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<br />.' <br /> <br />e <br /> <br />. <br /> <br />Area Manager <br />DRAFT <br /> <br />14 <br /> <br />Information on radio-tagged adult pikeminnow during fall suggests that pikeminnow seek out <br />deep water areas in the Colorado River (Miller et al. 1982, Osmundson and Kaeding 1989), as do <br />many other riverine species. Pouis, fUllS, wid other dup water areas, c3peciu11y in upstrea..'TI <br />reaches, are important winter habitats for pikeminnow (Osmundson et al. 1995). <br /> <br />On the Green River, tributaries are an important habitat component for pikeminnow (Holden <br />2000). Both the Yampa River and White River were heavily used by pikeminnow subadults and <br />adults, apparently as foraging areas (Tyus 1991). The tributaries were the primary area of <br />residence to which the adults returned after spawning. Tributaries to the San Juan River no <br />longer provide habitat to adults because they are dewatered or access is restricted (Holden 2000). <br />Pikeminnow utilized the Animas River in the late 1800s,and this river may still provide suitable <br />habitat; however, the present pikeminnow population is downstream from the mouth of the <br />Animas River about 50 miles (Holden 2000). <br /> <br />Very little information is available on the influence of turbidity on the endangered Colorado <br />River fishes. Osmundson and Kaeding (1989) found that turbidity allows use of relatively <br />shallow habitats ostensibly by providing adults with cover; this allows foraging and resting in <br />areas otherwise exposed to avian or land predators. Tyus and Haines (1991) found that young <br />pikeminnow in the Green River preferred backwaters that were turbid. Clear conditions in these <br />shallow waters might expose young fish to predation from wading birds or introduced, sight- <br />feeding, piscivorous fish. It is unknown whether the river was as turbid historically as it is today. <br />For now, it is assumed that these endemic fishes evolved under natural conditions of high <br />turbidity; therefore the retention of these highly turbid conditions is probably an important factor <br />in maintaining the ability of these fish to compete with non-natives that may not have evolved <br />under similar conditions. <br /> <br />Population Dynamics <br /> <br />Due to the low numbers of pikeminnow collected in the San Juan River, it is not possible to <br />quantity population size or trends. Estimates during a seven-year research period between 1991 <br />and 1997 suggest that there were fewer than 50 adults in a given year (Ryden 2000a). The ability <br />of the pikeminnow to withstand adverse impacts to its populations and its habitat is difficult to <br />discern given the longevity of individuals and their scarcity within the San Juan River Basin. At <br />this stage of investigations on the San Juan River, the younger life stages are considered the most <br />vulnerable to predation, competition, and habitat degradation through contamination. Population <br />level response times to rebound from these impacts may take several years or more. <br /> <br />Between 1991 and 1995, 19 (l7 adult and 2 juvenile) wild pikeminnow were collected in the San <br />Juan River by e1ectrofishing (Ryden 2000a). Wild adult pikeminnow were most abundant <br />between RM 142 (the former Cudei Diversion) and the Four Comers at RM 119 (Ryden and <br />Ahlm 1996) and they primarily use the San Juan River between these points (Ryden and Pfeifer <br />1993, 1994, 1995a, 1996). The multi-threaded channel, habitat complexity, and mixture of <br />substrate types in this area of the river appear to provide a diversity of habitats favorable to <br />pikeminnow on a year-round basis (Holden and Masslich 1997). <br /> <br />OD1409 <br />