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z 60 <br />0 <br />~ 50 <br />U) <br />40 <br />0 30 <br />z 20 <br />0 10 <br />cc: <br />W 0 <br />HABITAT TYPE <br />Figure 14. Habitat composition (percent total area) of the four 15-mile reach study sites during <br />periods of moderate summer flow levels and low flow levels. <br />During low flow periods, no use of backwaters, eddies or pools was observed. Slow runs and fast <br />runs, not preferred at moderate flow levels, were preferred when water was low (Fig. 15). <br />Lowered suitability of backwaters, eddies and pools may force fish into suboptimal habitats (slow <br />and fast runs) that they otherwise would not select. Change in use of habitats may result from <br />changes in quality of physical habitat features such as depth, dissolved oxygen, etc., or changes in <br />biotic interactions. Displacement of squawfish from backwaters during low flow conditions is <br />likely in response to reduced depth and cover. Displacement from eddies and pools, generally <br />deeper than backwaters, is more difficult to explain. Movement into runs from eddies and pools <br />may be precipitated by movement of other fish: they may be following their displaced food fishes, <br />or avoiding those fish with which they negatively interact. <br />It is likely that during times of very low flow, much of the reach becomes unsuitable to squawfish <br />as well as to other fishes. In the Grand Valley, the Colorado River has few instream boulders or <br />undercut banks and little overhanging vegetation that typically provide cover for fish in many <br />western streams and rivers. Here, depth, turbidity and perhaps agitation of the surface in some <br />areas are the only sources of cover. Native fish have made the most of this limited cover by <br />developing cryptic coloration and countershading as a mechanism to minimize detection by <br />predators. Our observations indicate that large portions of the 15-mile reach are shallow at low <br />water and when the water becomes clear, there is little cover for squawfish or the fish they feed <br />upon. Fish normally distribute themselves so that competition for food in any one locality is <br />reduced. A loss of cover from low flows may lead to displacement from established home feeding <br />ranges. If such conditions are temporary, fish are apt to become stressed when they are concen- <br />trated in limited areas of suitable cover; if the conditions are more permanent, a loss of carrying <br />30 <br />GP BA ED PO SR FR RI RA