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-11- <br />through September in Hawaii. Shelton (1964) shoed evidence of a second <br />complement of ova developing in spawned female shad in central Arizona <br />lakes but only in part of the population of shad two years and. older. He <br />further demonstrated that shad from different age-groups spawned at different <br />times. Fish of age groups III-IV had a spawning peak about April 10, while <br />age-group I fish had a spawning peak in late May, more than a month later. <br />Shad spawning in Lake Powell began in early May in the extreme upper <br />reservoir and continued down lake. This proogressive spawning pattern was <br />similar to that described by Netsch et al. (1971) in Beaver Reservoir, Arkan- <br />sas. At Beaver Reservoir the shad popultion peaked four-eight weeks after <br />spawning, then numbers decreased rapidly due to mortality. Occasionally, <br />followed due to continued spawning. At Lake Powell, the <br />secondary peaks, <br />population peaked in mid-July, approximately` six weeks after spawning began. <br />At all sampling areas, numbers then decreased and later increased again (Fig- <br />ure 3). Continued spawning definitely occurred at Wahweap and Bullfrog as <br />shown by larval occurring in trawl catches in August and September. <br />Smallest shad captured per tow at Wahweap was consistently near 15 mm, which <br />was about the smallest fish that could be captured in the trawl. Larval <br />shac' were taken less often at Bullfrog indicating a weaker continued spawn <br />than at TTahweap (Figure 2). Prolonged spawning was Tot noted at Hite, <br />size of the smallest fish steadily increasing after July. Anderson (1973) <br />found that density dependent factors including crowding and nutritional con- <br />ditions affected spawning of gizzard shad (Dorosoma cepedianum). Strong <br />year classes of shad were produced frOM low density populations of adult shad <br />and. wok year classes from high density adult populations., Heidinger and <br />Imboben (1974) demonstrated that young-of-the-year shad introduced into a <br />shad-free enviror ent could mature and successfully spawn the same year. The