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• <br />e <br />Reduced stream flows may also be a reason for reduced survival rates of Colorado <br />pikeminnow in the Green River Basin in 2000 to 2003. Base flows were relatively low in all <br />years from 2000 to 2003, and particularly so in 2002 (Fig. 2). For example, mean monthly flow <br />levels of the Green River at Jensen (gage # 09261000) in July in 2000, 2001, 2002, and 2003, <br />were the 8', 3"d,2 nd , and 7t' lowest, respectively, that have been recorded since gage operation <br />began in 1947; other years of low July flows were 1961 (5t'), 1963 (Is), 1989 (0), and 1994 (6``'). <br />Mean annual flows for 2000, 2001, 2002, and 2003 (water years) were the 19`h, 7t' , 2°a, and 9`h <br />lowest, respectively, that have been recorded in the 57 years of gage operation. Rates of decline <br />in abundance of adult and recruit-sized Colorado pikeminnow varied in the five different reaches <br />sampled in the Green River Basin from 2000-2003 and were highest in the middle Green River <br />reach and the White River. The exact mechanism for increased mortality due to drought is not <br />apparent either, but could be due to decreased habitat size, density-dependent effects of crowding <br />of large, piscivorous Colorado pikeminnow into reduced habitat and subsequent reduced food <br />supply, increased incidence of disease, or other unknown factors. This may be particularly true in <br />the White River, which is the smallest stream sampled during this study. <br />Reduced condition of Colorado pikeminnow in 2000 to 2003 compared to 1991 to 1999, <br />based on ANCOVA of length-weight relationships, may be responsible in part, for reduced <br />survival in drought-affected streams. Reductions in habitat size may have also reduced survival <br />by increased potential for encounters (direct predation or bite damage) with predaceous northern <br />pike, which are abundant in the Yampa River and present in the middle Green River reach. Given <br />rather severe reductions in stream flow, perhaps apparent reduced abundance of Colorado <br />pikeminnow in the Green River Basin during the study period is not surprising. <br />50