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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11/14/01 draft report, SChmI~t and Box
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of seine hauls with zero fish IresUllS in large standard errors in the estimate of mean CPE in the
middle Green River. These ~mors are SuffIcIently large to mask 10ngItudmal dIfferences In
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distribution of larval fish. I~ some years, however, the largest CPE are in reaches characterized by
restricted meanders, such as 'reach 265250, where shoreline complexity is large. However, field data
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also indicate that highest C~E occur in other reaches in some years.
The simulation mod~l also predicts that a very small proportion of the larvae remain in
backwaters in fall, and that ~ost larvae are transported downstream from the our study area within a
few weeks of the onset of d~ft. The model predicts that abiotic factors control drift rate, transport
into backwaters, and leakag~ back into the main flow and that most larvae drift beyond the study
area to the lower Green Rivh or perhaps the Colorado River in southern Utah.
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The model is based Fn parameterizing physical processes that well-describe longitudinal
dispersion in streams, and t~e model correctly predicts the initial fast transport of larvae
downstream. Thus, field satnpling in late summer does not describe any the dynamics of the initial
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processes that lead to signi~~cant transport beyond the study area.
Model results do no~ predict the variation in field-based CPE data, although the importance
of restricted meandering re~hes as nursery habitat for larvae is illustrated in some years. Poor
prediction may be the result of inadequacies of the model, a poor field sampling program, or aspects
of both. I
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Operations of Aamfng Gorge Damhave the potential to directly affect larval fish
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populations, The model pr~icts that the total population of larval fish in the middle Green River is
very sensitive to the dischatge of the Green River at the time larval drift begins. The proportion of
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larval fish retained in the stjudy area is much less when drift occurs during relatively high discharge.
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Channel simplification, resulting from climate change diversions, and dams throughout the
Green River basin have th4potential to affect the longitudinal distributionoflarval fish populations
because simplification red~ces shoreline complexity. The impact of decreasing the longitudinal
heterogeneity of channel organization due to channel simplification has the potential to decrease
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peak populations of larval fish in most restricted meandering reaches and to decrease the late
summer and fall populatiops of larval fish in a reach near Ouray, Utah (reach 265250).
Development of the simulation model shows that many aspects of the physical and
biological processes of la&al drift and tranSport into backwaters are poorly known. Discrepancy
between interpretations of1lsparse field data and model predictions suggest that sampling programs
for larval fish in the middlf Green River should be reevaluated and that field studies of the
processes that determine t~e distribution of larval fish Should be initiated. The model provides a
template of evaluating the!effects of river managementon an endangered and endemic fish of the
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Colorado River system. I
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