2004-06-29_PERMIT FILE - C1981038 (3)

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19 was very similar to their availability durins sutra e:. As in the suttuner, backwaters, eddies, and pools were the preferred types of habitat in the w'.-:er. However, whereas eddies were most preferred in summer, pools were most preferred in winter. Adult pikeminnow used fewer habitat types overall during winter than during summer. Although fast runs and riffles were used during the summer, they were not used during the winter. The colder water temperatures in winter which cause lower metabolic rates may account for the avoidance of hish velocity sites. Absolute area of pools increases as flows decrease and slow- runs lose velocity. Because Osmundson et al. (1990 did not sample low• flows in the winter, they could not determine if pools would still be preferred in the winter at lower flows. Spring (April-July): Osmundson and Kaeding (1989) reported that pikeminnow use of low velocity habitats such as backwaters and flooded gravel pits is greatest during the spring runoff. It is believed that pikeminnow use these habitats during the runoff to escape the high velocity, low temperature flows of the main channel. Because backwaters, flooded gravel pits, and other low velocity habitats aze considerably warmer than the main channel during the runoff, these habitats allow pikeminnow to extend their growing season substantially. The earlier watming of these habitats may also be important in enabling pikeminnow to reach spawning condition by the time flow and temperature in the main channel are optimum for spawning. Osmundson et al. (1995) reported that, in the 15-mile reach, the numbers of backwaters and flooded gravel pits increases with increasing spring flows. (Although the number of backwaters eventually decreases as increasing flows convert backwaters to sidechannels, the number of other low velocity habitats likely increases as increasing flows inundate additional bottomlands.) The decrease in the magnitude, duration, and frequency of high spring flows, then, decreases the quantity and the duration and frequency of availability of important low velocity, higher temperature habitat in the spring. This could be affecting pikeminnow• growth and spawning success. Also, the quantity and frequency of availability of inundated floodplain depressions used by razorback suckers for spawning is dependent on the magnitude and frequency of spring flows necessary to inundate these areas. The decrease in the magnitude and frequency of spring flows necessary to inundate floodplain depressions is believed to be lazgely responsible for poor razorback sucker spawning success. Bioloeical Environment Food supply, predation, and competition aze important elements of the biological environment. Food supply is a function of nutrient supply and productivity, which could be limited by the presence of contaminants. The modification of flow regimes, water temperatures, sediment levels, and other habitat conditions caused by water depletions has contributed to the establishment of nonnative fishes. Predation and competition from nonnative fishes have been clearly implicated in the population reductions or elimination of native fishes in the Colorado River Basin (Dill 1944, Osmundson and Kaeding 1989, Behrtke 1980, Joseph et al. 1977, Lanigan and Berry 1979, Hinckley and Deacon 1968, Meffe 198, Propst and Bestgen 1991,