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r Among native fishes in the lower Colorado River, the razorback sucker has received the most attention since the 1970s (Minckley 1983). Al- though annual spawning occurs, the population consists mostly of large, old adults, and there is no evidence of recruitment (Minckley et al. 1991). A large population appar- ently formed when Lake Mohave filled in the early 1950s, and the catch-per-unit effort indicated ex- Table 2. Population estimatesa for wild adult razorback suckers in Lake Mohave using Schumacher and Eshemeyer's multiple census (left) and Schnabel's annual census methods (center and right). Period Estimate Year Estimate Year Estimate 1980-1993 73,500° 1992 40,093 1993 23,118 1988-1993 59,500° 1994 21,292 1995 21,913 1991-1993 23,300° 1996 15,187 1997 11,122 1992-1999 18,248 1998 12,614 1999 9,086 a. Ricer (1975). b. From Marsh (1994). ceptional population stability through the late 1980s (Marsh and Minckley 1992, Marsh 1994). An estimated population of 73,500 wild adults in the period 1980-1993 dropped to 18,248 by 1992-1999. Annual estimates have consistently declined, from 40,093 in 1992 to 9086 in 1999 (table 2). Our success, and that of others, at rear- ing razorback suckers in isolated backwaters to sizes sufficient to avoid predators (Minckley et al.1991) led to a program to collect wild larvae from Lake Mohave and rear them in iso- lation for repatriation as subadults (Mueller 1995). The first repatriates joined breeding aggregations in 1993, and by 1999 they accounted for 12% of the population (figure 2). As noted above, the wild fish population was by then reduced to approximately 9000 individuals, to which Pacey and Marsh (2003) added an estimated 3000 surviving adult repatriates for an estimated total of approximately 12,000 adults. Prognosis. Our central argument is that quantitative recov- ery goals for the four big-river fishes (USFWS 2002a, 2002b, 2002c, 2002d) are grossly inadequate. An earlier, conceptual plan for managing lower Colorado basin native fishes (figure 3) included six levels of accomplishment, starting with pre- venting extinction (level I) and ascending to ultimate recov- ery (level VI, delisting, is removal from ESA protections and represents political recovery, USFWS 1996). Tn our view, the damage already suffered, coupled with predictable future de- mands on the lower river, makes achieving level rV (expan- sion to sustainability in natural habitats) or higher unlikely under present condi-100 tions. It is reasonable, however, to predict success to level III (stabilization), thereby ru 75 contributing significantly to species' per- petuation and, in concert with other S. 50- forts (e.g., USFWS 2002a, 2002b, 2002c, 2002d), perhaps to downlisting (from 25 endangered to threatened) and delist- ing. Native fishes can survive so long as appropriate habitat and management are 1991 1 provided and they are afforded commit- ments comparable to those for sport fish and wildlife. We define level III.(stabilization), the goal of species maintenance, not in terms of numbers of individuals but rather in terms of the genetic variability that existed a century ago in native big-river fish populations. This is the level of variabil- ity produced by the evolutionary process. If recovery plan amendments (USFWS 2002a, 2002b, 2002c, 2002d) are im- plemented, they will significantly erode this variability. Those plans may maintain some of the products of evolution for a time but will severely curtail the process. As Rolston (1991) convincingly argued, "It is not form (species) as mere mor- phology, but the formative (speciating) process that humans ought to preserve, although the process cannot be preserved without its products" (p. 103). Our recommendations delin- eate conditions under which these fishes may retain charac- teristics essential to the continuation of this formative process. Rationale for population goals No quantitative data, historic or otherwise, exist on original numbers of any native fish in any habitat of the Colorado basin. It is thus impossible to specify numbers required for downlisting or delisting that are based on restoration of his- torical population sizes. Estimates might be made using methods such as population viability analysis, but demo- graphic data are too sparse for accuracy or reliability. We therefore used three approaches to estimate the numbers of reproducing adults that are sufficient to sustain the four species: (1) qualitative observations, (2) genetic information, March 2003 / VoL 53 No. 3 • BioSdence 223 Figure 2. Untagged wild adults (open squares), tagged (recaptured) wild adults (open diamonds), and repatriated (recaptured) razorback suckers (filled squares) as a percentage of total catch in the springtime "razorback roundups" in Lake Mohave, Arizona and Nevada, 1991-1999. From Pacey and Marsh (2003).