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-- ~ I o ~ rI ~ ^ a o N ~ o N N ~ 1 ~ ~ n i ~ ~ ~ a ~ a ~o E ~.~ c, ~ ~ ` o ', ~ v~ d L -O . ~O N N '~ yr 4 ~<`2e ~' I .: `~ INTRnDUCED FISHES OF THE LC)WER COLORADO $5 Smallmouth bass stomachs contained large numbers of ephemeropteran nymphs and megalopteran larvae. Odonate naiads, including both damsel- and dragonflies, were taken by juvenile largemouth bass, warmouth, and green sunfish. Included were species of clambering damselfies typically found in beds of aquatic plants (see also Weaver and Zeibell 1976). Chironomids also were taken by the last three fishes. Bluegill depended strongly on Chironomids, along with zooplankton (Table 1 ). Redear sunfish ate chironomid larvae only inf•e- quently. Mouthbrooder and striped mullet contained a fe~v tiny chironomid larvae that ma}~ have been consumed along with detritus (especially in the latter). Mouthb- rooder, however, contained a' few terrestrial insects and a number of other benthic groups. Introduced palaemoneid shrimp, Palaemonetes paludosus, are common in the lower Colorado River, but contributed to the diet of only five fish species. Black crappie appeared to select the food item. Largemouth bass, warmouth, and green sunfish all fed on shrimp at about the same proportion. Channel catfish contained them at a frequency of only 3J%. Perhaps its semi-transparent body makes the shrimp relatively immune to all except special predators, especially when in dense aquatic vegetation. The introduced crayfish, Procambarus c/ark/, was a major food of almost all large carnivores, especially catfish and Smallmouth bass. Rainbow trout, carp, largemouth bass, warmouth, green sunfish, and black crappie also ate the deca- pod. Edwards (1974) reported crayfish from striped bass stomachs. Introduced softshelled turtle, Trionyx spiniferus, also depended heavily on them (present in 10 animals dissected, along with odonate naiads as the only other food item). Asiatic clams were eaten by carp, channel catfish, yello`v bullhead, redear sunfish, largemouth bass, and mouthbrooder (Table 1). !n all but redear sunfish, clams were digested without breakage of valves, with the shell simply passing through the intestinal tract. The redear sunfish is especially adapted for crushing mollusks, with molariform teeth on its pharyngeal bones; however, less than 20% of shells in redear stomachs had been physically damaged. Crushing of clams is obviously not requisite to digestion since shells in hindguts of redear sunfish and other species alike were devoid of flesh. Consumption of clams by carp was spectacularly high in some instances, with some fish containing more than 30. Some clams eaten by carp and channel catfish exceeded 2.5 cm across the valves, but most were less than 1.0 cm. Rainbow trout, two of the catfish, and all sunfishes excepting bluegill con- tained other fishes. Channel and flathead catfishes, largemouth bass, warmouth, and black crappie were the most piscivorous species. Edwards (1974) demon- strated that striped bass in the Colorado River were also voracious piscine predators (.Table 1 ). Threadfin shad was the exclusive fish eaten by trout. Largemouth bass also ate shad, followed by red shiner, unidentified sunfish, unidentified fish, and other largemouth bass. Channel catfish fed on shad and red shiner, unidentified cen- trarchids, and other unidentified fish. Flathead catfish ate mostly red shiner and mouthbrooder. Four flathead catfish each had eaten a single fish, each of a different species (threadfin shad, carp, channel catfish, and undetermined cen- trarchid).The warmouth was a specialized piscivore, feeding on poeciliids and red shiner. Green sunfish were more opportunistic, eating small carp and uniden- ,~