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62 Zooplankton in Lake Mohave was predomi- nated by Cladocera (Bosmina and Daphnia to- gether comprised nearly two-thirds of total num- bers) and Copepoda (about one-third of total, including nauplii, copepodites, and adults). Ro- tifera were of minor significance (Table 1). Total density averaged 1,54 organisms m-3. These same animals were most abundant in larval diets (Tables 1, 2), and relative abundances indicated positive selection for Bosmina and negative selection (or avoidance) of copepods (Table 1). Larval suckers showed positive selection for zooplankters 400 to 999 x 10-4 mm3 and avoidance of those 100 to ,99 x 10-4 mm3 (Fig. 2A), a' pattern nearly identical Table 1. Relative abundance of predominant zooplankters in water samples (p;) and larval razorback sucker stomachs (r;) and linear index of selectivity (L) in Lake Mohave and the Arizona Bay backwater, Arizona-Nevada. 1985. Prey items Lake Mohave Arizona Bay Backwater p; r; L A ri L Rotifera 5.5 1.2 -4.3 57.9 10.7 -47.2 Cladocera BVSnu.-ta 511.0 9V.3 J7.J 114 41.9 p 28.5 Daphnia 12.8 2.4 -10.4 12.8 17.7 4.9 (subtotal) (63.8) (92.7) (28.9) (26.2) (59.6) (33.4) Copepoda 30.7 1.2 -29.5 16.0 1.6? 0.2 Other <1.0 4.8 4.8 <1.0 13.5 13.5 Total n 345 165 382 1,020 Mean density (no. M-3) 1,554 377 Table 2. Food habits (percent occurrence and mean t SE number of items per stomach that contained food) of larval razorback sucker in Lake Mohave and the Arizona Bay backwater, Arizona-Nevada, 1985. Prey items Lake Mohave Arizona Bay Backwater Percent Mean Percent Mean Rotifera 1.6 1.0 ± 0 53 4.2 ± 7.0 Cladocera Alona Bosmina Ceriodaphnia Chydorus Daphnia Macrothrix Copepoda Ostracoda ` Hydracarina Insecta' Chironomidae Trichoptera Undetermined Organic matter Larvae examined Number (%) empty 21.8 6.4 1.6 2.4 1.6 8.1 - 3 1.0 ± 0 5.6 ± 5.0 55 10.4 ± 14.2 - 35 5.4 ± 4.5 - 5 1.0± 0 1.0±0 44 5.5± 4.9 3.0 ± 1.4 - - 1.0 ± 0 45 4.8 ± 9.5 - 4 4.3 ± 4.2 - 3 1.0± 0 - 5 1.0 ± 0 - 2 1.0± 0 - 9 10.7 ± 10.8 124 75 83 (67) 28 (42)