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backwaters (Kidd 1977, McAda and Wydoski 1980, Valdez et al. <br />1982a), but sometimes moved into faster, shallower habitats, <br />presumably to spawn (McAda and Wydoski 1980, Tyus and Karp 1989, <br />Tyus and Karp in press). In winter, razorback .suckers in the <br />Colorado River, Colorado, used deep (> 1.3 m), low-velocity (0.3 <br />m/s) habitat with silt and sand substrate (Osmundson and Kaeding <br />1989a). In the Green River, Utah, overwintering adults were found <br />over silt and cobble substrate where water was less than 1 m deep <br />and current velocity was less than 0.3 m/sec (Valdez and Masslich <br />1989). <br />Movement <br />Transport and movement of razorback sucker larvae in riverine <br />environments is poorly understood. In studies of other catostomid <br />larvae, low-velocity stream habitats such as backwaters and stream <br />margins were used extensively (Hinckley 1973, Scott and Grossman <br />1973). Downstream transport of larvae can occur when current <br />velocity exceeds swimming ability. Movement distance downstream <br />probably depended on size and developmental state of larvae, <br />stream size, current velocity, turbidity, channel morphology, and <br />perhaps water temperature (Bestgen et al. 1987). In a laboratory <br />experiment, two-week-old razorback sucker larvae were tested for <br />response to varying flow and light conditions (pers. comm., H. M. <br />Tyus, U. S. Fish and Wildlife Service). Most downcurrent movement <br />occurred during darkness in the fastest flow conditions, but <br />decreased as age, and presumably, size, increased. <br />29 <br />