9665

• UCREFRP Catalog Number: 9665
• Author: Schmidt, J. C. and J. B. Box.
• Title: Controls on the Longitudinal Distribution of Age-0 Colorado Pikeminnow in the Middle Green River, Colorado and Utah - Draft Report.
• USFW Year: 2001.
• USFW - Doc Type: Logan, UT.
• Copyright Material: NO

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295265 in 1991, reach 265250 in 1991 anA'992, an~ reach 240230 in 1991. ~us, CPE was
larger in 1991 than in 1990, despite the fact that the drift was twice as large in 1990 and the
~ hydrology was nearly the same')
).. ~ . Reaches are designated by distance of the reach boundaries upstream from the confluence
~ __ \ of the Green and Colorado Rivers. These locations are in river miles, because this river location
to ~ I system h~ long.been in use in the region. Thus, reach 310300 is 16.7 kIn long and is located
. J . i hetw_een nver mIles 310 and 300.
'4 \ ~ Other sampling provides conllicting perspectives on the relative size of larval populations
. between 1990 and 1991. JuvenIle plkemmnow samplIng mdlcate that recrUitment from the 1990
larval population was greater than from the 1991 population, because 41 juvenile fish were collected
in fall 1991 and none in fall 1992 (McCada, unpubl. data). However, Valdez and Cowdell (1999)
sampled 281 age-O pikeminnow in fall 1990 and 526 in fall 1991 in reach 265250, indicating larger
arval populations in 1991. >f ,.~ "}
The standard error of the mean CPE is typically large, because of the large variability in the
number of pikeminnows collected in each sampled backwater, the high proportion of seine hauls
with no fish, and the few number of backwaters sampled. Differences in the mean CPE of each
reach beyond the standard error only occurred in 1992 and 1993, when reach 265250 had higher
CPE than elsewhere. In other years, the reach standard errors were too large to identify distinctive
characteristics in the longitudinal distribution of larval pikeminnows. In these circumstances where
a small proportion of the population is sampled, a simulation model offers an attractive alternative in
the evaluation of population distributions and the factors that control that distribution.
Simulation Model of Drift and Transport into Backwaters

11/14/01 draft report, Schmidt and Box

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Overview
We developed a simulation model of larval drift and transport into backwaters in order to
predict the distribution of age-O fish. The simulation model integrates hydrologic, geomorphic, and
biologic factors and was written using Stella Research Software (High Performance Systems,
1996). The model is structured such that a cohort of larval fish enters the Green River each day
from the Yampa River, and these fish drift downstream through 10 reaches that are between 8 and

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~" 50 kIn long (Appendix A). Reaches were defined on the basis of geomorphic characteristics of the
~ ,r"\ channel, alluvial valley, and the location of tributaries. Since the YampaRiveris the only source of
. f ,r\) larval fish into the study area, we compared field measurements with model predictions.
J ;J' L j~. The predicted time it takes larvae to drift through each reach is determined by reach length,
) r:! I- flow velocity, and whether larvae remain in the main cunen! or are transported into backwaters. We
assumed that larvae drift at the mean velocity of the stream. We recognize that this assumption may
"f/ . \:" over-predict the rate of movement of larvae through reaches, but more accurate predictions of the
V.i<~te of drift necessitate field studies. .

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