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

11/14/01 draft report, Schmidt and Box

Estimates of the total number of larvae are based on drift samples collected in nets at the
mouth of the Yampa River by Bestgen et aI. (1998). Bestgen et al (1998) estimated the transport
abundance of drifting larvae in three daily drift net samples, adjusted to an hourly rate, by dividing
the number of larvae captured by the duration of the sample and the estimated proportion of the
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total discharge sampled. We estimated the total daily number of drifting larvae by multiply1ing
Bestgen et al. (1998)' s hourly estimated rate by 24. In some cases, hourly drift rates varied greatly.
Bestgen et al (1998) noted that there were 3 peaks of larval drift in 1990, and the peak drift rate on
these days was between 5,000 and 10,000 fish per hour. These rates result in estimates of between
120,000 and 240,000 fish entering the Green River on these three days.
Analysis of Field Data Concerning Longitudinal Distribution of Age-O Fish between 1990
and 1995
Although larvae are sampled as they enter the Green River in early summer, sampling of the
longitudinal distribution of larvae in backwaters is not undertaken until mid-September. We
analyzed available data (McAda et al. 1997) on the abundance ofIarvae in backwaters as measured
in September. The sampling protocol is to seine two backwaters per 8-km of the Green River, and
the data consist of the number of larval fish collected in each seine haul in each backwater and the

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total area seined (McAda 1993, Valdez and CowdellI999). Sampling was conducted in 7 reaches
of our study area and did not occur in the upstream 3 reaches. We calculated catch-per-unit effort
~ (CPE) for each backwater within a reach as the total number of larval pikeminnow collected in the
! J{' seine hauls divided by the seined area (McAda 1993, Valdez and Cowdelll999). The mean and
'J standard error CPE of each reach were computed. We chose mean CPE as an index of abundance,
~ because this index is routinely used by researchers in the upper Colorado River basin (McAda et al.
1997). We used the arithmetic mean rather than geometric mean CPE reported elsewhere (U. S.
Fish and Wildlife Service 1987), because the geometric mean reduces variance, and we were
interested in examinin,g-the variation associated with individual, backwater samples.
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The number b~~ fish that occur in backwaters in September is very small, and the
t\.;, number actually caught is even smaller. Of the 472 seine hauls in backwaters during the 6 years of
co~cem to this study, approximately 58 percent contained no fish. About 150 river km were
sampled eachyear; and 33 percent of the 8-km reaches in the 6 years of sampling contained no fish.
Of the 3.5 million fish (Table2) estimated to have drifted through the study area between 1990 and
1995, 1,201 were caught in seine hauls. Population estimates developed by McAda (1993) for the
number of larval fish in backwaters in September 1990 and 1991 are less than 1 percent of the total
number of larval fish estimated to have drifted downstream in those years.
The CPE of larval pikeminnow in backwaters in September varied from year to year, and
were greatest in 1991 and least in 1994 (Fig. 5). The largest CPE in any reach in any year was in
reach 265250 in 1993. Other large CPE's occurred in reach 300295 in 1990 and 1991, reach

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