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however, UDWR reported 1,315 adults in 2012. The core population estimates in 2011 and 2012 <br /> were 1846 and 1718,respectively(Figure 9). Population estimates in both Black Rocks and <br /> Westwater canyons declined dramatically during the first population estimation rotation in the <br /> late 1990s, but have remained relatively stable since that time. Colorado State University's <br /> recent robust population estimate analysis more clearly indicated that declines in the Westwater <br /> and Black Rock humpback chub populations are due to lapses in recruitment(i.e. adult survival <br /> rates have remained stable). Principle investigators agree that reinitiating an age-0 monitoring <br /> component is advisable. It should be noted that whatever is affecting humpback chub <br /> recruitment has not affected sympatric populations of native roundtail chub; roundtail chubs <br /> populations in both canyons have remained stable or have increased since population estimation <br /> started. In addition to the potential and recent negative interactions between humpback chub and <br /> nonnative predators discussed above,both the Westwater and Black Rocks populations are at <br /> risk of potential chemical contamination due to the proximity of a railroad located on the right <br /> bank of the Colorado River which at times transports toxic substances. <br /> Black Rocks& Westwater Canyons "Core Population" Estimates <br /> 9000 <br /> 4; 8000 <br /> E <br /> 2 7000 <br /> 41 <br /> V, <br /> r- 6000 <br /> 0 <br /> m 5000 <br /> C 4000 <br /> a <br /> 3000 <br /> c <br /> 2000 <br /> coy 1000 <br /> 0 T� <br /> 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 <br /> Figure 9. Combined population estimates for humpback chub in Black Rocks and Westwater <br /> Canyon based on a robust open model created by Dr.'s Bestgen and White,Colorado State <br /> University. The 2002 Recovery Goal downlist criteria for these combined("core population") <br /> estimates is 2,100 adults. <br /> The Cataract Canyon humpback chub population is small,with estimates of about 150 wild <br /> adults in 2003 and 66 in 2005 (Badame 2008). Estimates are difficult to obtain in Cataract; <br /> therefore, catch-per-unit-effort(CPUE)has been determined to be an effective replacement <br /> (began in 2008 on a 2-years-on, 2-years-off sampling regime). In 2011, UDWR reported that the <br /> Cataract population appears to be stable with CPUE ranging between 0.010 and 0.035 <br /> fish/net-hour. Despite additional effort to sample below Big Drop Rapid, no additional <br /> humpback chub were encountered in the new riverine habitat created by low Lake Powell levels. <br /> 27 <br />