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150 v Low 100 L rn 3 0 E 50 rn J.- 0 2 4 6 8 30 v Low E 25 ? Medium E O High 01 c: 20 w f - 15 0 o ?v? L 10 FEEDING BY LARVAL RAZORBACK SUCKERS 0 2 4 6 8 Week FIGURE 3.-Mean weight (A) and mean total length (B) of razorback suckers reared for 8 weeks in ponds fertilized at low, medium, and high levels at Dexter Na- tional Fish Hatchery, New Mexico, 1985. There also were no differences among treat- ments in selection of prey by taxon or size. Data were therefore pooled for analysis of larval food selection. First-feeding larvae selected rotifers and tiny chironomids. Copepods and chironomids were 347 eaten by intermediate-sized fish, and the largest fish clearly selected cladocerans (Figure 7). These taxonomic trends are primarily explained by prey size (Figure 8). Small larvae selected animals 0.1 mm or less in width (largely rotifers and chiron- omids). As larvae grew, they shifted to copepods (about 0.2 mm wide), and they ate organisms that averaged 0.3 mm (mostly cladocerans) when 20.0 mm TL or longer. From our data we do not know whether organisms wider than 0.3 mm might have been preferred by the larger larvae, because few invertebrates in our ponds exceeded this size. Ra- zorback sucker larvae may have selected the larg- est organisms that were also most abundant. Discussion Addition of nutrients to an aquatic system tends to increase density of primary producers and in- vertebrates (Geiger 1983; Geiger et al. 1985; Pace 1986), which in turn may enhance food-related survival and growth of fishes. Populations of algae and invertebrate consumers increased rapidly in our experimental ponds after fertilized and unfer- tilized ponds were filled. Invertebrate populations had broadly similar growth patterns in all ponds, although they varied considerably in detail. Fer- tilized ponds developed greater numbers and biovolumes than did untreated ponds. Numbers were positively correlated with fertilization rates, but biovolumes in untreated ponds and those fer- tilized at an intermediate level were similar and lower than in highly fertilized ponds. Inverte- brates were dominated by small-sized taxa, es- pecially early in the experiment. Invertebrates had greater size diversity when ponds were fertilized than when they were not. There were no significant differences in su ival of larval razorback suckers in the various reat- ments. Presence or absence of tiger salam nder larvae, which can be both competitors of fis lar- vae (Collins and Holomuzki 1984) and pred tors TABLE 3.-Frequency of occurrence of various food items in stomachs of razorback sucker larvae during th? first 7 d after larvae were stocked into ponds. Frequency data include larvae with empty stomachs. Week 1, day Larvae (N) Frequency of occurrence (%) in stomachs Phytoplanktona Diatoms Detritus Otherb Empty (' Yo) 8 8 50 38 50 0 25 9 10 30 50 50 20 50 10 68 22 47 51 10 40 12 30 25 67 80 63 17 13 13 46 38 62 69 15 14 43 23 49 51 88 5 I Excludes diatoms. b Includes rotifers, nauplii, cladocerans, invertebrate eggs, and chironomids