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Introduction Fishes of the Colorado River vary from cold water trout species found in the river's mountainous headwa- ters to uniquely adapted desert river species found at lower elevations. \'Vithin the study area, the Colorado River corridor between Glen Canyon Dam and the west- ern boundary of Grand Canyon National Park (hereafter Grand Canyon), the Colorado River was a seasonally warm and turbid river characterized by large seasonal variations in flow before it was altered by the closure of Glen Canyon Dam in 1963 (Topping and others, 2003). Although water temperatures fluctuated between 320F (ooC) during winter to a high approaching 860F (30oC) during late summer, several warmwater native fish spe- cies successfully inhabited this stretch of the river (Cole and Kubly, 1976). Because of the harsh environment created by dramatic seasonal fluctuations in the river's predam flow and temperature, only 8 of the 32 species of native fish historically found in the Colorado River were common in the Grand Canyon reach of the river. Other native fishes within the study area were restricted to small tributary streams or occurred only in transient or seasonal numbers. Of the eight fish species that were originally common to the study area, only four species are known to persist today. The number of species that made up the original fish community of the Colorado River was altered well before the construction of mainstem dams because of the introduction of nonnative fishes by early Euro- pean settlers. Nonnative fishes, from sport fishes to escapees from aquaria, have been intentionally and inadvertently stocked in the Colorado River for more than 100 yr (Mueller and Marsh, 2002). Today, non- native fishes originating in many parts of the world are found in the Colorado River. Table I contains a list of the native and nonnative fishes of the Colorado River in Grand Canyon. This chapter examines the status, trends, and recent condition of Grand Canyon fishes, focusing particular attention on the endangered humpback chub (Gila cypha) because of its prominence within the Glen Canyon Dam Adaptive Management Program (see Overview, this report). The chapter begins with a discussion of the conditions that led to the development of the Grand Canyon's unique native fish populations and then moves on to the reasons for their decline. The effects of the modified low fluctuating flow (MLFF) alternative on fish