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12 <br />Analysis of and/Critical Habitat 7ik ly to be Affe~$ <br />The Grand Valley area is occupied year round by Colorado pikeminnow. <br />The 15-mile reach is the section of the Colorado Rive:c extending from <br />the confluence of the Gunnison River upstream to the Grand Valley <br />Irrigation Company Diversion Dam. it has been identified as important <br />habitat for Colorado pikeminnow. Not only are densities of pikeminnow <br />especially high there (Figure 1), but the average size= of the fish <br />there is larger than in any other portion of the Colorado River <br />(Figure 2). <br />Radio-telemetry studies show upstream and downstream movement of adult <br />Colorado pikeminnow in the main stem Colorado River (INcAda and Kaeding <br />1991). The most dramatic movement was exhibited by a fish implanted <br />with a radio transmitter at Gypsum Canyon in upper Lake Powell on <br />April 5, 1982. The fish was contacted next in the lo•:~er Cataract <br />Canyon area on July 9, 1982. The next contact was made above the <br />Black Rocks area of Ruby Canyon, some 160 miles upstream. The <br />movement was accomplished in 41 days and is believed to be related to <br />spawning. At the end of September 1982, this fish was located in the <br />15-mile reach (river mile 178), nearly 200 river miles from its <br />furthest documented downstream location. <br />Other radio-tagged fish in the Colorado River have not displayed such <br />dramatic migratory behavior. Radio-telemetry studies conducted during <br />1982-1989, which focused on upstream reaches of the Colorado River in <br />and around the Grand Valley, provide the best indication of use of the <br />15-mile reach above the confluence of the Gunnison River. Movement of <br />these fish during a field season was generally limite3 to 25-30 miles <br />(Osmundson and Kaeding 1989, McAda and Kaeding 1991). <br />During 1986-1988, 17 adult Colorado pikeminnow were captured in the <br />15-mile reach during April-June and radio-tagged. The fish exhibited <br />a diversity of localized movements throughout the Grand Valley but <br />spent a major part of their time in the 15-mile reach. Two remained <br />in the reach throughout the estimated spawning period (Osmundson and <br />Kaeding 1989). <br />In a study by McP.da and Kaeding (1991), a suspected prespawning <br />aggregation of adult Colorado pikeminnow was observed in mid-July of <br />1982 at river mile 178.3 in the 15-mile reach. In the first <br />observation, three radio-tagged fish were tracked to one riverine pool <br />area, and nine adults at or near spawning condition were then captured <br />there after limited net sampling efforts. The aggregation occurred a <br />few days after mean daily water temperature had reached 20° C and <br />during a time when runoff flows were dropping off sharply. A second <br />aggregation was noted at river mile 175.3, 12 days after the initial <br />observation. Drifting trammel nets through an area occupied by two <br />fish equipped with transmitters yielded an additional male Colorado <br />pikeminnow in spawning condition. During this same time period, an <br />