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344 MARSH AND DOUGLAS TABLE 1.-Numbers and mean total lengths (ranges in parentheses) of introduced piscivores and their fish prey in the Little Colorado River, Arizona, July 1991-June 1995. Some prey (ND, not determined) were in states of digestion that prevented species identification, measurement, or both. Native taxa are indicated by an asterisk (*). Predator Species N Total length (mm) Taxon Prey fishes in stomachs N Total length (mm) Rainbow trout 3 356(332-398) Humpback chub* 3 43(40-45) 2 372(370-373) Speckled dace* 2 60(45-75) 2 353(335-370) Fathead minnow 5 42(30-60) 1 355 Common carp 1 60 1 375 Flannelmouth sucker* 1 78 1 350 Bluehead sucker* 1 ND 4 351(319-383) ND Cyprinidae 9 46(30-65) 1 341 ND Catostomidae* 1 43 10 345(313-389) ND 16 96 (34->150) Channel catfish 8 554 (375-790) Humpback chub* 22 115(85-200) 5 453(271-594) Speckled dace* 6 67(60-75) 1 482 Fathead minnow 1 60 1 476 Common carp 1 116 2 525(455-594) Hannelmouth sucker* 2 175(150-200) 4 599(582-605) Bluehead sucker* 5 211(150-250) 2 480(477-482) ND Cyprinidae 6 58(40-80) to 452(281-595) ND 11 100(58-200) Yellow bullhead 1 167 ND Cyprinidae 3 39(37-41) 1 171 ND 1 >100 Black bullhead 2 152(134-169) Humpback chub* 2 52(49-55) Brown trout 1 341 Speckled dace* 1 77 1 566 ND 1 118 ther their guts were removed and fixed in 10% formalin or whole fish (<100 mm) were preserved in 95% ethanol. The entire digestive tract was ex- amined and food composition was determined in the lab. Fish prey were identified and measured if enough of the specimen remained. Pharyngeal tooth counts aided in discriminating among cyp- rinid prey. We also examined stomach contents microscopically to determine if larval fishes had been eaten. Results and Discussion Humpback chub remains were identified in 13 (3.2%) of the 408 digestive tracts examined (Table 1). Overall, remains of native fishes were in 13.7% of 219 stomachs that contained food. Common carp Cyprinus carpio, speckled dace Rhinichthys osculus, fathead minnow Pimephales promelas, bluehead sucker Pantosteus (=Catostomus) dis- cobolus, flannelmouth sucker Catostomus latipin- nis, and unidentified fish remains were in stomachs of 51 predators, 5 of which also had eaten hump- back chub (Table 1). Mean length of humpback chub in stomachs (102 mm TL; N = 27) did not differ significantly (two-sample t-test, Snedecor and Cochran 1967) from that of other ingested fishes (92 mm; N = 74). No larval or small post- larval fishes were found, probably due to the tran- sient presence of this fragile stage in predator guts. Rangewide, the humpback chub is represented by several sustaining populations, but assessment of the effects of predation on early life stages on long- term population viability cannot be made until methods to quantify this predation are worked out. Stomachs of 3 of 174 rainbow trout (prey length 40-45 mm) and 2 of 12 black bullhead (49 and 55 mm TL) contained humpback chub; 22 rainbow trout had eaten other fishes. Speckled dace and an unidentified fish were in 2 of 10 brown trout stom- achs, and unidentified fish remains were also in 2 of 10 yellow bullheads. Eight of 202 channel cat- fish ate 1-7 chub each (85-200 mm). Fish remains were in 25 other stomachs. Channel catfish that ate fish averaged 500 rum TL, which was larger than catfish that had not eaten fish, a result sup- ported by Tyus and Nikirk (1990). Algae (primarily Cladophora) were the most common food of rainbow trout (47% frequency of occurrence), brown trout ate mostly terrestrial in- vertebrates (20%), and fish was the most common food of other predator species (10-20%; Table 2). Rainbow trout and channel catfish consumed a va- riety of other items including vegetation, amphi- pods Gammarus lacustris, aquatic insect larvae,