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Articles <br />Legal Objectives Procedure to be applied Evidence of succz65 <br /> <br />l Adequate numbers of <br />r <br />d <br />h <br />d f <br /> Prevent extinction ts; <br />Secure reproductive adu secure <br />o <br />is <br />wil <br />otherwise unknown <br /> confirm survivability of <br /> young to maturity <br />•II <br />sub- <br />sample of remaining <br /> Perpetuate existing <br /> <br />III <br />= Develop and/or create Sufficient numbers, <br />population structure, <br /> population(s) <br />Sta habitats of sufficient and genetic viability <br /> physical, chemical, and <br /> biological quality <br />I`7 <br />Resto=hab <br />Numbers of localities <br />and numbers of fish <br /> Expand population(s) <br />F and per locality <br /> Completion of life cycle <br /> Self-sustaining Promote natural without human inter- <br /> population(s) recruitment and dispersal vention; resumed <br /> to expand range ecosystem function(s) <br />a Recovery <br />Figure 3. US Fish and Wildlife Service conceptual plan for managing native fishes of the lower <br />Colorado River basin (USFWS 1996). <br />and (3) empirical data on survivorship, standing crops, <br />growth, and other population statistics. <br />Qualitative observations. We suggest that it may be justifiable <br />to use data for nonnatives as a surrogate for population sizes <br />of native fishes (large versus small), on the assumption that <br />today's food supplies in many places are quantitatively, if not <br />qualitatively, comparable with those of the past For example, <br />adult Colorado squawfish and nonnative flathead catfish are <br />large-bodied, ambush piscivores, so past numbers of Colorado <br />squawfish and present-day flathead populations maybe sim- <br />ilar. The giant native minnow originally occupied the entire <br />lower Colorado basin, including the delta and Gila River <br />drainage. Adults moved upstream to spawn in numbers srnf- <br />ficient to harvest as human and livestock food (Miller 1961, <br />Minckley 1973, 1991). Flatheads now usurp the minnow's <br />whole former range in the lower Colorado River, where sur- <br />face water remains. <br />Numbers of Colorado squawfish in approximately 550 <br />kilometers (km) of the Green River flowing through Col- <br />orado and Utah, where the largest wild population persists, <br />were estimated in six ways by Tyus (1991). The number of <br /> Plan and implement Representative sub- <br /> genetic management, population(s) secured or <br />genetic variability expand broodstock otherwise unknown <br />adults luiigtr than 40 ccntimctere (cm) total length (TT., the <br />distance from the snout to the tip of the depressed tail fm) <br />varied from 1.4 to RO fish per km (averaging approximately <br />17). This must be a minimum estimate of original abun- <br />dance, since squawfish now must contend with factors that <br />presumably reduce their abundance. These factors include <br />naturally cooler waters, which probably result in lowered <br />productivity (Kaeding and Osmundson 1988); new dams <br />and river regulation; and competition with shoreline com- <br />petitors not historically present (e.g., northern pike Esox Lu- <br />cius, for space at least). Comparable estimates for adult <br />flatheads are 156 to 259 fish per km in the Colorado River <br />main stem near Yuma, Arizona (Young and Marsh 1990), <br />and an average of approximately 70 fish per km in the Gila <br />River (Marsh and Brooks 1989). This means that approx- <br />imately 4 to 15 times more flatheads live in lower-basin <br />streams than Colorado squawfish in the Green River. Thus, <br />if past and present ecological situations indeed are com- <br />parable, even by an order of magnitude, large numbers of <br />Colorado squawfish occupied lower-basin streams in the <br />past. Ample literature exists on standing stocks of prey <br />needed to support predatory fishes, and forage is clearly <br />224 BioScience March 2003 /Val. 53 No. 3