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An Instream Flow Philosophy for Recovering Endangered Colorado River Fishes Harold M. Tyus U.S. Fish and Wildlife Service p.o. Box 25486 Denver Federal Center Denver, Colorado 80225 ABSTRACT: Remnant stocks of some endangered Colorado River fishes per- sist, including Colorado squawfish (Ptychocheilus lucius), humpback chub (Gila cypha), razorback sucker (Xyrauchen texanus), and bony tail (Gila elegans). Recovery of these species to a nonendangered status is problematic. Provision of flows to maintain habitats in which the fish are declining may do little to slow their decline or prevent their loss. It is argued that determination of instream flows for endangered fishes should be based on the premise that the fish are not occupying optimal, or perhaps even acceptable habitats, a philosophically dif- ferent approach than that addressed by prevailing flow methodologies. A re- covery rather than survival philosophy is recommended that includes devel- oping instream flow needs through a combination of empirical studies, simulations, and assessments. KEY WORDS: Endangered species, Gila elegans, Gila cypha, instream flow, pty- chocheilus lucius, stream habitat, Xyrauchen texanus. INTRODUCTION Severe, long-term, and adverse impacts this form of habitat deterioration, prompt- have changed historic fish habitats and ing the American Fisheries Society to state stocks in North America. Populations of that alteration of streamflows is the major native fishes are declining and some are factor in the loss of stream fishery re- faced with extinction. During the past 100 sources in North America (Peters 1981; Tyus years, 3 genera, 27 species, and 13 subspe- 1990). It is unknown whether the rate of cies of fishes have become extinct due to loss of these natural faunas can be reduced physical habitat alteration and concomi- because community-level disturbances of- tant fish introductions (Miller et a1. 1989). ten result from species extirpations. The An additional 364 rare fishes in Canada, fate of native Colorado River fishes in the the United States, and Mexico are threat- southwestern United States is of particular ened with extinction (Williams et a1. 1989). concern, because widespread and drastic In a 10-year review of fishes of threatened, habitat loss has had severe impacts on this endangered, or special concern listed by unique fauna (Carlson and Muth 1989). Deacon et a1. (1979), Williams et a1. (1989) The Colorado River basin, encompassing found that 139 new taxa warranted pro- about X2 of the land mass of the continental tection, but not a single species had been United States, is located in extremely arid removed from the original list by success- country (McAllister et a1. 1986) and its wa- ful recovery efforts. ters are a precious commodity. Waters of There are few rivers remaining in North the Colorado River were diverted for ag- America in which annual flow patterns ricultural use by Native Americans, but have not been severely altered. Stream these diversions increased exponentially fishes have been particularly affected by with subsequent settlement. Many such di- I Rivers. Volume 3, Number 1 Pages 27 -36 27 II~