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An understanding of the motivations guiding behavior and habitat use of individuals of the endangered fishes at different stages of development is highly desirable for determining the needs of each species. The task is difficult because the subjects are hard to locate and there are confounding factors, as mentioned previously. Nevertheless, such an understanding is needed for determining the potential role of tributary streams in the recovery effort. At present, the knowledge is incomplete.. The Colorado pikeminnow has been studied in greater detail than the other three endangered fishes, and aspects of its life cycle have previously been reviewed by several authors (e.g., USFWS 1991; Haynes et al. 1984, Tyus 1986,1990,1991ab, Nesler et al. 1988; Tyus and Haines 1991; and Muth et al. 2000). Its life cycle is relatively complex, including spatial separation of life stages and energetically-costly migratory behavior. These components, mostly involving widespread movements, are tactical parts of the life strategy that has maximized fitness of the Colorado pikeminnow over millions of years. In general, adults spend most of the base flow period (September -April) in main-channel habitats of upper river reaches that may extend upstream as far as the transition zone of cold water habitat. Each fish occupies a home range in which physical habitat conditions are variable, and habitat selection is probably related to prey abundance. Adults exhibit considerable tolerance to cold and remain active throughout the winter. Adult pikeminnow become very active in spring when snow melt causes the rivers to rise. Most of the adults move into seasonally inundated habitat, presumably t'o seek more acceptable habitat conditions and also to take advantage of terrestrial food sources. Rising flow, increasing temperatures and photo period, and other environmental influences stimulate gonadal development and reproductive behavior. During peak runoff, usually in May, adults begin migrating to spawning areas. Although not proven conclusively, we presume that homing, guided mostly by olfactory cues, takes the mature adults to spawning sites from which most emerged as sac-fry. Spawning activity occurs over a 3-4 week period when flows are declining after peak runoff, and when water temperatures typically are in the range of 22-25°C. Adult fish may use large backwaters, mouths of tributary streams, gravel pit ponds, or flooded bottoms for staging because ambient warming can produce higher temperature than in the mainstream river, and this may hasten the maturation of ova. Reproductively active Colorado pikeminnow utilize eddy habitat for staging and cobble bars for deposition of the adhesive eggs. After about 7-10 days newly emerged sac-fry drift downstream and reach nursery habitats, primarily shallow (ephemeral) backwaters and eddies in alluvial reaches. Backwater nursery habitat is created by the declining flows that follow peak runoff. If peak flows are low, few backwaters are formed. If flows remain high, potential backwaters are inundated. In the nursery area, the larvae and postlarvae feed on zooplankton and benthos. As the young fish grow larger, they begin to consume fish. Juveniles (60-200 mm) continue to feed actively throughout the winter but can withstand extended periods of starvation if food is unavailable. The juvenile fish occupy 13