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• 19 <br />was very similar to their availability during sum.:.er. As in the summer, backwaters, eddies, and <br />pools were the preferred types of habitat in the v~.-:er. HoN•ever, whereas eddies were most <br />preferred in summer, pools were most preferred in winter. Adult pikeminnow used fewer habitat <br />types overall during winter than during summer. Although fast runs and riffles were used during <br />the summer, they were not used during the winter. The colder water temperatures in winter <br />which cause lower metabolic rates may account for the avoidance of high velocity sites. <br />Absolute area of pools increases as flows decrease and slow runs lose velocity. Because <br />Osmundson et al. (1990 did not sample low flov<•s in the winter, they could not determine if <br />pools would still be preferred in the winter at lower flows. <br />Spring (April-July): Osmundson and Kaeding (1989) reported that pikeminnow use of low <br />velocity habitats such as backwaters and flooded gravel pits is greatest during the spring runoff. <br />It is believed that pikeminnow use these habitats during the runoff to escape the high velocity, <br />low temperature flows of the main channel. Because backwaters, flooded gravel pits, and other <br />low velocity habitats are considerably warmer than the main channel during the runoff, these <br />habitats allow pikeminnow to extend their growing season substantially. The earlier warming of <br />these habitats may also be important in enabling pikeminnow to reach spawning condition by the <br />time flow and temperature in the main channel aze optimum for spawning. Osmundson et al. <br />(1995) reported that, in the 15-mile reach, the numbers of backwaters and flooded gravel pits <br />• increases with increasing spring flows. (Although the number of backwaters eventually <br />decreases as increasing flows convert backwaters to sidechannels, the number of other low <br />velocity habitats likely increases as increasing flows inundate additional bottomlands.) The <br />decrease in the magnitude, duration, and frequency of high spring flows, then, decreases the <br />quantity and the duration and frequency of availability of important low velocity, higher <br />temperature habitat in the spring. This could be affecting pikeminnow growth and spawning <br />success. <br />Also, the quantity and frequency of availability of inundated floodplain depressions used by <br />razorback suckers for spawning is dependent on the magnitude and frequency of spring flows <br />necessary to inundate these azeas. The decrease in the magnitude and frequency of spring flows <br />necessary to inundate floodplain depressions is believed to be lazgely responsible for poor <br />razorback sucker spawning success. <br />Biological Environment <br />Food supply, predation, and competition aze important elements of the biological environment. <br />Food supply is a function of nutrient supply and productivity, which could be limited by the <br />presence of contaminants. The modification of flow regimes, water temperatures, sediment <br />levels, and other habitat conditions caused by water depletions has contributed to the <br />establishment of nonnati~~ tshes. Predation and competition from nonnative fishes have been <br />. clearly implicated in the population reductions or elimination of native fishes in the Colorado <br />River Basin (Dill 1944, Osmundson and Kaeding 1989, Behnke 1980, Joseph et al. 1977, <br />Lanigan and Berry 1979, Minckley and Deacon 1968, Meffe 1985, Propst and Bestgen 1991, <br />