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' sites in low-velocity habitats, where their movements were restricted to a <br />I relatively small area until spring. These habitats are often large <br />backwaters or river pools, covered with ice. Whether adults prefer ice cover <br />' or whether ice coincidentally forms on the low-velocity habitats selected by <br />fish is unknown. <br />Depth <br />' Depths of habitats selected by squawfish varied somewhat by season (Fig. 7). <br />Some shallow-water habitats were used during most of the year, and depths of <br />2.0-5.9 feet were most frequently used overall. However, the use of deep- <br />water habitats increased during June-August, the period of rapid decline in <br />river flow. In August and September, depths >6 feet were more frequently <br />used than were any of the other categories. The increased use of deep water <br />during the summer months is partly explained by the use of the plunge pools <br />of the Redlands and Price Stub diversion dams. During October-November, when <br />the irrigation season ends and many upstream diversions of water cease, flows <br />in the 15-mile reach increase. This increases the depth of backwaters and <br />pools. Some fish return to the 15-mile reach from adjacent reaches at this <br />time. Although the precise reason for this return movement is unknown, it <br />suggests that the availability of these deep-water habitats might have been <br />limiting during the previous summer months. <br />Temperature <br />Environmental temperature is very important to all cold-blooded organisms, <br />including fish. In upstream regions like the 15-mile reach, slow annual <br />growth of Colorado squawfish may result from seasonal temperatures that are <br />generally well below the physiological optimum of the species (Kaeding et <br />al. 1985). This is well established for other fishes. Slow growth decreases <br />24