1 Parasites of native and non-native fishes of the Little Colorado River, Grand Canyon, Arizona. Anindo Choudhury1, Timothy L. Hoffnagle2, and Rebecca A. Cole USGS-National Wildlife Health Center, 6006 Schroeder Road, Madison, WI 53711, U.S.A. 2Oregon Department of Fish and Wildlife, 211 Inlow Hall, Eastern Oregon University, La Grande, OR 97850 Corresponding author: Rebecca Cole, USGS – National Wildlife Health Center, 6006 Schroeder Road, Madison, Wisconsin 53711, U.S.A. Phone: (608) 270 2468. E-mail:
[email protected]. 1Current address: Division of Natural Sciences, St. Norbert College, 100 Grant Street, DePere, Wisconsin 54115, U.S.A. Running Head: Fish parasites in the Little Colorado River. 2 ABSTRACT: A 2-year, seasonal, parasitological study of 1435 fish, belonging to 4 species of native fishes and 7 species of non-native fishes from the lower Little Colorado River (LCR) and tributary creeks, Grand Canyon, Arizona, yielded 17 (possibly 18) species of parasites. These comprised 1 myxozoan (Henneguya exilis), 2 copepods (Ergasilus arthrosis and Lernaea cyprinacea), 1 acarine (Oribatida gen. sp.), 1 piscicolid leech (Myzobdella lugubris), 3 (possibly 4) monogeneans ( Gyrodactylus hoffmani, Gyrodactylus sp., Dactylogyrus extensus, and Ligictaluridus floridanus), 4 nematodes (Contracacecum sp., Eustrongylides sp. Rhabdochona sp., Truttaedacnitis truttae), 3 cestodes (Bothriocephalus acheilognathi, Corallobothrium fimbriatum and Megathylacoides giganteum), and 2 trematodes (Ornithodiplostomum sp., Posthodiplostomum sp.). Of these, Rhabdochona sp. is the only adult parasite native to the LCR. Infection intensities (worm burden) of Ornithodiplostomum sp and B. acheilognathi were positively correlated with length of humpback chub, Gila cypha. Adult helminths showed a high degree of host specificity, the exception being B.