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

Colorado River abundance, survival, and capture probabilities.—During 2005, sampling
resulted in capture of 426 stocked razorback suckers in the Colorado River, of which 145 (34%)
were adults (> 400 mm TL); a total of 12 adult individuals were recaptured among the five
sampling passes. Based on adult capture data, model M t + length (a model that allows capture
probabilities to vary with sampling pass and with fish length) produced an abundance estimate of
1,066 fish, although precision was low (95% confidence limits = 377–3,703) and likely a result
of relatively low probabilities of capture of 0.018 to 0.057 for fish that averaged 437 mm TL
(Osmundson and Seal 2009). Using the same model, an abundance estimate of 2,137 was
derived for all sizes of razorback suckers present (SE = 348, CV = 16%; 95% confidence interval
1,576–2,958).
Zelasko et al. (2009; 2010; 2011) found similarly low capture probabilities for razorback
suckers in both the Green and Colorado River systems, with highest capture rates coming from
the largest, most easily captured fish. These data demonstrate that Colorado pikeminnow
sampling can produce sufficient captures of fish to estimate population abundance but estimates
are imprecise and of limited value. Additional data from multiple years of sampling, and
additional captures from other sampling programs will likely increase precision of those
estimates because data can be borrowed across years to increase efficiency of the estimators.
The point here is that abundance and survival estimates can be produced but managers will need
to decide how much increased recapture effort is needed to satisfy the need for increased
precision and potentially reduced bias of the estimates.
Simulations to guide sampling effort and group stocking size
Simulation methods. —To assist with determining the levels of sampling needed to
increase precision and reduce bias of estimators, we simulated various sampling program data in
program MARK (White and Burnham 1999). Recall that the overall goal of the monitoring
sampling is to obtain relatively accurate and unbiased estimates of abundance, survival, or other
parameters of interest, and to estimate those parameters with a relatively high level of precision.
Those goals can be accomplished mainly by increasing recapture rates of marked fish, and the
simulations can be used to predict effects of variations in sampling programs on accuracy and
precision of estimates.
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