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<br />28 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I, <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />500 fish/mile is a plausible expectation for an average density in these locations. <br />Using 2,000 adult fish as a minimum abundance requirement and the aggregation <br />of these populations into complexes as proposed above, each one described in <br />Table 3 would have the physical habitat capacity to support a genetically viable <br />humpback chub population. Cataract Canyon, the smallest of the humpback chub <br />habitats with 12 miles of occupied habitat, would require 167 chubs/mile to meet <br />the abundance criterion. It is not known whether Yampa-Whirlpool-Split Mountain <br />canyons, Desolation-Gray canyons, and Cataract Canyon can support more than <br />the 500-1,500 humpback chub previously estimated for these locations even <br />though these equate to only 13-42 chub/river mile. Therefore, it is not feasible to <br />propose more than three of the five populations or population complexes are <br />necessary to meet the population abundance requirement for delisting. All five <br />populations are necessary for recovery, but the abundance criterion to satisfy the <br />hypothetical threshold for maintaining genetic diversity need be met only by 2-3 <br />populations for downlisting and delisting. <br /> <br />Considering the benefits of maintaining an effective population size above <br />1,000 adult fish, it is proposed that 1,250 adult fish be used as the lower boundary <br />of the 95% confidence intervals for those 2-3 populations meeting the 2,500 fish <br />abundance requirement. <br /> <br />The criterion for identifying and measuring trend in population abundance is <br />similar to that for Colorado pikeminnow. The time period defining the trend <br />analysis is eight years for delisting. An eight-year generation time was estimated <br />for humpback chub using a 4-year maturation time and 0.76 survival rate <br />(generation time = 4 + [1/( 1-0. 76)]. This regression of population estimates should <br />include a minimum of five estimates spanning the eight-year period for delisting. <br />This requirement applies to all five humpback chub population complexes. <br />Indication of a progressive year-to-year decline in the five estimates over the eight- <br />year period, even in the absence of statistical significance would also be <br />considered a failure to meet the trend criterion. <br /> <br />The criteria for annual recruitment and population structure are also similar <br />to those for Colorado pikeminnow, and are based on length frequency distribution <br />data obtained during standardized monitoring and population estimation (see <br />Table A-7). The requirement for each population parameter is specified by a range <br />that is reflective of the existing data and variability for each population. The length <br />frequency data for the Black Rocks only permitted the calculation of a three-year <br />average and only the Westwater Canyon data permitted the calculation of three- <br />year rolling averages specified above as a requirement (Table A-7). Recruitment <br />and population structure criteria are defined using different size categories for <br />Black Rocks, Westwater Canyon, and LC-Grand Canyon based on the different <br />characteristics of the length frequency distributions for each of these populations. <br />For downlisting and delisting requirements, the size defining recruitment for the <br />Yampa, Cataract, and Deso-Gray populations was set at 240 mm, similar to the <br />Slack Rocks population, due to lack of suitable data for these populations. <br />