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<br />experience a dramatic decline as it reaches its life expectancy (Table 7, Figure 7). Also, older fish may <br />fail to reproduce, further reducing the potential for population growth in the future. A negative response <br />is illustrated by the population shifting toward the right side of the length-frequency graph. Mean length <br />increases between 1998 and 2007 and remains relatively constant thereafter, as the population <br />becomes more skewed toward older age-classes. The number of age-classes decreases between <br />2003 and 2010, as the oldest age-classes begin dying off. <br /> <br />The neutral response is somewhat more difficult to defme (Table 8, Figure 8). While the existing <br />population ages, some recruitment into the adult population appears to be taking place; however, there <br />are no strong age-classes coming up nor any apparent trends indicating overall population growth. <br />However, there is no inunediate concern of a significant decline in population either, because the age- <br />class structure is diverse and is not shifting toward the right side of the graph. Colorado Pikeminnow <br />Y oung-of-the-year (YOY) Production <br /> <br />Catch per unit effort (CPE) data have been collected for YOY Colorado pikeminnow from 1982 <br />through 1997 (Table 9, Figure 9). Geometric means of these data are represented as gray-shaded <br />diamonds in Figures 9-12. In addition, from 1993 through 1997, standard error (SE) values also were <br />computed for YOY data (solid vertical I-bars in Figures 9-12). To establish a population baseline <br />value for YOY production, we calculated an arithmetic mean (5<=0.382733) and SE (,,0.252791) from <br />the annual geometric means of all existing CPE data. The population baseline value for pikeminnow <br />YOY production (5<+SE=0.635524) is represented by the broken horizontal line in Figures 9-12. This <br />value serves as the threshold used to detennine a positive, negative, or neutral response, based on the <br />frequency with which future CPE-SE values for YOY exceed this value. <br /> <br />Future CPE"SE data were simulated for 1998 through 2010 (Tables 10-12, Figures 10-12). In <br />Figures 10-12, simulated CPE geometric means are represented by diamonds with solid vertical I-bars <br />representing "SE values. In Figures 10-12, for years in which CPE-SE > 0.635524, CPE values are <br />represented by white diamonds; for years in which CPE-SE:s 0.635524, CPE values are represented <br />by black diamonds. We evaluated three different methods to compare simulated data in Tables 10-12 <br />with baseline population data: a cumulative average exceedance frequency based on the number of <br />years CPE-SE exceeds the threshold (population baseline) value divided by the total number of years <br />after 1997; a rolling 5-year average which looks back 5 consecutive years from the present to <br />determine the frequency with which CPE-SE exceeds the threshold value; and a rolling 10-year <br />average which looks back 10 consecutive years from the present to detennine the frequency with which <br />CPE-SE exceeds the threshold value. <br /> <br />YOY are considered to exhibit a positive response (Table 10, Figure 10) whenever values of CPE-SE <br />exceed the threshold in at least 20 percent of the years. In this example, the results are unambiguous. <br />Beginning in 1999, all three methods show a positive response (i.e., values ofCPE-SE exceed the <br />threshold value in 20 percent or more of the years). <br /> <br />6 <br />