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

numbers during the reproductive season. This was particularly evident in the middle Green
River, when the few remaining wild razorback suckers present in the late 1990’s could typically
be found only during spring on or near shallow spawning areas or in or near off-channel habitats
(Bestgen 1990; Bestgen et al. 2002). Those seasonal patterns may indicate that fish in other
seasons are spread out, difficult to detect because of the habitat they use (e.g., water depths that
do not allow efficient electrofishing), or both. Perhaps for those reasons, adult life stage
razorback suckers have sometimes been described as difficult to sample. Support for relative
difficulty of sampling comes from relatively low probabilities of capture for 1 st year and
subsequent life stage fish (Zelasko 2008; Zelasko et al. 2009; 2010; 2011), which were generally
< 0.04 (median = 0.0385, 0.002–0.128). Razorback suckers in the San Juan River had higher
capture probabilities (Bestgen et al. 2009), which may be indicative of the prevailing shallow
habitat through much of that river, and depth-related differences in capture probabilities.
Life history parameter estimation based on tag-recapture data was useful to assess
survival of stocked razorback suckers, and to refine stocking goals and procedures. Those
studies found survival of fish recaptured in a relatively short time after stocking (9–12 months)
was very low and was size-dependent, but after that survival was much higher and rates were
similar to that for wild fish (Bestgen et al. 2002; Zelasko et al. 2010). A main conclusion of
those analyses that were especially relevant for monitoring was that probabilities of capture (p’s)
were quite low, and much lower than was typically found for species like Colorado pikeminnow
or razorback suckers in different systems (Bestgen et al. 2007, Zelasko et al. 2009, Bestgen et al.
2009) even though data were derived from some of the same sampling programs. For example,
Colorado pikeminnow and many razorback sucker captures were from the same multi-pass,
multi-year (e.g., robust-design, Pollock 1982; Pollock et al. 1990) sampling programs in the
Green and Colorado rivers. Zelasko et al. (2010) identified low p’s as a concern and we excerpt
that here:
“Ultimately, increasing capture probability must become a priority if more
precise parameter estimation is desired. In mark-recapture studies, one aims to
capture the most individuals from a released cohort on the first occasion after
initial marking (stocking), which equates to high recapture probability.
Although this study improved on that aim compared to the previous analysis,
data were still collected from a variety of sampling programs where effort was
sometimes low after stocking substantial numbers of fish, and very few efforts
specifically targeted stocked razorback suckers. In contrast, species-specific,
Colorado pikeminnow abundance estimate sampling produced recapture
probabilities ranging from 0.01 to 0.20 in the Green River subbasin, 2000–2003
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