Biological Conservation 25 (1983) 135-159 Factors Resulting in Decline of the Endangered Sonoran Topminnow Poeciliopsis occidentalis (Atheriniformes: Poeciliidae) in the United States Gary K. Meffe, Dean A. Hendrickson & W. L. Minckley Department of Zoology, Arizona State University, Tempe, Arizona 85287, USA & John N. Rinne Rocky Mountain Forest and Range Experiment Station, Tempe, Arizona 85287, USA ABSTRACT The endangered Sonoran topminnow Poeciliopsis occidentalis has stead- ily declined in distribution and abundance in the past several decades, and currently survives in the United States only in several isolated localities in southern Arizona. This reduction is correlated with, and primarily attributed to, habitat destruction, and introduction and estab- lishment of mosquito fish Gambusia affinis and other exotic fshes. Topminnows have characteristically been reduced in number or replaced within a year or two of introduction of non-native fshes. Other native .fishes have experienced similar declines after introduction of exotics, and much of the endemic western ichthyofauna may be vulnerable to extir- pation in this manner. Predation by introduced fishes on natives appears to be a likely mechanism of replacement. INTRODUCTION The native ichthyofauna of arid western North America is depauperate in comparison with other regions of the continent. Much of this fauna is 135 Biol. Conserv. 0006-3207/83/0025-0135/$03"00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain