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<br />968
<br />
<br />OSMUNDSON AND BURNHAM
<br />
<br />be introduced, and floodplains may continue to
<br />constrict from construction of additional jetties
<br />and dikes and from further channel-stabilizing ef-
<br />fects of exotic riparian vegetation (see Graf 1978).
<br />Colorado squawfish, with their exceptional lon-
<br />gevity (Osmundson et al. 1997) and high fecundity
<br />(Hamman 1986), are well equipped to persist
<br />through long periods of adverse conditions. His-
<br />torically, their basinwide distribution no doubt
<br />provided adequate colonization sources if drought
<br />or other conditions temporarily rendered one or
<br />more tributaries uninhabitable. Today, habitat
<br />modifications occur rapidly and are likely to per-
<br />sist for long periods; in addition, colonization
<br />sources have been substantially reduced. Though
<br />the nearby Green River population may serve as
<br />a colonization source for the Colorado River (Gil-
<br />pin 1993), a balance between local extinction and
<br />colonization (see Gilpin and Hanski 1991) for this
<br />and other wild populations is unlikely if extinction
<br />is caused by a deterministic response to degraded
<br />habitat conditions (Harrison 1991; Thomas 1994).
<br />The same logic applies to stocking hatchery-reared
<br />Colorado squawfish to reestablish extirpated pop-
<br />ulations. Recovery to a viable, self-sustaining lev-
<br />el depends on first identifying and then amelio-
<br />rating or eliminating the ultimate limiting factors
<br />(Tear et al. 1995).
<br />For this population, a low frequency of strong
<br />year-classes currently appears to limit adult abun-
<br />dance. Although any increase in adult survival rate
<br />would be beneficial, the success of recovery efforts
<br />will largely depend on providing environmental
<br />conditions that increase reproductive success and
<br />survival of early life stages.
<br />
<br />Acknowledgments
<br />
<br />We thank Mike Tucker, Tom Fresquez, Dale Ry-
<br />den, Mike Montagne, and Bruce Bonar for assist-
<br />ing with data collection; Frank Pfeifer for admin-
<br />istrative support; and Bob Burdick (USFWS) and
<br />Bill Elmblad (CDOW) for sharing their respective
<br />data sets. We appreciate Karl Seethaler, Rich Val-
<br />dez, Chuck McAda, and Toby Mourning for al-
<br />lowing our use of their length-frequency data from
<br />1974 to 1976, 1979, 1982, and 1995, respectively.
<br />Kevin Bestgen, Tom Chart, Bruce Haines, Pat
<br />Martinez, Tom Nesler, Ron Ryel, Jack Stanford,
<br />and an anonymous reviewer provided valuable
<br />comments on various drafts. Ron Ryel also pro-
<br />vided assistance with statistical analyses. Funding
<br />was provided by the U.S. Fish and Wildlife Service
<br />and the Recovery Implementation Program (RIP)
<br />for Endangered Fish Species in the Upper Colo-
<br />
<br />rado River Basin. The RIP is a joint effort of fed-
<br />eral and state resource agencies, upper basin water
<br />and power user groups, and environmental organ-
<br />izations.
<br />
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