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Parasites of Native and Non-native Fishes of the Lower Little Colorado River, Arizona - 2000 Annual Report Changing the behavior of intermediate hosts is a well-documented strategy by parasites to increase their likelihood of being passed on to the next host (Beckage 1997). The catfish tapeworms (c. fimbriatum and M giganteum) use small fish (possibly humpback chub) as an intermediate host. Ornithodiplostomum sp. was found in 78% of the humpback chub examined. This parasite uses fish-eating birds as its definitive host, but uses small fishes as intermediate hosts. Additionally, Ornithodipostomum ptychocheilus (Radabaugh 1980) and other related trematodes (Crowden and Broom 1980; Brassard et al. 1982; Lemly and Esch 1984; Lafferty and Morris 1996) have been shown to alter host behavior in such a way as to increase the risk of predation and, thus, the likelihood of continuing the life cycle of the parasite. Cestodes may also be able to increase the likelihood of their intermediate host to be consumed by the next host by decreasing growth (Pennycuick 1971) or increasing oxygen demand (Smith and Kramer 1987), which expose the fish to an increased risk of predation (Milinski 1985). It may be that parasites that use humpback chub as an intermediate host areas or more important to the survival of this endangered species than the parasites that use them as definitive hosts. Future Studies Phase 1 of this study will continue with further field trips into the LCR scheduled for Marchi April and June 2001. Evaluation of all sampled parameters (parasites, tissues, fish diet, benthos, plankton, temperature, etc.) will follow the final field trip. Phase 2 has started (preliminary work has already begun) and will focus on the impact of B. acheilognathi on the growth rate of its host and acute pathogenicity in very young fish. We are concerned that field data may be skewed as to impacts of this parasite on the very young fish (i.e., young fish that are heavily infected may not survive long enough to be sampled). Experimental work will help us fill this information gap. Parasite impact on growth is relevant because of the potential to increase the window of vulnerability to predation for these fish. Additionally, vulnerability to thermal stress is greater in smaller fish so infected fish that disperse into the mainstem Colorado River may also suffer increased mortality. At least two introduced parasites (B. acheilognathi and L. cyprinacea) infecting humpback chub are restricted to the warmer LCR by the colder Colorado River (Carothers et al. 1981; Brouder and Hoffnagle 1997; qarkson et al. 1997; Hoffnagle and Cole 1999). T. truttae is not restricted by cold temperatures, since it is found in rainbow trout in the Colorado River at Hoffnagle et al. 2000 5