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188 VALDEZ AND MUTH 2004). Population estimates are available for the upper Colorado River subbasin for 1992-1994, 1998-2000, and 2003-2004; and for the Green River subbasin for 2000-2003. This history of es- timates is insufficient to determine if recent declines are attributable to normal population cycles or en- vironmental threats. The life history of Colorado pikeminnow is rea- sonably understood and environmental stressors that affect populations continue to be investigated. The upper basin recovery programs have implemented aggressive management actions to address threats, but response by endangered species may not be di- rect or immediate because of the complexity of en- vironmental correlates that affect population dy- namics. Application and evaluation of management actions are on parallel courses such that when threats are removed or minimized, populations are expected to increase. Humpback Chub According to the 2002 recovery goals (U.S. Fish and Wildlife Service 2002b), recovery and long-term conservation of humpback chub depend on: 1. Finalization and implementation of site- specific management tasks to minimize or remove threats to attain necessary levels of protection. 2. Maintenance of six genetically and demo- graphically viable, self-sustaining popula- tions, including five in the upper Colorado River basin and one in the lower basin. Each population is maintained such that: · trends in annual adult (age 4+) point esti- mates do not decline significantly, · mean estimated recruitment of age-3 naturally produced fish equals or exceeds average annual adult mortality, and · three genetically and demographically vi- able, self-sustaining core populations are maintained, such that each annual point estimate exceeds 2,100 adults. Concurrent estimates are not available for all five populations, but preliminary numbers of adults during 1998-2003 are Black Rocks (478-921); Westwater Canyon (2,201-4,744); Desolation/ Gray Canyons (948-2,193); Yampa Canyon (391); and Cataract Canyon (150). Population es- timates for humpback chub tend to be less precise than for Colorado pikeminnow because of the lo- gistical difficulty of sampling whitewater canyons inhabired by this species. As with Colorado pikeminnow, numbers of adult humpback chub apparently declined recently concurrent with ex- tended periods oflow flow and increases in non- native predatory fish. However, linkages between year-class strength and river flow are less clearly defined as for Colorado pikeminnow. A primary threat to humpback chub in the upper basin is pre- dation by channel catfish and smallmouth bass in Desolation/Gray and Yampa canyons. Efforts to mechanically remove nonnative fish from these population centers are ongoing and are being evaluated. Bony tail Bonytail is the most imperiled fish species of the Colorado River system. Wild populations are bio- logically extinct in the upper basin and only a few wild fish remain in the lower basin. According to the 2002 recovery goals (U.S. Fish and Wildlife Ser- vice 2002c), recovery and long-term conservation ofbonytail depend on: 1. Finalization and implementation of site- specific management tasks to minimize or remove threats to attain necessary levels of protectlon. 2. Establishment and maintenance of four ge- netically and demographically viable, self- sustaining populations, two in the lower Colorado River basin and two in the upper basin; one each in the Green River subbasin and upper Colorado River subbasin. Each population is maintained such that: · trends in annual adult (age 4+) point esti- mates do not decline significantly, · mean estimated recruitment of age-3 natu- rally produced fish equals or exceeds aver- age annual adult mortality, · annual point estimates for each of the four populations exceeds 4,400 adults, and