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68 <br />and indicates scene other factor such as esophagus diameter or mouth <br />breadth (Wankowski 1979) limits food size selection. <br />studies of lower Colorado River basin mainstream reservoirs (Table <br />4) indicate that rations of potential foods are highly variable <br />in time and space. Zoaplankton densities in Lake Mohave ranged frcam <br />27.3 to 44.6/L (X=26.3) at times of year when razorback sucker larvae <br />were present (February to April), and in the general areas of larval <br />occurrence in 1982 (Larry J. Paulson, pers. comm., unpubl. data). In <br />1985, however, density of zocplankton averaged only 1.5 organisms/L at <br />the specific times and places where larval razorback suckers were <br />present. Zocplankton densities reported in a Labe Mohave backwater <br />were even lower than in the main reservoir, averaging 0.4 potential <br />food items/L in six samples (Marsh and Langhorst 1988). Spatially, <br />annual means of about 1.8 to 45.5 organisms/L in Lake Mohave tend to <br />be lowest at stations near the inflow of hypolimnetic water from Lake <br />Mead and highest in the body of the lake (Cottonwood Basin) to near <br />Davis Dam (Paulson et al. 1980); larval production is concentrated in <br />Cottonwood Basin (Langhorst and Marsh 1986). <br />Wild-caught larvae averaged 10.6 mm TL in Lake Mohave in 1985, <br />and no growth was recorded despite a February-through-early-April <br />period of larval occurrence (Nam and Langhorst 1988). While it is <br />not ]mown how old these larvae were, fish of cagmrable sizes in ponds <br />at Dexter NFH were 14 to 21 d post hatching in high fertilization <br />treatments at invertebrate densities of 43.3/L, and 21 to 28 d old in <br />unfertilized ponds at 12.5 organisms/L. In the Lake Mohave backwater,