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m FOOD OF CHANNEL CATFISH IN THE COACHELLA CANAL, CALIFORNIA PAUL C. MARSH Center for Environmental Studies, Arizona State University, Tempe, AZ 85287 ABSTRACT Channel catfish, Ictalurus punctatus (75-414 mm total length), in the Coachella Canal, California, fed primarily upon Trichoptera (Smicridea utico, Nectopsyche sp.), Odonata, filamentous algae, chironomids, and aquatic Lepidoptera (Parargyractis confusahs). The high use of Lepidoptera larvae is heretofore unreported. Threadfin shad (Dorosoma petenense) and Asiatic clam (Corbicula fluminea), although locally abundant, we're not primary food items. These results emphasize the omnivorous, opportunistic nature of channel catfish feeding habits. INTRODUCTION. - Channel catfish, Ictalurus punctatus, were introduced into the lower Colorado River in 1892-3 (U.S. Fish and Wildlife Service 1981), were common by the early 1940's (Dill 1944) and now flourish throughout the system, including hundreds of kilometers of artificial waterways. There, as throughout its North American range, channel catfish provide an important sport fishery. Food habit studies on this species have been conducted at many localities representative of a wide variety of habitats (see Miller 1966 and Carlander 1969 for reviews). Channel catfish are generally omnivorous and opportunistic; young feed primarily upon aquatic insects and other benthic arthropods while larger individuals are increasingly piscivorous. Dill (1944) reported that channel catfish (mean length 155 mm) from the West Main Ditch, Imperial County, California, fed primarily upon chironomid larvae, caddisfly larvae, odonates, terrestrial insects and spiders, and "ooze." Catfish from the lower Colorado River fed variously upon sago pondweed (Potamogeton pectinatus), centrarchid fishes, crayfish, filamentous algae and small quantities of aquatic insects (Kimsey et al. 1957). Minckley (1981a) reported macrophytes, detritus, molluscs (Corbicula fluminea), crayfish, and fishes as predominant stomach contents from the same region. Channel catfish comprised about 51% of more than 9,000 fish taken in rotenone samples from the Coachella Canal, California, in November 1980, and provided an opportunity to obtain additional information on feeding relationships of channel catfish in the desert Southwest. The Bureau of Reclamation, Boulder City, Nevada, supported fish collection with funding, equipment, and man- power. William Rinne of that organization deserves special thanks. California Department of Fish and Game cooperated with permits and assistance. MATERIALS AND METHODS. - Channel catfish were obtained at three locations in the earth-lined Coachella Canal, Imperial County, California. Station A was a pool-like area and straight reach immediately below a concrete check-drop structure, Station B was a linear reach with no structures, and Station C was a double-barreled, concrete box siphon. Detailed descriptions of the study area and sampling stations were provided by Minckley (1981b). One hundred fifty-three specimens from the three stations were preserved in 10% formalin, measured (total length IT-L) to the nearest mm) and weighed (nearest 0.01 gm) in the laboratory. The digestive tract (esophageal septum to anus) was removed and esophagus plus stomach were separated from the intestine at the sphincter. Wet weights (nearest 0.01 gm) after blotting were obtained from stomach plus esophagus before and after removal of contents; weights of total contents were determined by subtraction. Food items from each stomach were examined at a magnification of 10X, identified and enumerated, and percentage volume of each constituent was visually estimated. Foods were analyzed independently for 2 to 5 major size groups of catfish (<100, 100-149, 150- 199, 200-299, and >299 mm TL) from each station. Intestinal contents of 20 fish representing all size groups were qualitatively examined for comparison with stomach contents. RESULTS. - Channel catfish ranged in length from 75-415 mm TL (x = 161) and in weight from 4.12-533.24 gm (x = 46.66).The fish were largest at Station A (x TL =168 mm, x wt = 52.79 gm), smallest at Station B (x TL = 105 mm, x wt = 13.20 gm) and intermediate at Station C (x TL = 129 mm, x wt =18.16 gm). All but two stomachs contained at least some food, most were full. Aquatic insects were the com- monest food; crayfish, fish, filamentous algae, and other items were relatively rare although in some individuals they con- tributed the major biomass. Foods eaten were generally similar at all sites (Table 1). A hydropsychid caddisfly larva, Smicridea utico, was the overwhelmingly dominant food. It occurred at greater than Marsh, P.C. 1981. Food of channel catfish in the Coachella Canal, California. Journal of the Arizona-Nevada Academy of Science 16:91-95. 91