K. R. Bestgen, K. A. Zelasko, and G. C. White. Monitoring ...

and absence of any fish stocked in 2007. Mixed patterns were also evident in the middle Green
River, as abundance in 2006 and stocking numbers in 2005 were lowest, and abundance
increased in 2007 consistent with higher stocking numbers in 2006. However, abundance
declined in 2008 by about 65%, even though stocking numbers were highest in 2007.
The large swings in abundance of razorback suckers among years would not be expected
in a stable population of wild fish. Even relatively small populations of wild razorback suckers
in the middle Green River in the 1980’s and early 1990’s did not experience dramatic population
swings, likely because of lack of recruitment and because the population was influenced mainly
by mortality of aging fish. One explanation for large population swings is the low survival rates
of fish just post-stocking, as amply demonstrated by Zelasko et al. (2010; 2011). Survival rates
must have been very low in the 2007–2008 interval for population abundances to decline so
dramatically in each reach. That relatively low flow year in 2007 may have had an adverse
influence on survival rates; Bestgen et al. (2010) did not estimate annual survival rates for
Colorado pikeminnow in the 2006–2008 period so no comparisons are possible. Another likely
explanation for apparently large swings in abundance among years is that high abundances in
2007 may have been biased by very low recapture rates; capture rates biased low would have the
effect of increasing population abundance because numbers of fish captured divided by an
unrepresentative and low p would increase apparent abundance. It is important to remember that
the abundance estimates presented here are for all life stages present; razorback sucker adults (>
400 mm TL) represented only 37% of fish captured in the 2006–2008 period so adult abundance
is substantially lower than the estimates portray.
Colorado River data, fish distribution, and macrohabitat use. —The 2005 sampling for
Colorado pikeminnow abundance estimation in the Colorado River also resulted in capture of
relatively large numbers of razorback suckers (Osmundson and Seal 2009). Distribution of
larger juvenile and adult razorback suckers in 2005 in the Colorado River was more concentrated
upstream, where about 60% of those fish were from the Palisade-to-Westwater reach
(Osmundson and Seal 2009, Figure 10). About 40% of razorback sucker captures were from the
more downstream reach Cottonwood Wash to the Green River confluence but < 1% of razorback
suckers were captured in the lowermost 46 miles from Potash to the Green River confluence.
Low abundance of razorback suckers was also noted in Westwater Canyon (e.g., RM 125–120,
in part) and in the most upstream 5-mile reach just below Price-Stubb diversion.
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