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<br /> <br />dri ft for a <br />od from July <br />r the 9-week <br />~er . <br /> <br />Diel Dri ft <br /> <br />Daily sdmpling by 4-hour periods, for most of the 9-week study, <br />revealed d definite pattern in diel drift (Figure 4). Highest drift rates <br />(0.037 fish per cubic meter) occurred during the period 2000 to 2400 hours, <br />while the lowest density (0.0088 fish/cubic meter) occurred in the preceding <br />period of 1600 to 2000 hours. Although this represents a four-fold increase <br />in drift denSity, analysis of variance revealed no significant differences <br />(p>0.05) between any time periods. <br /> <br />uly when the <br />apid warming <br />er d i scha rge <br />bout 284 cms <br />seen during <br />to l70C and <br /> <br />The largest numbers of drifting larvae were noted during very dark <br />evenings (2000-2400 hours) preceding the appearance of a full moon. This <br />observation and the high night-time drift densities suggest that the larval <br />fishes are photosensi live, and many are actively capable of entering and <br />escaping the surface drift currents. The absence of a significant diel <br />dri ft pattern may be attributed to the effect of frequent. turbidity and <br />sediment load on in-stream light penetration and intensity. <br /> <br />.h per cubic <br />averaged 234 <br />IIgust 8-14), <br />d 209.5 cms. <br />tempera tu re <br />lures duri ng <br /> <br />The 1 ack of signi ficance in di.el dri ft at the Parachute Si te might be <br />dttributed to pool ing of samples collected over the 9-week period. This <br />caused high seasonal and diel variation in densities. To reduce this <br />effect, the diel sample effort at the Pal isade Site was designed to sample <br />drift over a short time period (see Results of Palisade Study). <br /> <br />A comparison of the sizes and developmental phases of larvae collected <br />in drift nets with those collected concurrently in shoreline seine hauls <br />indicates that, for certain species, only a well-defined size group was <br />drifting, regardless of sample time. The native species (flannelmouth <br />sucker, bl uehead sucker, roundta il chub, and speckl ed dace) were captured at <br />the highest frequency in drift during their mesolarval phase of development <br />(Figure 5). A relative increase in late mesolarvae and metalarvae in <br />shorel ine seine haul s was accompanied by a concurrent decrease in these <br />forms in drift samples. The introduced white sucker, also a riverine <br />species, showed the same propensity to drift as mesolarvae. However, the <br />fathead minnow, an introduced nonriverine species, exhibited no distinct <br />pattern of drifting by length (Figure 6). <br /> <br />- <br />~ <br /> <br />! <br />.. <br />~ <br />! <br />~ <br />; <br /> <br />100 <br /> <br />'so <br /> <br />iOO <br /> <br />so <br /> <br /> 0.Ol5 <br /> O.Ol <br />.. <br />.. <br />.. <br />I 0.025 <br />.., <br />~ <br />0 <br />~ 0.02 <br />.. <br />.. <br />.. 0.015 <br />c: <br />.. <br />Q <br />c: <br />.. <br />.. O.CI <br />'" <br /> 0.005 <br /> 0 <br /> 0 <br /> <br />DO <br /> <br />;0 <br /> <br />I to water <br />! upper <br /> <br />length of Drifting Fishes <br /> <br /> <br />'\ <br />, <br />, <br />, <br />, <br />\ <br />, <br /> <br /> <br />1\\..----- ...-- <br /> <br />---A.. <br />P.lts.de SIte ~......... , <br />...~ <br /> <br />.. <br />I <br />, <br />I <br />I <br />I <br />I <br />I <br />, <br />, <br /> <br />0400 <br /> <br />0800 <br /> <br />1200 <br /> <br />1600 <br /> <br />2000 <br /> <br />2400 <br /> <br />TI.e PerIods <br /> <br />FIGURE 4. Mean densities of drifting larval fish during sh time periods at <br />the Parachute and Pal i sades study sites. <br /> <br />571 <br />