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Appendix C <br />Evaluation of the link between adult population and larval drift abundance <br />We made an attempt to link the larval drift catch with Colorado pikeminnow fecundity, sex <br />ratios and population size in the Colorado River. Osmundson and Burnham (1996) estimated the <br />adult population of Colorado pikeminnow in the Colorado River between Westwater (RM 125) <br />and RM 185 (upstream barrier) as 253 fish, or 4.1/mile. Extrapolated out to the lower 112 miles <br />below Westwater, this gives a population of 712 adults, although this is conservative estimate, <br />since ISMP catch rates of adult pikeminnow are consistently higher in the Colorado River below <br />Westwater than above. The IMO (Interim Management Objectives) (Lentsch et al. 1998) gives the <br />sex ratio as 6:1, male to female, or perhaps 100 females, and the average eggs per female as <br />10,000. The average number of eggs from a large (2.7 kg) female in a hatchery is 122885, or <br />44372 eggs/kg (Shazon Coates, DNFH, personal communication). However, the average weight <br />of pikeminnow collected during ISMP from 1992-1996 was only 1.1 kg, for an average number of <br />50000 eggs per fish which is the figure we used for this exercise. For the purposes of this <br />discussion we have assumed a 50% survival rate for the first 18 days post spawn, which is the <br />average numbers of days between estimated spawn and drift capture date. We also assumed not all <br />females spawn each year, and assigned a value of 75% participation, which is higher than-that used <br />by Lentsch et al. (1998). Based on these assumptions, we can estimate the total number of larvae <br />that were produced in one year. Multiplying this number by the percentage of the total river <br />volume sampled, we can estimate how many larvae we should have collected. Surprisingly, <br />considering the many poorly substantiated assumptions, the number of larvae estimated to be <br />collected is close to that actually collected (Table C1). <br />Extrapolating in the opposite direction, beginning with the actual larval catch and percent <br />of river sampled, we can look at the number of females and total adults contributing to the larval <br />production, based on an increasing skewed sex ratio (Table C2). In high production years, the <br />estimated number of adults at sex ratios greater than 3:1 is much higher than the 700 estimated <br />by Osmundson (1986). This indicates that one or more of the six assumptions is in error. Either the <br />population estimate is in error, sex ratios are not as skewed, larvae aze not randomly distributed, <br />and we catch a disproportionally high percentage of the drifting larvae, or female fecundity, egg <br />survival, and/or participation may be greater. Obviously, there is a need for further research on all <br />aspects of this question. <br />51 <br />