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Life History Requirements A thorough knowledge of life history requirements is essential for guiding recovery ~ efforts, because it establishes the environmental (abiotic and biotic) conditions that each life history stage needs for survival -and growth. Ideally, one would begin by assembling a comprehensive spatial and temporal map of habitat use, incorporating information such as the path and timing of migrations, location and time of spawning, location of nursery areas and time of occupation, and the habitats occupied by juveniles . and adults at different times of the year. In riverine habitat, the timing of most life history events is closely connected with flow in the river (Tyus and Karp 1989, Tyus 1990; Figure 2). The effects of annual changes in temperature and photoperiod, which also may be involved in the timing of life history events, are very difficult to separate from flow events like spring runoff. It seems likely that there may be interactions among the environmental variables that provide cues for life history events. Flow also plays a ~ significant role in the availability of certain types of habitat (e.g., habitat in the floodplain will only be inundated during peak flows), and in the physical dimensions of habitat (higher flow usually means deeper, wider habitat). Superimposed on the spatial and temporal map of physical habitat are the biological ~ dimensions of habitat, which are defined largely by predator-prey or competitive interactions. The endangered fishes must have access to an abundance of suitable food species, but not be exposed excessively to predation (both topics will be addressed in more detail in a later section). ~ A less obvious biological aspect that influences habitat selection and use is learned and/or instinctive (genetic) behavior. These behaviors tend to have a phylogenetic basis, and thus are commonly shared among related taxa. Examples include a propensity for selecting certain habitats, prey selection, extent and direction of migrations, and orientations to flow, temperature or substrate. Learned responses, ~ such as imprinting, are essential to some migratory species (e.g., acipenserids, clupeids, salmonids, and catostomids), which may rely on subtle environmental cues, such as chemical composition of the water, to guide them back to the spawning areas from which they emerged several years earlier. Concern about the role of these cues is raised whenever the natural habitat, or access to it, is altered. ~ Determining the life history requirements of endangered species is inherently difficult because so-few individuals exist in the wild. There are simply not enough opportunities 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 adult Colorado ~ pikeminnow are now found in deep runs, is it because eddies or "slack" waters are no longer available? Or, is it because they have been displaced from other, more suitable, habitat by aggressive, introduced fishes? 8