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<br />generation to generation, one needs information at the same <br />life stage (e.g., adult breeders) to encompass all factors that <br />influence it. Nonetheless, if 33% of female razorback suckers <br />contribute progeny each year, N of the 150,000 fish would very <br />likely exceed the stated goal of 5% (50,000) of the ancestral <br />1,000,000 fish. The annual yield of 10 progeny per female in <br />our model maybe too conservative, but it was held at that level <br />because of the uncertainties regarding N and concerns about <br />production and harvest discussed below. <br />We address these unknowns directly by promoting large, <br />panmictic populations in the channel plus connectives, from <br />which brood fish may be drawn. Doubling the number of <br />hectares, females, or young per female in our model results <br />in an order-of-magnitude change in estimated production. <br />However, unlike these parameters (some of which, such as area <br />dedicated for off-channel habitat, can be costly), altered sur- <br />vivorship results in logarithmic adjustments in population size. <br />An increase from 5% to 15% in first-year survival of repa- <br />triates, for example, results in an estimated 40% increase in <br />N in the channel plus connectives, from approximately <br />54,000 to 5,000 and 150,000 to 210,000 at 5 and 20 years, re- <br />spectively, in the examples given above. Increased survivor- <br />ship, with a reduction in investment in the number of isolated <br />habitats, brood fish, and other necessities, is thus the way to <br />succeed. <br />Completing the cycle. Step-like relationships exist between <br />body size of repatriates at the time of release and their sur- <br />vival in both rivers and reservoirs (figure 7). Thus, the young <br />should be nurtured toward the largest possible body size be- <br />fore they are transferred to the river. Given the major role of <br />size-dependent predation in limiting reestablishment of Col- <br />orado River fishes, transferring a few large (i.e., > 35 cm TL) <br />individuals is far more productive than repatriating many <br />smaller fish that would be devoured by predatory nonnative <br />fishes. <br />At moderate densities in lower-basin waters, young razor- <br />back suckers can reach more than 30 cm and bonytail more <br />than 25 cm TL in the fast year of life. By their second yeas', <br />they commonly reach 45 cm and 30 or more cm TL, respec- <br />tively. These growth rates allow for annual or biannual har- <br />vesting. Growth rates of repatriates in Lake Mohave are sim- <br />ilar, and substantial growth continues with increasing age <br />(figure 8). As demonstrated by survival of repatriates (figures <br />7 and 9), and in view of undetectable adult mortality in- <br />ferred from long-term catch-per-unit efforts in Lake Mo- <br />have between the 1970s and the onset of population collapse <br />in the late 1980s (Pacey and Marsh 2003), razorback sucker <br />of approximately 30 cm TL and longer are essentially immune <br />to existing predators. First-year survival of approximately <br />5% for repatriates in the wild increases to approximately <br />80% in subsequent years (figure 7, top graph), correspond- <br />ing roughly to increases in average TL from 30 to 35 cm at <br />repatriation to 45 cm 2 to 3 years later (figure 9). These <br />trends should apply as well for Colorado squawfish, although <br />that species has a slower growth rate than razorback sucker <br />Figure 7. Estimated survival of repatriated subadult <br />razorback suckers in Lake Mohave (Arizona and Nevada) <br />and San Juan River (New Mexico and Utah) that is based <br />only on fish greater than 29 centimeters total length at <br />time of stocking. The top graph ie based on raw data, <br />with no assumptions applied. The bottom graph is based <br />on the assumption that no individual is recaptured twice; <br />as a result, all fish captured once are theoretically lost <br />from the population. <br />(Minckley et al. 1991, Osmundson et a1.1997). Chub, which <br />more rarely exceed 35 to 40 cm TL as adults, may or may not <br />be more vulnerable to such size-structured predation. What- <br />ever the case, young should be nurtured toward harvest and <br />repatriation at the largest possible body size. <br />In practice, time of transfer from off-channel habitats is con- <br />trolled by growth, which is partially a function of population <br />size, which in turn is related to recruitment. Harvest and <br />transfer should be balanced with production of appropriate- <br />sized fish, but care must be exercised not to allow off- <br />channel populations (brood fish plus progeny) to exceed <br />carrying capacity, which would cause stunting. Potential dele- <br />terious impacts on off-channel populations, such as parasitism, <br />can be avoided by maintaining relatively low-density, highly <br />productive stocks by appropriate harvest and translocation to <br />230 BioSdence • March 2003 / Vol. 53 No. 3