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Office, Washington, 1982. Predicting at- ,es 85-88 in Forest and ate. University of Min- Miscellaneous Publi- d D. L. Tweed. 1983. esearcher-defined simi- bility research. Journal -262. ,? ec 7 o f North American Journal of Fisheries Management 25:547-556, 2005 Article] ® Copyright by the American Fisheries Society 2005 DOI: 10.15 77/M04-123.1 Repatriation as a Management Strategy to Conserve a Critically Imperiled Fish Species PAUL C. MARSH,* BRIAN R. KESNER, AND CAROL A. PACEY School of Life Sciences, Arizona State University, Post Office Box 874501, Tempe, Arizona 85287-4501, USA Abstract.-A repatriation program to conserve critically imperiled razorback sucker Xyrauchen texanus, an endemic fish of the Colorado River basin in western North America, was initiated in 1990. The species, once widespread and abundant, now is extirpated from most of its range because of human-induced factors. Natural recruitment to wild populations rangewide is largely precluded by nonnative predation. The largest remaining wild population occurs in Lake Mohave, Arizona and Nevada, but its numbers have declined dramatically over the past decade, such that the genetic legacy of the species may soon be lost. As part of a cooperative repatriation program, more than 440,000 naturally produced razorback sucker larvae were harvested and grown in protective cus- tody; from these, nearly 58,000 marked juveniles were released into the lake between 1993 and 2002. Annual repatriate population estimates (modified Petersen method) ranged from 1,017 to 2,494 and poststocking survivorship (Program MARK) ranged from 2% to 6% for the period 1999- 2002. Total length at release was the most important determinant of poststocking survival, which more than doubles for releases averaging 350 mm compared with those averaging 300 mm. Achiev- ing the program goal of reestablishing an adult population of 50,000 individuals depends upon increasing repatriate survival. Restoring populations of extirpated species to historical habitat is a fundamental strategy toward recovery of imperiled taxa as diverse as wolves, salmon, and condors (Bangs et al. 1998; Young 1999; Meretsky et al. 2000). Such efforts ideally are made after removal or amelioration of threats that resulted in the species elimination in the first place. In other instances, the goal may be perpet- uation of a vulnerable gene pool even though threats remain, as in the cases of wild lake trout Salvelinus namaycush in Lake Superior (Wilberg et al. 2003) and the California clapper rail Rallus longirostris obsoletus in central California (Har- ding et al. 2001) or razorback suckers Xyrauchen texanus in the Colorado River basin, the subject of this paper. The razorback sucker is a large, endemic big- river fish of the Colorado River basin, historically widespread and abundant throughout the basin, ranging south from Green River, Wyoming, to the Colorado River delta in northwestern Mexico (Minckley 1983). The species was described as in decline more than 30 years ago because field col- lections generally contained only large adults (Minckley 1973), and this pattern has been con- sistently observed over ensuing decades (McAda * Corresponding author: fish.dr@asu.edu Received September 17, 2004; accepted October 18, 2004 Published online May 11, 2005 and Wydoski 1980; Minckley 1983; Marsh 1994; Modde et al. 1996; Minckley et al. 2003). The largest remaining population of razorback suckers inhabits Lake Mohave, a mainstream Colorado River reservoir in Arizona and Nevada (Marsh and Minckley 1989; Marsh et al. 2003). Small popu- lations and scattered individuals are encountered elsewhere (e.g., Marsh and Minckley 1989; Modde et al. 1996; Holden et al. 2001). The population of razorback suckers from which the Lake Mohave stock was derived historically numbered in the hundreds of thousands (Minckley et al. 2003), but today fewer than 3,000 individuals are thought to persist (Marsh et al. 2003). Even though spawning and larval production still occur, juvenile recruitment is precluded by nonnative predation (Marsh and Langhorst 1988; Minckley et al. 1991; Marsh and Pacey 2005). Nearly 2 de- cades ago, some members of the population were estimated to be older than age 40 (McCarthy and Minckley 1987), and without recruitment the wild stock soon will be lost, and the species will not be far behind. Loss of any species is tragic in itself, but the unique and once abundant and wide-rang- ing razorback sucker also is among the most ge- netically diverse vertebrates ever studied (Dowling et al. 1996a, 1996b) and, thus, represents an ir- replaceable resource of both intrinsic and scientific value. During the 1980s, as an alternative to federal 547