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-5- <br />One sonar transect was run each sample night before trawl samples were <br /> <br />taken. The boat was operated at an idle speed of 800 RPM for a period of <br />eight minutes, while sonar transects were run over the trawling course. <br />Number and biomass of shad. captured per standard tow were ca pared with <br />erhograns showing fish trace density in the epilimnion. <br />The trawl selectively captured larval and juvenile shad with adults <br />taken infrequently. Larval and juvenile shad were differentiated by total <br />length after criteria presented by Barnes (1977). Larvae were designated as <br />shad smaller than 25 mm. All shad collected in the trawl were imnediately <br />preserved in a 10 percent formlin solution. Number of fish, eight of <br />catch, average total length, and range were determined for each haul. <br />Results <br />The first indication of threadfin shad spawning activity during 1977 <br />was the capture of larval shad on May 19, near Dark Canyon and the inlet of <br />the Colorado River. Surface temperature at that time was 64 F. Shad were <br />observed spawning in the ipper San Juan arm on May 22. Spawning was first <br />observed at Bullfrog on May 29 (63 degrees F) and at Wahweap on May 31 <br />(66 degrees F). Spawning systematically progressed from the inlet down lake <br />toward the dam. Spawning was last observed on Jime 14 (76 F) at Bullfrog- <br />Average. length of shad captured per standard toga increased caith time <br />at all sampling locations (Figure 2). The length of the smallest fish cap- <br />tured per tows constantly increased with time at all sampling locations (Fig- <br />ure 2). The length of the smallest fish captured per tow constantly increased <br />after July 18 at Hite, while size of the smallest fish captured at Bullfrog <br />showed a slight decrease after that date (Figure 2). The smallest fish cap- <br />tuned at Wahweap stayed relatively constant throughout the sampling season.