9499

• UCREFRP Catalog Number: 9499
• Author: Osmundson, D. B.
• Title: Removal of Non-native Centrarchids from Upper Colorado River Backwaters, 1999-2001
• USFW Year: 2003.
• USFW - Doc Type: Summary of Results.
• Copyright Material: NO

hard-bottomed jon boat for transport up and down river to backwater sites. Two netters
walked ahead of the floating barge with a positive probe in one hand and a net in the
other, while a third person pushed the barge and netted fish that surfaced behind the front
two netters. Stunned fish were netted and placed in a tub for later enumeration. Non-
native minnows, identified while in the water, were not netted (non-native minnow
removal was conducted separately as part of a different Recovery Program project; see
Trammell et al. 2002). Captured fish were identified and measured for total length. In a
few cases, where an unusually large number of fish were encountered, complete counts
were made, but individual lengths were measured from sub-samples only. Native
species, incidentally captured, were released; all nonnatives were disposed of on site or,
in the case of large bass, eviscerated and placed on ice for later transferral to the
Colorado Division of Wildlife.
Effectiveness of removal efforts was evaluated by examining trends in
electrofishing capture rates during the course of the three-year effort, with the
assumption being that removal, if effective, would manifest itself in the form of declining
capture rates of centrarchids. Such a result would be expected if centrarchids were
removed faster than they could replace themselves. Effort per backwater was not
quantified in a way that would allow statistical comparisons of catch rate among
sampling seasons. However, for individual backwaters that were sampled on several
occasions, barge-shocking technique was consistent enough that effort could be
considered roughly similar from one seasonal effort to the next. Also, average number of
fish captured per backwater was used as a rough index of reach-wide relative abundance
to allow among-season comparisons. Use of this index decreases variability in total
catch associated with unequal numbers of backwaters electrofished per season.
In addition to examining catch rates associated with this effort, catch rates of
centrarchids from two other studies were also examined for comparison. These other
studies employed seining as a means to capture fish: the Interagency Standardized
Monitoring Program (ISMP) annual fall YOY surveys from 1986 to 2001, and a non-
native minnow removal effort conducted by Trammell et al. (2002) in the Grand Valley
during 1999-2001. Also, because the goal of the project was to increase survival of
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