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

on capture rates of untagged razorback suckers, which is an important part of recovery goals that
require recruitment at levels that offset mortality rates (USFWS 2002). However, the restrictive
assumption of that approach is that all untagged fish are wild-produced, which we know is likely
not true, because of stocking of “excess” fish in various places, as well as tag loss, tag failure,
improper tag scanning and other issues that result in a non-detection of a once-tagged fish.
Because recapture rates are already relatively low, careful tag detection and data recording
becomes even more vital to effective parameter estimation. Zelasko et al. (2010) summed up
this issue regarding their use of tagged fish recaptures as follows:
“Of the 4,010 total razorback sucker capture events we examined, at least
275 records (6.9%) had PIT tag errors which made them unusable.
Another 328 capture records (8.2%) did not have associated stocking
data, which could be the result of captures of wild untagged fish, captures
of hatchery-reared untagged fish, loss of tags, failure of equipment to
detect tags, or failure to scan fish prior to stocking. Accurate tagging, tag
detection, and data recording are minimal requirements to understand
provenance of captured fish (hatchery or wild) and recruitment rates of
razorback suckers.”
Some of those errors or problems can be accommodated in an analysis informed by things like
tag loss rates, but presence of large numbers of untagged fish as juveniles, when large numbers
of unmarked early life stages are released one to several years prior is problematic and not likely
treatable in an analysis. Those factors will frustrate attempts to estimate recruitment rates of
wild fish until they are minimized to a much greater level.
Another approach that would aid identifying actual wild-produced juveniles from
untagged or otherwise unrecognizable hatchery fish would be to employ trace element analyses
of structures such as scales. Investigators have used this technique to differentiate wild from
stocked fish of other species and preliminary analyses are underway using scales from San Juan
River razorback suckers (Flem et al. 2005, S. Platania, pers. comm.). Preliminary results show
clear differences between element concentrations in scales consistent with when fish were in the
hatchery and were then subsequently captured in the river after a reasonably long time interval.
Verification of the number and, potentially, rates of wild fish recruitment would be very useful
and presumably require little additional work other than the analysis of the scales themselves,
since non-regenerated scale samples could be collected when untagged fish were noted in other
ongoing sampling programs.
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