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<br />of a negative population response. Age-class structure is more subjective and, therefore, must be <br />viewed in context with the other indices. Ifboth the other indices are positive, we would expect age- <br />class structure to show a positive response, as well. However, it provides a reasonable snapshot of the <br />status of a population, when there are no discernible trends in the other indices. <br /> <br />Adult Population Size <br /> <br />Following the Interagency Standardized Monitoring Program (ISMP) protocol, catch per unit effort <br />(CPE) data were collected for adult pikeminnow every year from 1986 through 1997. Standard error <br />(SE), which is a measure of variability in CPE data, also was computed for each of these years (fable <br />1). Figure 1 presents these data graphically, where mean CPE values are represented as white <br />diamonds with ",SE as solid vertical I-bars. In addition, for 1993-95, the adult pikeminnow population <br />was estimated at 600 (dark gray squares) with a 95% confidence interval (CI) of ol150. This CI <br />indicates that the actual adult pikeminnow population is likely to fall between 350 (600-250) and 850 <br />(600+250) with 95% certainty. <br /> <br />To determine the response of the adult population, positive and negative threshold values were <br />established (broken vertical lines in Figures 1-4). The threshold for a positive population response is <br />the baseline population estimate plus two times the CI (600+(2*250) = 1100), while the threshold for a <br />negative population response is the population baseline minus the CI (600-250 = 350). Population <br />estimates> 1100 represent a positive response; population estimates < 350 represent a negative <br />response; and population estimates 2: 350 and:S 1100 represent a neutral response. Numerical <br />examples of positive, negative and neutral population responses are tabulated in Tables 2-4. In Figures <br />2-4, future CPE values are represented as gray diamonds, while positive, negative and neutral future <br />population responses are indicated as white, black and light gray squares, respectively. <br /> <br />In the positive example (Table 2, Figure 2), the initial population estimate (850) in years 1998-2000, <br />would be neutral because 850 is less than 1100, the positive threshold value, and greater than 350, the <br />negative threshold value. However, in years 2003-05 and 2008-10, population estimates are 1300 and <br />1400, respectively. Since these values exceed 1100, both estimates express a positive response. When <br />the population estimate increases above 1100, a new population baseline is established at the higher <br />population level. <br /> <br />In the negative example (fable 3, Figure 3), the initial population estimate (400) indicates a neutral <br />response, because 400 is greater than 350 and less than 1100. But population estimates for years <br />2003-05 (220) and 2008-10 (150), indicate a negative response, because both estimates are less than <br />350, the negative threshold value. <br /> <br />In the neutral example (fable 4, Figure 4), population estimates are greater than 350 and less than <br />1100 for all years. Therefore, future population responses are neither positive nor negative. <br /> <br />4 <br />