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Native Fishes In Arid Lands: A Dwindling Resource of the Desert Southwest Photographs by John N. Rinne INTRODUCTION The American Southwest is usually defined to include all or parr of 12 States in western United States and Mexico (Fig. 1), much of which is lowland desert intetcu pted and diversified by isolated mountains and plateaus. Evaporation exceeds ptecipitation so greatly that surface water should the- oretically not exist. Nonetheless, major rivers such as the Colotado and Yaqui formerly collected sufficient water to flow all the way through the arid lowlands to the sea. Just a few decades ago, western streams swarmed with fishes that had survived millennia of temarkable geologic and climatic changes. They had adapted to survive the special conditions in canyon-bound rivers that varied from raging torrents in flood to isolated pools in drought. Some had specialized to withstand high summer temperatures and salts concent:rated by evaporation in desert lakes and marshes. However, when technological humans moved in droves to the warm, arid region, native fishes began to disappear. Humans need lots of watet toO, and typically far more than required [Q suPPOrt native fishes. Early diversions for mineral and cattle production and irrigated agriculture were followed by giant mainstream dams that not only stored and controlled runoff, but produced electricity as well. This abun- dant source of powet led to expansion of industry, and to retirement communities, recreation, and more urbanization-the "Sun Belt" enterprises of today. All these demand water for drinking, watering lawns and golf coutses, and other uses-as well as for more and more electrical power. As a result, native fishes are being exterminated. Destruction of aquatic habitats, changes from natural to artificial conditions, and predation and competi- tion by alien species enhanced by artificial condi- rions, aU combine to desrroy rhem. Many are nearing extinction, some are already gone, and neither legis- lation, nor determined attempts at conservation by agency, academic, a nd other managers has succeeded in reversing the trend. The only chance seems to lie in an emergence of public opinion that recognizes native fishes as valuable resources and demands their conservation. Text by W. L. Minckley THE SETTING Elevations in the region vary from sea level to more than 3600 m. Precipitation is equally variable, averaging less than 5.0 cm per year in the most severe desert, to more than 128 cm at the higher elevations. Most of the vegetation zones of North America are represented, from patches of tundra on the highest peaks, downhill through extensive conifer forests then woodlands and grasslands, to end with desertscrub on the basin floors. The region is also hydrologically complex. Most major drainage basins are composites .of once- independent systems brought together by ctustal movements or volcanic activity. Most of the Sonoran Desert region, emphasized here, is characterized by through-flowing rivers passing from mountains to the sea. Closed basins (Fig. 2), from which wai~;' escapes only through evaporation or seepage, are common in the Mohave, Great Basin, and Chihuahuan deserts. Coverage is restricted to watersheds west of the Continenral Divide, which ulrimarely drain to the Pacific Ocean through the Colorado River, Rio Yaqui, and lesser streams between those two major basins (Fig. 1). Origins of the Desertlands Desert conditions are new, in geologic time, to much of rhis area. Studies of fossil plants and animals have convincingly demonstrated that only a few thousand years ago woodlands and prairies flourished where desert now exists. Because trees and grasses require more water than cacti and shrubs, available precipitation and runoff were either greater in amount, far different in distribution rhroughout the year, or both. Cycles of drying alternating with periods of substantial moisture have progressed toward more and more frequent and severe drought. The region is now more arid than ever before. Over a longer time scale, complex geologic activity shaped the land surface to help set the stage for development of southwestern deserrs. The coastlines of Mexico and California slid northwest along the San Andreas and relared faults in response to 1 01093