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66 original habitat in backwaters and channels of turbid, unpredictable, desert rivers and the time of year when their eggs hatch. Larvae appear mostly in February and March in Lake Mohave. under natural conditions in the pristine Colorado River system, this timing would have corresponded to later parts of a long period of low flow in summer, autumn, and winter (Turner and Rarpiscak 1980), and may have been a time of relatively high potential food supply in sandy- bott?d, plankton poor habitats. Snowmelt typically resulted in maximum yearly discharge in the Colorado mainstream during May and June, during which time backwaters and channels would be flooded and reshaped. The juvenile razorbacks, at this time 40 mm TL, or larger, may have used backwater habitats or moved into smaller rivers and creeks, or downstream to wider, slower-flowing areas. Unlike survival, rates of growth of razorback sucker larvae in high- and medium-treatment ponds were significantly greater than in unfertilized ponds (Fig. 6). Nonetheless, larvae larger than 12.0 mm TL from all ponds consistently had about 100 organisms in their digestive tracts, apparently maintaltli.Tlg a near maximum fullness with no significant increase in numbers of items eaten by shifting to larger foods with growth (Fig. 8). Assuming 100% success, an individual larva would have been required to search only 4.2 or 2.3 _ i 41100 organisms in medium- and high-treatment ponds, respectively. Two or three times that water volume, 8.0 1+1100 food items, and thus more effort, would have been required in an unfertilized pond, so the