9588

• UCREFRP Catalog Number: 9588
• Author: Bestgen, K. R. and e. al.
• Title: Population Status of Colorado Pikeminnow in the Green River Basin, Utah and Colorado.
• USFW Year: 2005.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

reduced the possibility of movement to the small areas that were not efficiently sampled. Static
population size was also ensured in the period encompassed by the within-year sampling
occasions, because of the large size of the study area. The only location of
emigration/immigration for fish was from the distant downstream Colorado River. The
likelihood of substantial movement from there is much reduced at that time of year because fish
tend to occupy small and stable home ranges (Tyus 1990; Irving and Modde 2000). This notion
was further supported by the low number of Colorado pikeminnow moving (Vl s) between river
reaches within a sampling season (unpublished data). The short period encompassed by the
sampling occasions also ensured that mortality and recruitment were minimized.
The assumption of homogeneity of capture probabilities of individuals is unlikely to be
fulfilled completely except in all but the most restricted conditions. However, we minimized
heterogeneity due to fish size effects by adding the covariate TL to analyses where appropriate,
which adjusted capture probabilities over the range of fish lengths encountered. This was
important because sampling crews noted that large fish were sometimes more difficult to capture
than small ones. We investigated the likelihood of a population composed of groups of
individuals with inherently different capture probabilities but found no support for such based on
preliminary analyses using mixture models (Pledger 2000). We also demonstrated that initial
capture probabilities were equal to recapture probabilities among the short-term sampling
occasions (i.e., pk = ck) by including an additive parameter to the top model (additive model is #4
in the set). The additive model suggested that recapture probabilities were slightly (usually about
0.001) but not importantly higher that initial capture probabilities. Minimal differences in initial
capture rates and recapture rates suggested that confounding due to factors such as fish avoidance
of sampling boats did not introduce heterogeneity into capture probabilities, sop's and c's were
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