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36 The State of the Colorado River Ecosystem in Grand Canyon Canyon, as well as for the two most common nonnative species, rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta). Decline of Native Fish Introductions of Nonnative Fishes There are a number of reasons for the decline of native fishes, including the potential effects of nonna- tive fish species. Nonnative fish have been found in the Colorado River since the 1800s (Minckley, 1991). These species are potential predators of and competitors with native fish and include common carp (Cyprinus cmpio), fathead minnow (Pimephales promelas), plains killifish (Fundulus zebrinus), rainbow trout, brown trout, red shiner (Cyprinella lutrensis), and channel catfish (Ictalurus punctatus). Nonnative species may share rearing habitats used by native fish, habitats which include complex shorelines, tributaries, backwater areas, and eddies. The presence of warm water, cool water, and cold water nonnative fish species in the Colorado River is an issue of consider- able importance (U.S. Department of the Interior, 1995) because there are now nonnative fishes that may negatively interact with native fishes under virtually any temperature regime and in any habitat of the river. Today, the Colorado River has nearly twice as many nonnative species (60) as native species (32); in the Grand Canyon reach of the river the situation is even more extreme, where the ratio of native to nonnative spe- cies is more than 4 to 1 (Valdez and Carothers, 1998). The introduction of nonnative species to the Colorado River, both intentionally and unintentionally, was well underway before 1900. As such, the ratio of nonnative to native fishes was high in Grand Canyon before the construction of Glen Canyon Dam. For example, the National Park Service introduced both brown trout and rainbow trout to tributaries like Bright Angel Creek in the 1920s to provide sport fishing opportunities (Valdez and Carothers, 1998). Because of the continuous nature of the river and its tributaries before dam building, spe- cies introduced almost anywhere in the basin had the potential to find their way to the Grand Canyon por- tion of the river, and many did. Before Glen Canyon Dam, the Grand Canyon reach was dominated by a single introduced species, the channel catfish (Valdez and Carothers, 1998). Following construction of the dam in 1963, Federal and State agencies again introduced rainbow trout below Glen Canyon Dam to establish and maintain a sport fishery in the 15-RM reach between the dam and Lees Ferry. This stocking continued for more than 30 yr, until the mid-1990s. Numerous other spe- cies of nonnative fishes were also introduced into Lake Powell and Lake l'vIead to create or enhance recreational fishing (Mueller and Marsh, 2002). The effects of nonnative fish on native species, including predation and competition, are important considerations when evaluating any management action intended to benefit native fishes. These considerations are particularly important given the proximity of Lake Powell and Lake Mead, reservoirs with diverse nonna- tive fish populations, to Grand Canyon. Any manage- ment action intended to improve habitat conditions for native warmwater fishes also runs the risk of providing additional habitat that is suitable for nonnative predators and competitors. Nonnative fish predators currently in the Grand Canyon reach of the Colorado River include striped bass (iV/orone saxatilis), channel catfish, largemouth bass (i\1icropterus salmoides), green sunfish (Lepomis ryanellus), brown trout, and rainbow trout. Currently, nonnative coldwater species (trout) are abundant, while the nonna- tive warmwater species exist in relatively low numbers. Glen Canyon Dam Effects The predam success of nonnative species was, in part, due to the fact that the river was generally what fishery biologists term a "warmwater habitat." The annual temperature cycle of the Colorado River through Grand Canyon was similar to temperate lakes and streams at lower elevations, where temperatures ranged from cold or cool in winter to warm in summer. Native species require warmer temperatures to spawn and reproduce successfully. This seasonal pattern also allowed many of the introduced species to complete their life cycle. One of the major impacts of Glen Canyon Dam on the Colorado River was the change in water tempera- ture to a relatively cold, steady temperature that favored coldwater species like trout over native fishes and intro- duced, warmwater species. \Vhile most of the warmwa- ter species can survive in these colder waters, they cannot reproduce and do not grow well, having been adapted to at least seasonally warmer temperatures. Other possible effects of dam operations on the riverine environment that may affect fishes include increased water clarity, altered flow patterns, and reduced sediment. All species that are native to Grand Canyon evolved in highly turbid environments, so the clear water released from the dam may favor nonnative