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

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Palisade to Loma and collected 62 largemouth bass and 644 green sunfish. Also during 1999-2001,
Osmundson and Pfeifer (200 1) used electrofishing in the Colorado River from Palisade to Westwater
Canyon and collected 580, 1872, and 1,448 largemouth bass and 3078, 4062, and 3244 green
sunfISh per year, respectively. Bestgen and Bundy (2001) suggested largemouth bass abundance
increased over the 1993 to 1998 period to 0.5 fish/10m2 based on ISMP catch rates; green sunfish
abundance varied over the 1990s with significant peaks noted 1992 and 1998 at 1.5-2.5 fish/10m2,
and the three nonnative cyprinids decreased in abundance from a 1989 peak of over 140 fish/10m2
for both red shiner and fathead minnow to lows in 1995-1999 of <30 fish/10m2. Despite this
decline, nonnative cyprinid abundance was still one to two orders of magnitude greater than that
for nonnative centrarchid gamefish.

Fall seine collections suggested considerable variability in relative abundance both within
and among juvenile centrarchid and ictalurid species present in backwater habitats. The majority
of each gamefish species sampled came from only 1-3 backwaters out of the 11-20 sampled per
year. For centrarchids, these results suggest reproductive success, if it occurs in the river, may
occur only in 10-15 % of the backwaters each year. For successful nesting and reproduction to
occur in the river, centrarchid spawning sites should be low-velocity habitats with water levels that
remain stable long enough, and water temperatures are wann enough for a satisfactory period of
time to permit incubation and hatching by these species. These conditions seem unlikely in the
upper mainstem Colorado River since fluctuating spring and summer flow conditions occur and
probably induce high annual mortality for young and juvenile centrarchids in backwater habitats
that can convert to flowing secondary channels with storm input, or become isolated and dried up
under decreasing flows. Some years may be more conducive to centrarchid reproductive success
than others in riverine habitats.

Reproductive success and survival of centrarchids in the Colorado River may be limited
due to unsuitable habitats for adults, and upon unstable quality and quantity of spawning and
nursery habitat in the river for young or juveniles. This inference is supported by backwater
results here showing relatively low catch rates of young and juvenile fish overall and length
frequency distributions showing relatively few adult fish present in the riverine habitats presumed
to best suited to these centrarchids. Habitat suitability for both green sunfish (Stuber et al. 1982a)
and largemouth bass (Stuber et al. 1982b) indicate the riverine environment of the Colorado River
is often suboptimal for both species. Optimal riverine habitat for green sunfish is characterized
by at least 50% pool area and 35-75 % instream cover. Temperature preference is around 28C and
adults avoid temperatures ranging outside 26-31C when possible. Only at the Stateline gauge in
1989 did mainchannel temperatures exceed 25C, and only for about 20 days. Though often found
in riverine environments, green sunfish prefer current velocities below 10 cm/sec. Both embryos
and fry of green sunfish prefer temperatures from 18-27C and current velocities below 8-15
cmlsec.

Largemouth bass do not prefer riverine environments unless the current velocity is slow,
water clarity is relatively high, and aquatic vegetation is present (Stuber et al. 1982b). Pool area
above 60 % and stream gradients below Im1km are preferred. Largemouth bass are considered

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