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

eproduction is observed and when larvae are more abundant. Nevertheless, findings will be
useful to evaluate population status and response to management actions, including flow
management. We suggest additional sampling and research to better understand fish distribution
and entrainment into floodplain wetlands, capture efficiency, and genetic verification of
identified larvae.
Valuable capture-recapture data for large juvenile and adult razorback suckers in the
Green and Colorado River systems is available from existing sampling programs, particularly
that for Colorado pikeminnow Ptychocheilus lucius. That sampling occurs on a three-year on,
two-year off cycle and will form the foundation for monitoring larger-bodied razorback suckers.
However, that sampling by itself is insufficient to effectively monitor survival and population
abundance of large juvenile and adult razorback suckers and additional sampling and recaptures
of tagged fish are needed in those years to increase recapture rates. We analyzed habitat types of
captures made in spring in each system, and additional sampling should minimally focus on
slackwater channel margin habitat including flooded tributary mouths, backwaters, and eddies in
each system. Additional active sampling (e.g., electrofishing) as well as passive sampling (e.g.,
fyke/trap nets) is proposed. Monitoring of concentration areas, especially those near spawning
areas, via PIT tag detector arrays would be especially useful because those may maximize
captures when fish are concentrated and reduce potential handling effects and disturbance. A
last resort would be more active sampling directly over spawning areas, an option which should
be explored only if other techniques do not yield sufficient recapture information. Additional
sampling effort should overlap sampling for Colorado pikeminnow closely in time and space so
that it can be incorporated into abundance estimation data; data useful for estimation of survival
estimates can be collected over a broader time frame and is not as restrictive as that for
abundance estimates. Simulations were also conducted to guide the minimum levels of sampling
needed to raise probabilities of capture to 1) reduce bias of parameters derived from recapture
data, and 2) increase precision to levels that provide useful estimates. We also make
recommendations for frequency and type of data analyses to make best use of data gathered in
the future. Implementation of this monitoring program should increase the ability of managers to
make informed decisions regarding the status of razorback suckers in the Green and Colorado
River systems, which should assist in evaluation of conservation and recovery status of the
species.
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