9528 ‎(2)‎

• UCREFRP Catalog Number: 9528
• Author: Martinez, P. and N. P. Nibbelink.
• Title: Colorado Nonnative Fish Stocking Regulation Evaluation.
• USFW Year: 2004.
• USFW - Doc Type: Grand Junction, Laramie.
• Copyright Material: NO

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"effective distance" is minimized. A. Martinez (CDOV~ has a strong database on many ponds,
and some evaluation on the effectiveness of treatments in controlling nonnative fish in ponds.
4. Nonnative fish densities in backwaters, pre- and post-regulation periods
' Figure 23 shows the sampling locations for three intensive nonnative fish sampling or
removal efforts in backwaters of the ISA of the Colorado River from 1997 to 2001. Note that the
only dataset that contains information pre-regulation is the Bundy and Bestgen (2001) dataset,
which has less spatial coverage than the more recent data sets of Trammell et al. (2002) and
Osmundson (2003). This situation posed potential problems in examining trends over the years,
but nonetheless, visual comparisons can be made in reaches of the river that have been resampled
' in later years.
Table 14 shows the percent composition for several nonnative fish species in backwater
collections of Bundy and Bestgen (2001), Trammell et al. (2002), and Osmundson (2003) within
the ISA. Fathead minnow was clearly the most abundant nonnative present in backwaters at
more than 50% over all sampling efforts. Fathead minnows were not caught during the sampling
1 efforts of Osmundson (2003) due to the fact that it was an electrofishing effort which favored the
capture of medium- and large-sized species. Green sunfish, largemouth bass, black bullhead and
white sucker were the next most prevalent species, respectively, based on the samples from these
' studies collected by various methods.
Relative densities of five nonnative fish species of interest (largemouth bass, green
'' sunfish, fathead minnow, bluegill and black crappie) were based on the first-pass sampling
efforts of Bundy and Bestgen (2001) and Trammell (2042) and are mapped in Figures 24a-e for
1997-2001, and for the pre- (1997-1998) and post-regulation time periods (1999-2001). $ach
sampling approach varied in intensity, resulting in large variation in the number of individuals
caught for any particular species. In general, these figures indicate no obvious change in the
densities of each species over the years, but they do indicate that the highest densities are shifting
locations from year to year. This could have been a direct result of removal efforts which may
have dampened populations in the subsequent year in particular locations, but these efforts
appeared to have had little effect on the overall populations of nonnative fishes in the river.
' Further, there appears to have been little change in the composition offish species with respect
to native vs. nonnative proportions from 1997-2001 (Figure 25).
Proximity analyses
1. Comparison of stocking events vs. high backwater densities ("high" densities are locations
that fall in the top 10% of density for the particular species being considered)
Proximity analyses in the ISA, although expected to potentially reveal patterns not visible
to the naked eye, indicated a similar lack of pattern as did the visual analysis. Figure 26 shows
that the proximity of high largemouth bass density in backwaters to largemouth bass stockmg
locations was not different from a comparison of random backwater locations with low/absent
abundance of largemouth bass to the same stocking locations. Fathead minnow also showed no
pattern, and as previously mentioned, green sunfish were not stocked.
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