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• ? )& D-7 Y?11nc??2?1 e4 a? I q b4b A Conservation Plan for Native Fishes of the Lower Colorado River W. L. MINCKLEY, PAUL C. MARSH, JAMES E. DEACON, THOMAS E. DOWLING, PHILIP W. HEDRICK, WILLIAM J. MATTHEWS, AND GORDON MUELLER The native fish fauna of the lower Colorado River, in the western United States, includes four "big-river" fishes that are federally listed as endan- gered. Existing recovery implementation plans are inadequate for these critically imperiled species. We describe a realistic, proactive management program founded on demographic and genetic principles and crafted to avoid potential conflicts with nonnative sport fisheries. In this program, native species would breed and their progeny grow in isolated, protected, off-channel habitats in the absence of nonnative fishes. Panmictic adult populations would reside in the main channel and connected waters, exchanging reproductive adults and repatriated subadults with populations occupying isolated habitats. Implementation of the plan would greatly enhance recovery potential of the four listed fishes. Keywords: conservation, management, generics, endangered fishes Among the native fishes of the Colorado River are four "big-river" species: humpback chub (Gila cypha), bonytail (Gila elegans), Colorado squawfish (Ptychocheilus lu- cius), and razorback sucker (Xyrauchen texanus). Once gen- erally widespread and abundant (Minckley 1973), these species are now critically imperiled. Water development- damming rivers, creating impoundments and cold tailwaters, degrading habitats, and desiccating long reaches-and the introduction and establishment of a suite of nonnative species have adversely affected the native fishes. Severely reduced in abundance and range and under continuing threats, the four big-river species are now federally listed as endangered (USFWS 2002a, 2002b, 2002c, 2002d). The Colorado River drains a part of the American West renowned for its natural beauty, open space, and biodiversity. The whole region is arid, and water is critically limiting. Tens of millions of people rely on the river for water and electri- cal power, and as a result it is one of the most controlled rivers on Earth (Fradkin 1981). In addition to its direct impor- tance to human well-being, the Colorado River is critical to continental biodiversity. Its path through some of the driest, hottest North American deserts forms a mesic, north-south corridor for innumerable organisms. Further, the Colorado River system supports a unique biota of its own. Historical species-level endemism for fishes is approximately 75% for the ancient, long-isolated watershed (Carlson and Muth 1989). The lower Colorado River main stem, defined as the reach downstream from Glen Canyon Dam (figure 1), is the geopolitical focus of this article. Biotic elements of concern Thirteen fishes, including 10 freshwater species (table 1), constituted the original fish fauna of the lower, Colorado River main stem. Three largelymarine taxa from Mexico's Sea of Cortez (machete, striped mullet, and spotted sleeper) and woundfin were extirpated from the lower river main stem be- fore 1900; roundtail chub and Colorado squawfish are also gone. Humpback chub, speckled dace, flannelmouth sucker, and bluehead sucker persist in the Grand Canyon; bonytail, razorback sucker, flannelmouth sucker, and desert pupfish still live downstream, largely because of management action on their behalf. The four big-river fishes-bonytail, humpback chub, Colorado squawfish, and razorback sucker-are the bio- logical subjects of this article. Over the past century, the original fish fauna of the lower Colorado River has largely been replaced by nonnative species (table 1), especially downstream from Hoover (formerly Boulder) Dam. Rainbow trout live in the cold water below dams; threadfin shad, largemouth bass, black and white W. L. Minckley, now deceased, was professor emeritus, Paul C. Marsh (e-mail: fish. dr@as"du) is an adjunct professor, Thomas E. Dowling is a professor, and Philip W. Hedrick is Ullman Professor of Biology in the Department of Biology, Arizona State University, Tempe, AZ 85287. James E. Deacon is Emeritus Distinguished Professor of Environmental Studies at the Univer- sity of Nevada, Las Vegas, NV 89154. William J. Matthews is a professor of zoology at the University of Oklahoma, Norman, OK 73018. Gordon Mueller is an ecologist with the US Geological Survey, Biological Resources Division, Denver, CO 80225. m 2003 American Institute of Biological Sciences. March 2003 / Vol. 53 No. 3 • BioScience 219