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FEEDING BY LARVAL RAZORBACK SUCKERS <br />10.00 F <br />E 1.00 <br />E <br />E 0.10 <br />0 <br />0.01 <br />A <br />- ----------- <br />10 20 30 <br />103 <br />102 <br />Q) <br />10 <br />z <br />100 <br />10-t <br />B <br />10 20 30 <br />Total fish length (mm) <br />FIGURE 5.-Relationship between loglovolume (A) <br />and logo number (B) of organisms eaten and total length <br />of razorback sucker larvae in ponds at Dexter National <br />Fish Hatchery, New Mexico, 1985. Dotted lines indicate <br />95% confidence intervals. <br />maximum annual discharge in the lower Colorado <br />River mainstream in May and June, when back- <br />waters and channels alike would have been flood- <br />ed and reshaped. Juvenile razorback suckers, by <br />then estimated to be 40.0 mm TL or larger, may <br />have used backwaters, moved into smaller rivers <br />and creeks, or moved downstream to wide, slower- <br />flowing reaches. <br />Unlike survival, growth rates of razorback suck- <br />ers were significantly greater in high- and medi- <br />um-fertilization treatments than in unfertilized <br />ponds. Nonetheless, larvae larger than 15.0 mm <br />TL from all ponds consistently had about 100 or- <br />ganisms in their guts. As larvae grew, they appar- <br />ently maintained gut fullness by shifting to larger <br />foods, not by eating more prey. A larva would <br />have had to search only 2.3 or 4.2 L to encounter <br />100 organisms in ponds with high or medium fer- <br />1.5 <br />E 1.0 <br />E <br />t <br />v_ <br />3 <br />i 0.5 <br />4 <br />0.0 <br />349 <br />GAPE <br />j MAX <br />MEAN <br />MIN <br />10 15 20 25 <br />Total fish length (mm) <br />F[GURE 6.-Relationships between maximum (MAX), <br />minimum (MIN), and mean (MEAN) body widths of <br />prey organisms in larval razorback sucker guts and total <br />length of larvae from ponds at Dexter National Fish <br />Hatchery, New Mexico, 1985. The relationship between <br />calculated mouth size (GAPE) and total length is also <br />plotted on the same scale as prey width. Dotted lines <br />indicate 95% confidence intervals. <br />tilization, respectively. A search of two or three <br />times that volume (8.0 L) would have been re- <br />quired to yield 100 organisms in an unfertilized <br />pond. The energetic costs of searching for dis- <br />persed food may account for differential growth <br />among treatments; however, we did not calculate <br />evacuation rates in this study and therefore cannot <br />estimate consumption rates. The success of larvae <br />in capturing encountered prey also is unknown. <br />Razorback sucker larvae smaller than 12.0 mm <br />TL positively selected small organisms of 0.1-mm <br />body widths and did not take larger potential foods. <br />As larvae grew, larger organisms were preferen- <br />tially taken until mean widths of animals ingested <br />were about 0.3 mm in fish of about 20.0 mm TL. <br />Prey of appropriate sizes to be ingested were al- <br />ways present in ponds, and there was little appar- <br />ent taxonomic preference that could not be ex- <br />plained by a combination of size and abundance. <br />Selection of most organisms was random as mea- <br />sured by the linear selection index, and such pos- <br />itive or negative selection as occurred was scarcely <br />significant-except for cladocerans, which were <br />positively selected by larger fish. Seifert (1972) re- <br />ported a similar diet of rotifers (first) and cladoc- <br />erans (later) for larval white suckers, but noted an <br />overall indiscriminant feeding behavior. Wild ra-