of maximum drift to insure sufficient numbers of fish for the analysis. Paired shoreline surface samples, taken at about 2-hour intervals, indicate that drift densities were highest between midnight and 0400 hours (0.023 fish per cubic meter) and between 2000 hours and midnight (0.021). About 60 percent of the drift was recorded during these two time periods. The number of drifting fish between 0400 and 2000 hours was relatively consistent, varying between 0.009 and 0.004. The lowest drift density was observed between 1600 and 2000 hours. An analysis of variance revealed a significant difference (:,=0.01) between the hourly means. By using Fisher's least significant difference procedure for multiple comparisons, it was determined that there were significant differences (p<O.OI) between the two maximum values and the four minima. This indicates that drift densities at the Pal isade Site varied with time of day, with the highest densities occurring between 2000 and 0400 hours. .. .... ... .!. Vertical Distribution Of Drift .. GO .. 2 u .. o Paired sets were made with shoreline drift nets during a 24-hour sample effort to assess the vertical distribution of larval drift. Sixteen samples were taken from each of two levels in the vertical water column. level one was the uppermost 30 cm of water, incl uding the water surface. level two extended from 30 to 90 cm below the surface of the water with the net frame resting on the river bottom. An analysis of variance revealed no significant difference (p=0.15) between the mean drift density i~ the upper (0.014 fish per cubic meter) and lower (0.009 fish per cubic meter) levels. It appears from this analysis that there is uniform mixing of drifting larval fishes to a depth of at least 90 cm in the vertical water col umn along the shoreline. 900 750 . .... \ '. 600 DISCUSSION 450 Although the species composition of shoreline surface drift and midchannel surface drift was similar, fish densities in the former were nearly seven times greater, suggesting that the majority of larval fish drifting in the upper Colorado River do so along the shorel ine; however, in neither of these studies was the midchannel benthic zone sampled. Diel patterns in densities of surface drift indicate that the larvae may drift on the surface duri ng darkness (2000-0400 hours) and descend to dri ft in the benthic zone .or to rest in the substrate during dayl ight. Use of the midchannel benthic zone will not be known until effective quantitative drift samples can be taken in this deep, high-velocity area. 300 \ 150 0 .EIl ared to water the upper Drifting larvae were first seen at both sites three weeks before peak densities when the water temperature first reached 160C during a rapid warming period. Over 60 percent of the drift occurred during a 2 to 3 week period in mid to late July when water temperatures ranged from 18 to 210(, and river discharge dropped rapidly after spring runoff. Drift lasted for 9 to 10 weeks from the first of July to the first of September. From these observations, it appears that water temperature greatly influences the timing of larval drift in the upper Colorado River. It is not only the temperature at the time of hatching that appears critical, bu.i: ;llso the temperature during spawning, and egg development. A second pul se of drifting larvae at the Palisade Site in 1984 suggests that the spawning event for that year was spl it in two by a short period of unfavorable water temperature. 575