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food sources) in which streams provide the environmental (abiotic and biotic) conditions
<br />that each life history stage needs for survival, growth, and reproduction. This requires a
<br />knowledge of habitat use, incorporating information such as the path and timing of
<br />migrations, location and time of spawning, location of nursery areas, time of occupation,
<br />habitats occupied by juveniles and adults at different-times of the year, and other
<br />pertinent information. The amount of such information varies greatly among the four
<br />endangered fishes, -from excellent information on Colorado pikeminnow to mostly '
<br />anecdotal accounts for the bonytail (e.g., see reviews in recovery plans: USFWS
<br />1990ab, 1991, 1998; surveys: Quartarone 1993; and other information: Osmundson et
<br />al. 1997,1998; Osmundson and Burnham 1998; and Muth et al. 2000).
<br />In their natural riverine habitat, the timing of most life history events for the four
<br />endangered fishes is due to interactions among the environmental variables that
<br />provide cues, and of these, flow is an important component (Nester et al. 1988;Tyus
<br />and Karp 1989, 1991; Tyus 1990; Figure 2). Seasonal changes in temperature and
<br />photoperiod also are likely involved in the timing of life history events, but are difficult to
<br />separate from flow events such as spring runoff. Flow also plays a significant role in
<br />the availability of certain types of habitat (e.g., habitat in the floodplain will only be
<br />inundated during peak flows), and in the physical dimensions of habitat (higher flow
<br />usually means deeper, wider habitat). However, relatively simplistic attempts to provide
<br />habitat utilization profiles as a way of describing usable area of habitat for the
<br />endangered fishes as a function of flow has been frustrated due to fish use of different
<br />habitat parameters (e.g., water depth or velocity) depending on: location (e.g:,'river),
<br />time of year or life stage, and by use of habitats such as eddies and backwaters that
<br />are not amenable to flow models (Valdez et al. 1986, 1990; Tyus 1992).
<br />Superimposed on the spatial and temporal map of physical habitat are biological habitat
<br />components, which are defined largely by predator-prey or competitive interactions.
<br />The endangered fishes must have access to an abundance of suitable food species,
<br />but not be exposed excessively to predation. In some locations, high predation by
<br />introduced nonnative fishes may be the greatest impediment to expanding or
<br />reestablishing populations of the endangered fishes (e.g., Marsh and Brooks 1989,
<br />Marsh and Douglas 1997).
<br />A less obvious biological aspect that influences habitat selection and use is learned
<br />andlor instinctive (genetic) behavior. Major behavioral attributes tend to have a
<br />phylogenetic basis and are commonly shared among related taxa. Examples include a
<br />propensity for selecting certain habitats, prey selection, extent and direction of
<br />migrations, and orientations to flow, temperature or substrate. Learned responses such
<br />as imprinting, are essential to some migratory species (e.g., acipenserids, clupeids,
<br />salmonids, and catostomids), which may rely on subtle environmental cues, such as
<br />chemical composition of the water, to guide them back to the spawning areas from
<br />which they emerged several years earlier (e.g., see reviews by Hasler and Scholz 1983;
<br />McKeown 1984, Smith 1985). Concern about the role of these cues is raised whenever
<br />the natural habitat, or access to it, is altered.
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