9591

• UCREFRP Catalog Number: 9591
• Author: Nesler, T. P.
• Title: Interactions Between Endangered Fishes and Introduced Gamefishs in the Colorado River, Colorado, 1986-1991.
• USFW Year: 2002.
• USFW - Doc Type: 91-29,
• Copyright Material: NO

relationship of native fish species and nonnative cyprinids, respectively, with high flows, rather
than species interactions,

One of the Recovery Program concerns of the ISMP seine sampling design has been its
ability to accurately represent abundance of rare fish species inhabiting backwaters. Two
investigations conducted subsequent to this study employed more extensive depletion sampling and
multiple gear types. These results are also available for comparison with relative abundance
results for this investigation. Backwater depletion sampling with seine, fyke net, and
electrofishing was conducted to evaluate ISMP as a tool specifically for monitoring changes in
centrarchid species abundance, and to evaluate the removal of nonnative fish species from
backwater habitats (Bundy and Bestgen 2001). These authors concluded that ISMP seine sampling
underestimated the number of backwaters occupied by largemouth bass and green sunfish by about
50 % and underestimated the density of these species when present 68 % of the time. Bestgen and
Bundy (2001) noted ISMP abundance estimates for red shiner, fathead minnow and sand shiner
were also underestimated by 21-34%, Using Bestgen and Bundy (2001) as a more accurate picture
of relative abundance of these fish species in Colorado River backwaters from Palisade to Loma
(RM 185.4-152,0), young and juvenile nonnative gamefish were 5.4-5.6 percent of the total
sample in 1997 and 1998 and nonnative cyprinids ranged from 85-88 % , In both of those years,
green sunfish was the most abundant gamefish sampled.

Seine removal of nonnative fish from backwaters in the Colorado River from Palisade to
Loma in 1999 through 2001 indicated nonnative fish species comprise 82-96 % of the fish sampled
(Trammell et al. 2002). The three nonnative cyprinid species made up 94-99 % of the nonnative
fish population, and gamefish species made up 0.3-1.4%. Green sunfish and black bullhead were
the most abundant gamefish species.

The relative abundance of nonnative gamefish species and centrarchids in particular may
have peaked in 1997 at 11 % (McAda et al. 1998) or 5.4% (Bestgen and Bundy 2001). Regardless
of the sampling approach employed, relative abundance estimates for gamefish young-juveniles
range from < 1 to 6% of the backwater fish community, and nonnative cyprinids range from 85-
99 % , Green sunfish and black bullhead were usually the most common gamefish species
collected.

While percentages of nonnative gamefish and cyprinids remain similar across time and
studies, the total number of centrarchids collected per year varied considerably. In this
investigation from 1986-1991, 104 bass and 354 green sunfish were captured in seine hauls in 95
backwater sites; and 15 bass and 52 green sunfish were captured in block-and-shock sampling of 44
backwater sites (day or night sampling). Bestgen and Bundy (2001) and nighttime sampling of
backwaters demonstrated that the results of this investigation probably under-represented centrarchid
abundance. From 1997-1998, Bestgen and Bundy (2001) collected 321 tol,366 largemouth bass and
1,522 to 2,176 green sunfish in 45 backwater sites providing 15,978-16,229 d of available habitat.
From 1999-2001, Trammell et al. (2002) used backwater seining only in the Colorado River from

30

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I