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predators like trout, which are adapted to hunting in clear water (Valdez and Ryel, 1995). Similarly, the post- dam river hydrology is different from the predam river with respect to daily flow variation, flood frequency, and seasonal pattern and magnitude of maximum and mini- mum flows (Topping and others, 2003). These alterations in flow patterns potentially affect the spawning cues, habitat use, and distribution of native fish, as well as the suitability of mainstem Colorado River rearing habitat, in ways that are largely unknown and potentially com- plex (Korman and others, 2004). Finally, as Glen Canyon Dam blocks the majority of sediment transported by the Colorado River to the upstream portions of Lake Powell, the nearshore physical habitat available to native fish is fundamentally different from the predam river (Goeking and others, 2003; also see chapter I, this report). Except for temperature, the other potential effects of the dam that are mentioned here are based on inferences about what is known regarding fishes from other river systems. Little direct scientific evidence from the Colorado River itself exists regarding these effects, and there remains considerable uncertainty regarding the potential effects of management actions associated with these factors (' Valters and others, 2000). Other Factors New fish parasites in the system, changes in tribu- tary hydrology, and alterations in the food base that support fish populations are additional environmental factors that may be affecting native and nonnative fish species in Grand Canyon. Asian tapeworm (Bothriocepha- lus acheilognatln), a parasitic cestode, is a prominent exam- ple of a recently introduced parasite. Introduced into the United States in the 1970s with imported grass carp (Ctenopharyngodon idella) from China, the Asian tapeworm was discovered in 1990 in the Little Colorado River, which is an important spawning area for humpback chub (Choudhury and others, 2004). The tapeworm can cause mortality, but most often it is responsible for reduced growth and poor condition of infected fish. This para- site is currently restricted to the Little Colorado River because cold mainstem temperatures preclude comple- tion of its life cycle. The Little Colorado River is also an example of a tributary system in which upstream water use and development have changed the amount and timing of flows reaching the Colorado River. Such changes could affect fishes in the Little Colorado River and throughout Grand Canyon, especially below the tributary. Fishes of Grand Canyon 37 Status and Trends Until the I 990s, there were few attempts to monitor the status and trends of fishes in Grand Canyon. Infor- mation before the mid- to late-1980s was anecdotal and was provided by explorers, river runners, and occasional scientific expeditions. As a result, few data are available for the first 20 yr after Glen Canyon Dam was closed. Early fish collection efforts were reviewed by Valdez and Carothers (1998), and where appropriate these earlier data are used in comparison to current data for fishes in Grand Canyon. Efforts to estimate population size or relative abun- dance of fishes in Grand Canyon began under Glen Canyon Environmental Studies Phase II when private consulting firms, university researchers, the U.S. Fish and Wildlife Service (USFWS), and the Arizona Game and Fish Department conducted surveys and under- took population estimates in the mainstem Colorado River and in the Little Colorado River. Beginning in 1997, these eHarts became the responsibility of the U.S. Geological Survey's (USGS) Grand Canyon Monitoring and Research Center, which has worked cooperatively on monitoring activities with the U.S. Fish and Wildlife Service, the Arizona Game and Fish Department, and consulting firms (S'''CA Environmental Consultants, Inc., and Ecometric Research). For the purposes of monitoring the study area is divided into three seg- , , ments: the Lees Ferry reach (15 RM of Colorado River corridor from Glen Canyon Dam to Lees :Ferry); the mainstem Colorado River (downstream of Lees Ferry, RM 0 and the Paria River to RM 226 at the conflu- , ence of Diamond Creek); and the Little Colorado River (the 8.7 mi (14 km) of the tributary upstream from the mainstem). The status and trends of fish found in each of these reaches will be discussed separately. Humpback chub are discussed in a separate section. Lees Ferry The Lees Ferry reach of the river is managed pri- marily as a rainbow trout sport fishery. The Lees Ferry reach is known as a tailwater trout fishery because it occurs downstream from a large dam where deepwater discharges afford cooler water temperatures that allow coldwater species like trout to survive. In fact, trout not only survived in the Lees .Ferry reach following their initial stocking in 1964 but also nourished in the new habitat created by Glen Canyon Dam. The Lees Ferry