9640 - Laserfiche WebLink

Home Browse Search

. U When recovery efforts were begun, the abundance of predatory, nonnative species in the lower river made realizing these goals seem highly unlikely. Re- cruitment failure was poorly understood, so mil- lions of larval razorback suckers per year were cul- tured in hatcheries for stocking in southern Arizona (Inslee 1982, Hamman 1987), with essentially no success (Minckley et al. 1991, Hendrickson 1993). Stocking juvenile, hatchery-reared razorback suckers at somewhat larger sizes yielded the same result (Langhorst 1989, Marsh and Brooks 1989). Data for bonytail were similar. Each year, Lake Mohave brood fish produced bonyta.il progeny in 0.04-hectare (0.1 acre) hatchery ponds, but approximately 200,000 fish that were repatriated between 1981 and 1990 es- sentially disappeared. Survival, although detected, P, Breeding population in isolated, off-channel habitats Repatriated subadults Reproductive adults Panmictic population in channel plus connectives was low (< 0.001%); the total length of these fish at the time they were stocked rarely exceeded 10 cm. When predation on larvae and juveniles was iden- tified as the limiting factor (Minddey 1983,1991), ef- fortwas shifted to circumventing its impacts (Minckley et al. 1991, Pacey and Marsh 2003). Few problems existed with adults. Large individuals of all four taxa persisted under di- verse conditions where nonnatives were common, and all but humpback chub were known to successfully reproduce in farm ponds, under hatchery conditions, and elsewhere (Marsh and Pacey 2003). To speed the process for the razorback sucker, by then beginning its decline, we captured wild larvae directly from Lake Mohave, reared them in isolation from predators, and repatriated subadults back to the reservoir. In 1993-1995, stocking razorback sucker in small (0.05 to 0.17 ha, averaging 0.13 ha), predator-free habitats resulted in an average survival rate of 22% (0% to 81%) from larva to subadult. As noted before, repatriates entered the breeding population 2 years after the program was begun, and in 1999 they constituted approximately 12% of the reproductive adults. Hatchery-cultured larval and juvenile bonytail were added, grew well, and were also repatriated (table 4) with lesser success. On the basis of a model created in part from these ob- servations, managing 100 ha with 50 females per ha yield- ing 10 young per female provides 50,000 subadults per year for transfer to the channel plus connectives. If 5% survived the first year after repatriation and 80% survived each succeeding year of freedom, and if the same production- transfer rates continued, approximately 54,500 adults would theoretically be present in the channel plus con- nectives after 5 years. N, would stabilize at approximately 60,000 adults in about 20 years (in our model, adults are programmed to die at 35 years of age). In a second example, if a goal was 5% of the N estimated to produce today's genetic legacy for razorback sucker (about 1,000,000 females; table 3), 50 females per ha, an average of 10 progeny per female per year, and 250 off-channel hectares might approach that figure. Under the last scenario, 125,000 juveniles would be available annually for transfer, Figure 6. Schematic interrelations between lower Colorado River off- channel habitats and channel plus connectives. and at survival of 5% the first year and 80% thereafter, a Nc of approximately 139,000 adults would exist in the channel plus connectives at the end of 5 years; N would stabilize at approximately 20 years with close to 150,000 fish (approxi- mately 50% female). Unknowns. We have insufficient data to quantify the rela- tionship of N and N,, and because each female produces a vast surplus of ova, gross production of progeny may be only indirectly related. Demographic data from radiotelemetry studies and genetic data from an experiment examining pro- duction of razorback sucker progeny using mtDNA analysis independently suggest that individual razorback sucker females may not spawn every year. However, available survival estimates include only part of the life cycle and do not include most of the survival com- ponents for breeding adults. To properly measure N, from Table 4. Numbers of bonytail repatriated to Lake Mohave from natural reproduction in p onds at US Fish and Wildlife Service Dexter National Fish Hatchery, New Mexico, 1981-1997. Number of Total length (millimeters) Years fish Average Maximum Minimum 1981 26,817 1028 - - 1981-1982 14,700 102 - - 1985 12,618 102 - - 1987-1988 34,011 140 - - 1988-1989 15,540 102 - - 1989-1990 44,678 90 - - 1990-1991 9,283 102 - - 1991-1992 6,617 72 - - 1992-1993 17 167 259 95 1993-1994 7 243 265 227 1994-1995 12,507 105 322 101 1995-1996 131 308 368 154 1996-1997 784 279 420 225 102 mm = "fingerling" size; other data are actual measurements. a . March 2003 / Vol. 53 No. 3 • BioScience 229