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A/i J Transactions of 'the American Fisheries Society 126:343-346, 1997 © Copyright by the American Fisheries Society 1997 , Predation by Introduced Fishes on Endangered Humpback Chub and Other Native Species in the Little Colorado River, Arizona PAUL C. MARSH Center for Environmental Studies and Department of Zoology, Arizona State University Tempe, Arizona 85287-3211, USA MICHAEL E. DOUGLAS Department of Zoology and Museum, Arizona State University Tempe, Arizona 85287-1501, USA Abstract.-Fishes in the Little Colorado River in the Grand Canyon, Arizona, were sampled monthly from July 1991 to June 1995 as part of a study of the ecology of endangered humpback chub Gila cypha. Diets of five introduced predatory fish species were examined. Stom- ach contents varied among species and were low in di- versity and dominated by algae (primarily Cladophora), aquatic insects, and fishes. Humpback chub plus other native species were a significant component of the diet (13.7% frequency of occurrence among 219 of 408 stom- achs that contained food). Predation mortality from in- troduced fishes may significantly affect the native spe- cies by depleting numbers and reducing recruitment. Interactions between native and introduced spe- cies have been implicated in extirpations of indig- enous fishes around the globe (reviewed in Cour- tenay and Stauffer 1984; Welcomme 1988; Pollard 1989; Rosenfield and Mann 1992). Native fishes of the Colorado River basin of western North American have been affected similarly. In the ba- sin, an historically depauperate ichthyofauna of 36 species (many polytypic) has been subjected to a suite of about 70 nonnative fish species brought intentionally or inadvertently to the region. Co- incident with these introductions and with habitat alteration resulting from development of water re- sources, 3 native species now are extinct and 22 others are listed as endangered or threatened. Pre- dation and competition by introduced fishes have contributed to the present faunal status (Dill 1944; Miller 1946, 1961; Minckley and Deacon 1968, 1991; Moyle et al. 1986), but only recently have these species introductions become a focus of con- servation planning for native fishes. Endangered humpback chub Gila cypha is a me- dium-sized minnow that persists in only seven reaches of four rivers of the Colorado River basin because of habitat loss and modification, hybrid- ization with congeners, and interactions with non- native fishes (USFWS 1990). The largest remain- ing population is in the Little Colorado River, the major tributary to the Colorado River in the Grand Canyon, Arizona (Kaeding and Zimmerman 1983; Douglas and Marsh 1996). We examined predation by nonnative fishes on humpback chub and other native species, and as- sessed the potential impact on humpback chub. Rainbow trout Oncorhynchus mykiss and channel catfish Ictalurus punctatus are common and brown trout Salmo trutta, black bullheads Ameiurus me- las, yellow bullheads A. natalis, and striped bass Morone saxatilis are uncommon in the mouth and lower reaches of the Little Colorado River. Chan- nel catfish predation on humpback chub has been documented from stomach contents (C.O. Minck- ley, U.S. Fish and Wildlife Service, personal com- munication) and has been suggested by crescent- shaped bite marks on humpback chub (Kaeding and Zimmerman 1983; Karp and Tyus 1990). However, there are no published data on predation on humpback chub or other native fishes by the introduced species. Methods Fishes were captured in the lower 15 km of the Little Colorado River about monthly from July 1991 to June 1995. Hoop nets (0.8 or 1.2 m in diameter, 2.4 or 3.0 m long, 4 or 6 hoops, single or double throat) were deployed in all available habitats deeper than about 0.4 m. Trammel nets (7.6-45.7 m long, 1.8 m deep, 1.3-3.8-mm inner and 30-mm outer meshes) were set in water deeper than 0.5 m in the Little Colorado-Colorado River confluence area, and occasionally near Powell and Salt canyons, (3.0 and 10.6 river kilometers up the Little Colorado from the confluence). We also an- gled sporadically throughout the stream with bait- ed hooks and artificial lures to collect additional fish. Fish were identified, counted, measured for total length (TL, mm), and weighed (g). Native species were released near the point of capture after pro- cessing. Nonnative fishes were sacrificed and ei- 343