PARASITES OF FISHES IN THE COLORADO RIVER AND SELECTED TRIBUTARIES IN GRAND CANYON, ARIZONA Chad M. Linder, Rebecca A. Cole*À, Timothy L. Hoffnagle`, Bill Persons§, Anindo ChoudhuryI, Roger Haro, and Mauritz SternerÀ Department of Biology, University of Wisconsin–La Crosse, 1725 State Street, La Crosse, Wisconsin 54601. e-mail: rcole@usgs.gov ABSTRACT: As part of the endangered humpback chub (HBC; Gila cypha) Adaptive Management Program, a parasite survey was conducted from 28 June to 17 July 2006 in 8 tributaries and 7 adjacent sections of the main stem of the Colorado River, U.S.A. In total, 717 fish were caught, including 24 HBC. Field necropsies yielded 19 parasite species, 5 of which (Achtheres sp., Kathlaniidae gen. sp., Caryophyllaidae gen. sp., Myxidium sp., and Octomacrum sp.) are new records for Grand Canyon, Arizona, U.S.A. Spearman’s correlation coefficient analyses showed no correlations between parasite burden and fork length for various combinations of fish and parasite species. Regression analyses suggest that no parasite species had a strong effect on fish length. The most diverse parasite community (n 5 14) was at river kilometer (Rkm) 230, near the confluence of Kanab Creek. The most diverse parasite infracommunity (n 5 12) was found in the non- native channel catfish (CCF; Ictaluris punctatus). Overall parasite prevalence was highest in CCF (85%) followed by that in HBC (58%). The parasite fauna of humpback chub was mainly composed of Bothriocephalus acheilognathi and Ornithodiplostomum sp. metacercariae. The Colorado River (COR) in Grand Canyon, U.S.A., and its biota have been dramatically changed by the completion of Glen Canyon Dam (GCD) in 1963 (National Academy of Sciences, 1991). Before 1963, temperature, discharge, and turbidity of the COR varied seasonally. After 1963, seasonal flow rates and temperature were stabilized, daily flow rates became more variable, and turbidity decreased. Coinciding with the closing of GCD, 5 species of native Colorado River fish were declared federally endangered or threatened (USFWS, 2003). Today, among approximately 20 non-native fishes, only 4 of the original 8 native species remain, i.e., bluehead sucker (BHS; Catostomus discobolus), flannelmouth sucker (FMS; Catostomus latipinnis), speckled dace (SPD; Rhi- nichthys osculus), and humpback chub (HBC; Gila cypha, which is federally endangered (http://endangered.fws.gov/federalregister/ index.html). HBC is the only extant species of those originally listed, and it can be found in 6 populations throughout the span of the COR. The largest extant population of HBC is found within the lower COR in and near the confluence of the Little Colorado River (LCR) in Grand Canyon. Above GCD in the upper COR, HBC is found in 5 smaller, distinct populations at Black Rocks, Westwater Canyon, Cataract Canyon, Desolation/Gray Canyons, and Yampa Canyon (USFWS, 1990). Due to perennially cold, clear water in the COR, native fish in Grand Canyon now successfully recruit primarily in tributaries of the COR (Valdez and Ryel, 1995; AGFD, 1996), of which the LCR is the largest and the main spawning area for all native species (AGFD, 1996; USFWS, 2006). Introductions of non-native fishes and their parasites also are implicated in the declining numbers of humpback chub (Meretsky et al., 2000). At least 14 exotic non-native parasites are known to infect native fishes of Grand Canyon, although the only extensive survey has been in only the lower 18 km of the LCR (Choudhury et al., 2004). Three of these parasites, Asian fish tapeworm, Bothriocephalus acheilognathi (Cestoda); anchor worm, Lernaea cyprinacea (Copepo- da); and Ornithodiplostomum sp. (Trematoda), are particularly worrisome, because they are pathogenic and infect HBC with a greater intensity than any other fish in the system (Brouder and Hoffnagle, 1997; Hoffnagle and Cole, 1998; Choudhury et al., 2004). Larvae of Ornithodiplostomum sp., when found encysted in the brain of fish, were shown to alter behavior and increase mortality (Radabaugh, 1980; Sho and Goater, 2001). The definitive hosts of various Ornithodiplostomum sp. are piscivorous birds (Hoffman, 1999; Woo, 2006). Both B. acheilognathi and L. cyprinacea, which use fish as the definitive hosts, have been reported as pathogenic and potentially fatal (directly or indirectly) to fish of various age classes (Scha ¨ pper- claus, 1986; Hoffnagle et al., 2006). Previous fish parasite studies have been limited to a particular species or sampling area (Carothers et al., 1981; Hoffnagle and Landye, 1998; Choudhury et al., 2004). To facilitate native fish recruitment, a proposal for the installation of structural modifications (temperature control devices) on GCD is being considered (USBR, 2004). Such a device would attempt to raise main-stem COR temperatures. If water temperature in the main stem increases, it is possible that parasite species that can complete their life cycles only in tributaries due to the warmer tributary water temperatures could establish in the main stem. As a result of these concerns, we conducted a parasite survey of native and non-native fishes in the 7 major tributaries and selected reaches of the main stem of the COR in Grand Canyon, known to be important to native fishes. The main objective of the study was to document parasite prevalence and distribution in the COR locations important to native fishes so that parasites that are reproducing in the tributaries could be documented and the potential ramifications of parasite movement into the main stem could be evaluated. MATERIALS AND METHODS Study area The study area includes 7 tributaries and 7 adjacent main-stem COR sections in a 362-km stretch from Lees Ferry (Rkm 0) to Diamond Creek (Rkm 362) (Fig. 1; Table I). The main-stem COR was sampled at Rkm 98, 138, 174, 230, 254, 319, and 362; tributaries in proximity to those main- stem sections are the Little Colorado River (LCR), Bright Angel Creek (BAC), Shinomu Creek above (SHOA) and below (SHOB) the falls, Kanab Creek (KAN), Havasu Creek above (HAVA) and below the falls (HAVB), and Diamond Creek (DIA). With the exception of the Paria River (PAR), every main stem section was sampled within 2 km, both upstream and downstream from the associated tributary confluence. Received 18 May 2011; revised 17 July 2011; accepted 27 July 2011. * To whom correspondence should be addressed. DOI: 10.1645/GE-2538.1 { U.S. Geological Survey, National Wildlife Health Center, 6006 Schroeder Road, Madison, Wisconsin 53711. { Oregon Department Fish and Wildlife, 203 Badgley Hall, Eastern Oregon University, LaGrande, Oregon 97850. } U.S. Geological Survey, Grand Canyon Monitoring and Research Center, 2255 North Gemini Drive (Mail Stop 9394), Flagstaff, Arizona 86001. ISt. Norbert College, DePere, Wisconsin 54115. J. Parasitol., 98(1), 2012, pp. 117–127 F American Society of Parasitologists 2012 117