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. . 4 biological opinion: decreases or no change in populations may indicate that additional depletions are not justifiable. Examples Colorado pikeminnow from the Upper Colorado River were used in the examples described below because data are readily available to establish reliable baselines for each of the three indices: adult population size: adult length- frequency: and young-of-the-year. Similar assumptions could be made for the humpback chub, as well. when sufficient data are available. Individual criteria or threshold values were selected for each of the indices by which the pikeminnow population response would be measured: however. these could be modified, if new data suggest more appropriate value(s), . We did not evaluate nor recommend a formula by which these three indices miqht be applied in determi th need to ei onsult n, That is. whet tree indir.es would be weiqhted diffprpntlv or equally. or if one. two, or all three indices would hove to demonstrate a neqative response before reinitiation would be required. o{{vr ~ 1;1- Adult Population Size Catch per unit effort (CPE) data were collected for adult pikeminnow every year from 1986 through 1997. Standard Error (SE) was computed for each of these years, as well (Table 1). Figure 1 presents these data graphically. where mean CPE values are represented by diamonds and SE by solid vertical 1- bars_ In addition. for the years 1993-95. the adult pikeminnow population was estimated at 600 (open squares) with a 95 percent confidence interval (CI) of !250 (dashed vertical lines). The basis for determining the nature of the population response (positive. negative or neutral) is the relationship between the baseline population estimate, !CI. and future population estimates, !CI. That is. if the future Pop,!CI overlaps the baseline Pop.!Cl. a neutral response would be indicated. If future Pop,!CI lies entirely above the baseline. a positive response would be indicated. and if it lies entirely below the baseline. a negative result would be indicated (Figure 1). For example, the baseline Pop.!CI is 600!250, so a population response would be positive if the future value of Pop.-CI > 850 (600+250). while the response would be negative if the future value of Pop.+CI < 350 (600-250). Future values of Pop.+CI ~ 350 and Pop.-CI ~ 850 represent a neutral response.