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obtained from fish stockings (e.g., Pacey and Marsh 1998). Thus, biotic and abiotic factors influencing decline of bonytail are not understood. Very few fish have been reported in the UCR basin and it is not known if the fish was ever widely abundant, although they were reported "numerous" in the Green River in Dinosaur National Monument (DNM; Vanicek and Kramer 1969). The last bonytail reported from the UCR basin was captured in 1984 near Black Rocks (Kaeding et al. 1986). Some bonytail have persisted in large reservoirs of the lower basin (e.g., Lake Mohave and Lake Mead), indicating an ability to live in lacustrine habitat (Minckley 1973, Valdez and Clemmer 1982, Foster et al. 1999). Results of telemetry studies have demonstrated that the fish can make substantial movements over short time frames, but may remain in certain areas for weeks (Marsh and Mueller 1999). Adult bonytail introduced .into the upper Green River in 1988 and 1989 exhibited crepuscular movements, and were relatively quiescent during the day and night (Chart and Cranney 1992). Studies are in progress to determine basic ecological requirements that may be needed for successful reintroduction (Growl et al. 1996). ENVIRONMENTAL CONSTRAINTS General The four endangered fishes successfully adapted to the many environmental constraints imposed by the historic Colorado River system. However, in the present, altered system, new constraints have been added with little time for the fishe~'to adapt to them, and some constraints, such as dewatering and high dams, cannot be overcome by adaptive capability of the fishes. Recovery of threatened or endangered (i.e., listed) species to aless-endangered status requires, at the least, reduction of pervasive threats, increased geographic distribution, and increases in population abundance, all of which will aid to increase population viability. For most listed species, the removal of threats is usually addressed immediately, while determining how to increase distribution and abundance. Reduced to its simplest element, if recruitment to the breeding population does not equal or exceed loss to all sources of mortality, other factors being equal, the population will decline. For most endangered species, loss has significantly exceeded recruitment in the recent past. Using the Colorado pikeminnow as an example, recruitment has failed in over 80% of its historic range (USFWS 1991). For the bonytail, it has been 100%. For all of the four listed fishes, successful recovery will require increasing recruitment (survival of young to complete the life cycle as reproducing adults) and maintaining populations of adults. This may be viewed as two recovery goals: to increase relative.survival of offspring and to increase carrying capacity (and longevity) of adults. Both will increase standing crops providing other constraints, or limiting factors, do not come into play. The factors contributing to recruitment gain and loss may be abiotic (physical or -- chemical), biotic, or both. Physical factors could include the quality or abundance of habitat required for one or more life history stages. For example, loss of habitat through channelization, or degradation of substrate by sediment accumulation, will 15