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Parasites of Native and Non-native Fishes of the Lower Little Colorado River, Arizona. 2000 Annual Report Introduction The Colorado River in Grand Canyon and its biota have been dramatically changed by the closure of Glen Canyon Dam in 1963 (National Academy of Sciences 1991). Today, among approximately twenty non-native fishes only four of the original eight native species remain (bluehead sucker Catostomus discobolus, t1annelmouth sucker C. latipinnis, humpback chub Gila cypha, speckled dace Rhinichthys osculus); and one of these (humpback chub) is federally endangered (change citation to federal register). Due to perennially cold, clear water, native fish in Grand Canyon now reproduce only in tributaries ofthe Colorado River (Valdez and Ryel 1995; Arizona Game and Fish Department 1996), of which the Little Colorado River (LCR) is the largest in Grand Canyon and the main spawning area for all native species (Arizona Game and Fish Department 1996). Non-native fishes introduced into the Little Colorado and Colorado river systems have brought various parasites with them. At least four exotic parasites are known to infect fishes of the LCR. Two of these parasites, Asian fish tapeworm Bothriocephalus acheilognathi (Cestoda) and anchor worm Lernaea cyprinacea (Copepoda) are particularly worrisome, since they infect humpback chub at a higher rate than any other species in the system (Brouder and Hoffnagle 1997; Hoffnagle and Cole 1999; Hoffnagle et al 2000). Both B. acheilognathi and 1. cyprinacea have been reported as pathogenic and potentially fatal (directly or indirectly) to fish of various age classes (Schapperclaus 1986). Bothriocephalus acheilognathi, in particular, has caused high mortality in native fishes that it has infected outside of its native range (Hoffman and Schubert 1984). These parasites cannot complete their life cycles in the mainstem Colorado River under present, cold water conditions. However, they can be transported by infected individuals to other suitable tributaries, such as Kanab Creek. Information on parasite distribution, host affiliation and impact of water temperature regimes has been requested by managers for making decisions regarding the future operation of Glen Canyon Dam and the proposed multi-level intake structure. This study is the first survey of the parasites of all fishes of the Little Colorado River and will provide information on prevalence of parasites and diseases present in the Little Colorado River drainage, with emphasis on native fishes and those species likely to be injurious to native fishes. Herein we report the results of our second year of sampling. Hoffnagle et al. 2000