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A,
<br />Natural populations elsewhere in the Colorado
<br />River system are non-sustaining and small or have
<br />been extirpated. Fish are encountered only occa-
<br />sionally in other portions of the lower Colorado
<br />River (Marsh and Minckley 1989; Minckley 1991)
<br />and, except for a few repatriates, the species has
<br />been extirpated from the Gila River system, Ari-
<br />zona, for nearly 50 years (Minckley 1973, 1983).
<br />Only about 100 fish are thought to persist in the
<br />Green River, the largest remaining population in
<br />the upper Colorado River basin (Bestgen et al.
<br />2002), and that population is also declining (Mod-
<br />de et al. 1996; Bestgen et al. 2002). The razorback
<br />sucker population of Lake Mohave, which histor-
<br />ically may have been derived from a source that
<br />numbered in the millions (Garrigan et al. 2002;
<br />Minckley et al. 2003), is now considered the last
<br />remaining storehouse of genetic diversity for the
<br />species (Dowling et al. 1996a, 1996b). As this
<br />adult population dwindles, the genetic variability
<br />of the species also declines.
<br />As a proactive effort consistent with recovery
<br />goals for razorback suckers (USFWS 1998, 2002),
<br />and with a comprehensive conservation plan for
<br />native fishes in the lower Colorado River (Minck-
<br />ley et al. 2003), the NFWG captures larval razor-
<br />back suckers, rears them to a size relatively safe
<br />from predation (30 cm or longer), and returns them
<br />to Lake Mohave. The goal of this program, initi-
<br />ated in 1991, is to establish a population of at least
<br />50,000 repatriated razorback suckers, in order to
<br />replace the aging and nearly extirpated adult stock.
<br />To date, approximately 70,000 fish have been re-
<br />leased back into the system, but there have been
<br />only 811 recaptures. This result may reflect the
<br />time needed for the fish to grow to maturity and
<br />become available for capture on the spawning
<br />grounds (McCarthy and Minckley 1987; Pacey and
<br />Marsh, unpublished data).
<br />It is hoped that the intensive conservation effort
<br />based upon repatriated fish will contribute toward
<br />perpetuation of the razorback sucker and its ge-
<br />netic variability until such time as threats to the
<br />species' survival can be ameliorated or removed.
<br />Only then can self-sustaining populations be re-
<br />established into historical habitats and thus put this
<br />critically imperiled species on the road to biolog-
<br />ical recovery.
<br />Acknowledgments
<br />Native Fish Work Group members representing
<br />Arizona State University, Arizona Game and Fish
<br />Department (AZGFD), U.S. Bureau of Reclama-
<br />tion (USBR), U.S. Fish and Wildlife Service
<br />NOTES
<br />1255
<br />(USFWS), Nevada Department of Wildlife
<br />(NDOW), U.S. National Park Service (USNPS),
<br />the U.S. Geological Survey Biological Resources
<br />Division (USGS-BRD), and others are acknowl-
<br />edged for their continuing logistic and program-
<br />matic support. Thanks are extended to all partic-
<br />ipants in field operations, with special appreciation
<br />for the leadership roles of Tom Burke (USBR),
<br />Mike Burrell (NDOW), Andy Clark (AZGFD), C.
<br />0. Minckley (USFWS), W. L. Minckley (ASU),
<br />Gordon Mueller (USGS-BRD), and Kent Turner
<br />(USNPS). Robert W. Clarkson and Thomas E.
<br />Dowling improved an earlier version of the man-
<br />uscript. Permits from AZGFD, NDOW, USFWS,
<br />and USNPS authorized collections. The USBR,
<br />Boulder City, Nevada, provided funding for this
<br />project.
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