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Summary and Conclusions <br />Decline of razorback sucker toward extinction is a function of recruitment failure throughout <br />its range. No verified recruits to an estimated 55,000 adults have been found among almost <br />12,500 fish tagged or otherwise handled in Lake Mohave, Arizona-Nevada, since 1973 <br />(Marsh, 1994). Large numbers of young hatch each year, but soon disappear to predation, <br />perhaps mediated by nutritional constraints soon after yolk absorption. Existing adults <br />hatched around 1954, coincident with impoundment and prior to prevalence of non-native <br />predators. Based on timing of disappearance of other reservoir stocks in the lower Colorado <br />basin (approximately 40 years after impoundment; Hinckley, 1983), the Lake Mohave <br />population should crash at 'any time. <br />MtDNA diversity in razorback suckers of Lake Mohave is remarkably high, with an <br />average of 0.68 haplotypes per individual. The population must therefore be comprised of <br />direct descendants of an exceedingly lazge, diverse, panmictic population inhabiting the lower <br />Colorado basin in pre-development times. Only natural recruitment can maintain existing <br />genetic variability. A population crash will result in significant loss of diversity, and the <br />possibility seems remote of solving the recruitment problem before the remaining population <br />collapses. Our efforts thus can augment the numbers of individuals, but can only maintain <br />some portion of the genetic variability that now exists. Unfortunately, the level. of potential <br />conservation of genetic variability is inverse to the numbers of fish produced by each of three <br />available means of maintaining population size. Lazge numbers of razorbacks (of relatively <br />law variability) may be hatchery cultured, fewer fish (with more variability) can be produced <br />by natural spawning in predator-free isolation, and fewer yet (with the most variability) are <br />available by collecting wild-spawned larvae and rearing them in protected sites. We <br />nonetheless strongly recommend the last option be selected, and advise that such.a program <br />28 <br />