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47 <br />channel, while introduced species were not. No particular mechanism <br />was reported as a possible cue to withdraw to the main channel,' <br />however. <br />o Highly adapted species tend to be less tolerant to change than species of a <br />more generalist nature. <br />Holden (1973, 1974) reported fishes of the and Southwest (including the <br />Colorado River Basin) to be few in number but to be highly adapted to <br />the environmental extremes encountered in the region. The same high <br />degree to which the native fishes have adapted has also made them less <br />able to tolerate modification of this environment. Introduced species <br />tend to handle environmental modification better since they are not so <br />specialized. <br />Habitat Availabilitv <br />All four rare fishes. <br />o Habitat availability varies with discharge and river segment characteristics but <br />some general relationships are common to most sites. <br />Wydoski (1980) presented availability of several habitat types at different <br />discharges for the White River, Colorado. Results were reported to be <br />representative of general flow-habitat relationships for many rivers. As <br />flow increases, deep fast habitat increases considerably, and riffle areas <br />increase moderately. Habitat types which decrease in area with