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324 RAZORBACK SUCKER SPAWNING <br />1994 <br />(A <br />MC <br />3 <br />O <br />LL <br />s00 <br />500 <br />400 <br />300 <br />...v.. vw~,no~yc <br />200 -'' Temperature <br />I Number of fish <br />100 p° <br />0 <br />Apr May <br />1995 <br />~~ <br />0 <br />soo <br />500 <br />400 <br />300 <br />200 <br />100 <br />0 <br />Apr May Jun <br />18 <br />15 <br />12 <br />N <br />7 <br />.-~ <br />9 ~ <br />a <br />C <br />6 ~ <br />3 <br />0 <br />18 <br />15 <br />12 <br />9 ~ <br />at <br />a <br />E <br />6 ~ <br />3 <br />0 <br />5 <br />4 ~ <br />y <br />l1 <br />w <br />3 ° <br />at <br />2 ' <br />z <br />1 <br />0 <br />5 <br />4 s <br />3 ° <br />at <br />.n <br />E <br />2 ' <br />z <br />1 <br />0 <br />Ficutm 4.-Comparison of the number of adult razorback suckers (with implanted radio transmitters) in the Escalante <br />spawning area with water temperature and discharge during May and June 1994 and 1995. <br />Spawning migration of the quillback Carpiodes cy- plained by the product of the two variables (Barton <br />prinus was initiated by increased discharge once 1980). The importance of increasing flows to ini- <br />temperatures exceeded 5°C (Parker and Franzin tiating spawning migrations has been reported for <br />1991). Discharge explained greater variation in the many fish species (e.g., Libosvarsky 1976; Wel- <br />number of migrating white suckers Catostomus comme 1985; Masse et al. 1991). Northcote (1984) <br />commersoni in an Alberta tributary than temper- suggested that a hierarchy of environmental fac- <br />ature, although the greatest variation was ex- tors stimulated fish migration in rivers. In some <br />1 <br /> <br /> <br />