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

during the same sampling pass, 933 fish were recaptured only once, while 71 unique fish were
recaptured 146 times (67 of which were recaptured twice and 4 were recaptured three times). Of
those 1,004 individuals, 118 (11.8%) were ≥ 400 mm TL at stocking (an adult fish, USFWS
2002), but 374 (37.2%) were ≥ 400 mm TL upon recapture, indicating growth of some
individuals between time of stocking and last recapture but relatively few were stocked as adults.
Maximum change in length over the 2006–2008 period was 74 mm TL; that 350 mm TL
individual was captured on the first sampling occasion in 2006 and was 424 mm TL when
recaptured on the last sampling occasion in 2008.
Change in length data were somewhat suspect as 269 of 1,004 recaptured fish (27%)
were apparently shorter than when released by an average of 9.6 mm TL (1 to 60 mm); that total
and mean value did not include two fish where a recording error was assumed (e.g., 440 at
release, 342 at recapture). In addition, 17 of the 1,004 recaptured fish (1.7%) did not have a
length recorded at the time of stocking. Given the value of length and change in length data to
inform size-dependent changes in capture or survival rates, to estimate growth rates between
recapture intervals, or to estimate condition indices, and the considerable expense associated
with culture of fish in hatcheries and recapture of stocked fish in the wild, we encourage
additional precautions to obtain the most accurate length data possible. This might be in the
form of more training, consistency checks, quality control in data recording, or other measures.
Below we use the 2006–2008 razorback sucker capture data to estimate recapture and
survival rates, and abundance. We do this mainly to illuminate expectations for the utility of the
razorback sucker monitoring data that will be gathered in future Colorado pikeminnow sampling
efforts; the reader should exercise caution in interpreting the estimates provided because they are
unreliable in most cases and instead focus mainly on the message that the type of data gathered
in those three years, in and of itself, may be insufficient to adequately estimate demographic
parameters of interest for razorback suckers in the Green River Basin, and perhaps the Colorado
River, Colorado and Utah, as well. It is recognized that additional 3-year-segments of data (e.g.,
2006–2008 and 2011–2013) will allow for more precise estimates of survival, because data are
then potentially borrowed across all years to estimate probabilities of capture and survival.
However, estimates of abundance and more importantly, their potential for bias and low
precision, are not likely to improve without addition of sampling occasions, or more recaptures
within occasions.
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