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I I I I I I I I I I I I I I I I I I I Colorado River Colorado pikeminnow Population Estimates Doug Osmundson, Ken Burnham, and Ron Ryel SUMMARY Abundance of the Colorado River population of Colorado pikeminnow Ptychocheilus lucius, a federally listed endangered species, was monitored from 1991 to 2000. Two multi-year data collection efforts were made: the first, from 1991 to 1994, and the second, from 1998 to 2000. The study area included occupied habitat within the mainstem Colorado River from its confluence with the Green River upstream to the Price Stubb Diversion Dam at Palisade, Colorado, excluding Westwater Canyon. The upper reach, from Westwater Wash to Palisade, was sampled three times annually; the lower reach, from the Green River confluence to Cisco, Utah, was sampled twice annually. A combination of trammel-netting backwaters and electro fishing shorelines was used to capture as many Colorado pikeminnow as possible during each pass. Fish were measured and marked with uniquely coded PIT tags. Capture-recapture data were used to develop estimates of population abundance using closed-model methods (Program CAPTURE). Annual estimates of whole-river population size (all fish> 250 mm TL) averaged 613 during the early study period and 778 during the more recent study period. Annual estimates of adults (> 500 mm TL) averaged 372 during the early study period and 534 during the recent period, representing a 44% increase in the adult population. However, differences between the two periods were not statistically significant due to fairly wide confidence intervals. For the upper reach, estimated probability of capture (P) averaged 0.12. When annual point estimates (no measure of variance) were regressed against year, a significant increase in both total fish and adults was indicated. An experimental exercise in predicting change in adult abundance using population point estimates, growth rates, length frequency, and survival estimates was attempted with mixed results. The estimated number of individuals capable of growing to adult size by the following year was calculated and added to the current-year' population point estimate; the number of adults expected to die based on the average annual mortality rate was then subtracted. The resulting predictions of adult numbers in the following year was calculated for all years for which . necessary values were available. These were compared against point estimates for those years. There was good agreement between the values in some years but not in other years. Discrepancies suggested that either the method, or the point estimates against which the results were compared, is not reliable enough to forecast trends. Continuation of the current capture-recapture methodology is recommended for monitoring this population. The three-year-on, three-year-off sampling regimen should also be continued. To tighten confidence intervals around the abundance point estimates it is strongly recommended that sampling should be intensified by employing two concurrent sampling crews so that four passes annually can be accomplished in each reach during the eight weeks of spring nmoff. To detect trends in abundance over time, some measure of variance around the point estimates should be included in the analysis. However, if linear regression is used, itis recommended that regressing three-year running averages of point estimates be used rather than regressing point estimates directly; this method appears to be more sensitive in detecting trends.