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

We assumed that the proportion of the pikeminnow population with the ability to swim was
directly proportional to the growth rate of the fish. The proportion of the population without
swimming ability was assumed to decrease linearly based on
p = 100 - [-36.35 + (4.45 * l)] (3)
where p equals the proportion of the cohort without swimming ability and I equals fish length, in
millimeters. This function was applied to all larvae that drifted on the same day. Length of larval
pikeminnow drifting in the Green River was estimated from a growth equation proposed by
Bestgen et al. (1997), assuming that water temperature was 18.2 degrees C in all years.
These growth rates predict that larval fish in Green River backwaters in late summer are
between 25 and 41 mm in length, and this estimate is somewhat less than that of Valdez and
Cowdell (1999) who estimated that larval pikeminnow in backwaters were between 21 and 80 mm
between 1990 and 1994. Thus, we underestimated the rate at which larval pikeminnow gain
swimming ability. The rate at which pikeminnow cohorts gain swimming ability is, in fact, related
to many phenomena in a complex way, and (3) is a simplification of a very complex process, For
example, Bestgen et al. (1997) point out that "a variety of factors besides temperature may affect
growth rates [of larval pikeminnow, including] longevity of backwaters [which leads to increased]
production of food for larval fish.." We varied the parameters of (3) in simulation runs and
determined that the model is generally insensitive to this function, within a reasonable range of
parameter values.
We assumed that the retention of larvae within backwaters was incomplete and that some
larvae reenter the main flow. The nature of this relationship is unknown, and we assumed that this
reentry rate, which we termed leakage, was computed by
L - a (NA)(IR)b (4)
where L is the total number of larvae leaking into the main channel during a time step, a is the total
number of larvae in the backwater, IR is an assumed proportional leakage rate, and NA and b are as
defined above. As larval pikeminnow grow and gain swimming ability, the leakage rate declines
because they may intentionally stay in backwaters (Paulin et al. 1989).
The model and sensitivity of its parameters
The model predicts thata day' scohort of larvae drift downstream and that the concentration
of larvae in the main flow longitudinally disperses because some larvae ate transported into
backwaters. Because natural processes result in the entry of larvae into the Green River at widely
varying rates during a two-week period, we illustrate the predictions of the model for a hypothetical
cohort of 100,000 larval fish entering the Green River on one day, and use the hydrology of 1992.
This simple, hypothetical scenario allows evaluation of the model' sassumptions and predictions.
The peak concentration of the longitudinally-dispersing cohort decreases downstream, and
the time it takes the cohort to pass a point increases (Fig. 8). The relatively few available

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