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Determining habitat requirements of endangered species is inherently difficult: the large habitats they occupy are difficult to reproduce experimentally and few individuals exist in the wild. In addition, it has been difficult to associate individuals of different life history stages with preferred habitat. The situation in the UCRB is further complicated by extensive alterations to physical and biological characteristics of the natural habitat. Thus, for example, if studies of adult Colorado pikeminnow indicate preference for deep runs, is it because eddies or "slack" waters are not available? Or have the fish been displaced from other, more suitable, habitat by physical changes or by introduced fishes? There are legitimate concerns whether present-day field studies of rare species will provide an accurate representation of their life history requirements, however there is no practicable alternative but to make the best use of such information. Field studies conducted where a species is relatively abundant, and where the habitat is altered least (all habitat now occupied by the endangered fishes has been altered some), are most likely to provide an accurate view of life history requirements (discussed in Tyus 1992). The optimal remaining habitat is closest to the conditions in which the native species evolved and presumably are the conditions in which the species are most likely to maintain an adaptive advantage over introduced species. Unfortunately, there has been little synthesis of life history requirements throughout the range of the endangered fishes. As a result, localized studies provide a narrow geographic focus that can lead to a fragmented view of recovery needs. Although local adaptations can and do occur, recovery efforts could benefit from a synthesis of general life history needs. "` Various locations in the UCR differ greatly in the degree of anthropogenic alteration. Thus, all are not suitable for determining life history needs of the fishes. Criteria are needed for selecting areas that may provide the most representative information; a relatively abundant population in which successful recruitment is occurring would seem to be ideal. However, it is not possible to find one location where all of the endangered fishes are presently most abundant. The Green River basin has long been identified as perhaps the most suitable location to determine management measures necessary for recovery of Colorado pikeminnow and razorback sucker because it supports the largest riverine populations of these species. Furthermore, flows of two major tributaries, the Yampa and White Rivers, remain largely unregulated. Similarly, the Little Colorado and Colorado Rivers in the Grand Canyon .and Black Rocks-Westwater canyons of the upper Colorado River are the logical places to study the life history needs for the humpback chub because there are large populations sustained by natural recruitment. Information obtained from "optimal" situations can be supplemented, albeit cautiously, with observation from sites where habitat has been altered, but where the species is relatively abundant. A good example of the latter condition is Lake Mohave, AZ-NV, which supports the largest- extant population of razorback sucker. The bonytail presents a special challenge because it is not sufficiently abundant anywhere in nature to afford opportunities for meaningful study of "natural" behaviors. 12