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14 I I I I I I I I I I I I I I I I I I I population estimates and 95% confidence intervals in Table A-1, the Green River population (including the Yampa and White estimates) exceed the requirements just described. The Colorado River population must increase by 600-700 fish. Expansion of the Colorado River population into the additional 100 river miles within the Rifle-Debeque reach and the Gunnison River via passage and translocation is therefore necessary and warranted. The delisting criterion for identifying and measuring trend in population abundance is straightforward and simplistic. The requirement calls for a stable or increasing trend in population estimates based on the slope of the regression line described by the point estimates being greater than or equal to zero. Statistical significance is measured at a probability level of 0.10. This requirement permits fluctuation in population level and even short-term decreases. A decreasing slope that becomes statistically significant at p=0.10 would be unacceptable. This regression of population estimates over time should include a minimum of seven estimates spanning a 12-year period for the delisting criterion. The 12-year trend period for delisting was used because it establishes a stability criterion over one generation time for the species. A generation time of 12 years was estimated from Seber (1982) based on 5 years to maturity and a 0.86 survival rate (Generation time = 5 + [1/(1-0.86)]. This trend requirement applies to population segments in rivers where standardized monitoring and population estimation have occurred previously. Indication of a progressive year-to-year decline in the seven estimates over the 12-year period, even in the absence of statistical significance would also be considered a failure to meet the trend criterion. The criteria for annual recruitment and population structure are also straightforward and simplistic. Each is based from the length frequency distribution data obtained for late-juvenile and adult fish during standardized monitoring and population estimation primarily, but may include all sampling efforts resulting in the capture of late-juvenile and adult Colorado pikeminnow. Each requirement for each population parameter is specified by a range that is reflective of the existing data and variability within each population segment. Using a five- year rolling average as the estimation period, the percent range specified provides some flexibility for annual variation but strives for persistence and stability in each. populations' reproductive success, growth, recruitment to adult stage, and survival. The White and San Juan rivers have the same requirements for recruitment and population structure while the requirements for Green, Colorado, and Yampa rivers are slightly different to account for observed characteristics from existing data. Delisting requirements differ from downlisting requirements for these parameters. For delisting, recruitment percentage is required to equal or exceed 10% for the Yampa, White and San Juan rivers, and equal or exceed 15% for the Green and Colorado rivers four times within the 12-year monitoring period. The purpose behind this requirement is to achieve a state of reproductive vigor and environmental conditions wherein strong year classes are produced periodically to sustain population growth or persistence. The production of these strong year classes are evident from existing length frequency distribution and recruitment