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Status of the Native Fish Fauna of the Moapa River (Clark County, Nevada) JEFFREY N. CROSSI Department of Biological Sciences, University of Nevada Las Vegas, Nevada 89154 ABSTRACT The Moapa River of eastern Clark County, Nevada was surveyed in January 1974 and August 1975 to determine the status of the native fishes. Thirteen species were collected; four are native to the Moapa River. Two nonnative species not previously recorded were collected. The abundance and range of two native species, Moapa coriacea and Gila robusta, decreased since the river was last surveyed (1964-68). A combination of factors likely account for this: habitat modification, poor water quality, and the presence of nonnative fishes. The Moapa River of eastern Clark County, Nevada is the southernmost remnant of the Pluvial White River; Hubbs and Miller (1948) and Miller and Hubbs (1960) discuss the physiography and fish distribution of the Pluvial White River system. The purpose of the present study was to determine if the status of the native fishes of the Moapa River had changed since the river was last extensively surveyed in 1964-68 (Deacon and Bradley 1972). STUDY AREA The Moapa River flows 40 km southeasterly through the Moapa Valley from a warm spring source to the Overton Arm of Lake Mead (Fig. 1) with an average gradient of 1.6 m/ km. The valley is arid to semiarid [ mean annual precipitation is 12.55 cm (Anonymous 1962) ]. Prior to construction of Hoover Dam and filling of Lake Mead (circa 1940) the Moapa River was a tributary to the lower Virgin River. More than 20 warm springs, believed to be 1 Present address: College of Fisheries, University of Washington, Seattle, Washington 98105. point discharges for carbonate rock flow systems, are the primary source of water for the Moapa River (Miffin 1968). The springs emerge through alluvial deposits over a 2.2-km reach of the upper Moapa Valley. The aggre- gate mean discharge is approximately 1,400 liters/s; temperature ranges from 26.7 to 32.2 C (Eakin 1964). Flash floods and snowmelt also contribute to runoff. The mean annual discharge of the Moapa River was 42.0 X 106 m3/yr near Moapa (1945-1960) and 41.0 X 106 m3/yr near Glendale (1950-1960) (Anon- ymous 1962). Meadow Valley Wash, the only significant tributary to the Moapa River, flows exclusively during periods of high runoff. The headwater springs are perennially clear. Turbidity and total dissolved solids generally increase downstream (Tables 1 and 2). Tem- perature decreases downstream. Leaching of irrigation water likely accounts for the de- crease in water quality in the middle and lower river (the Moapa Valley below Glendale is more highly developed agriculturally than the upper Moapa Valley). During the 1974 study the entire stream- flow was diverted through an irrigation canal 503