9406

• UCREFRP Catalog Number: 9406
• Author: Osmundson, D. B.
• Title: Importance of the '15-Mile Reach' to Colorado River Populations of Endangered Colorado Pikeminnow and Razorback Sucker.
• USFW Year: 2000.
• USFW - Doc Type: Grand Junction.
• Copyright Material: NO

flooded off-channel habitats where they
concentrated during spring (Kidd 1977,
Valdez et al. 1982). As recently as the late
1980s, razorback sucker were still occa-
sionally captured in the 15-mile reach,
primarily during their spring spawning
season (Osmundson and Kaeding 1991). In
the 1990s, only four razorback sucker were
captured in the Colorado River and these
were found in the reach immediately down-
stream of the 15-mile reach. Adult Colo-
rado pikeminnow, though rare, are rou-
tinely captured in the 15-mile reach
throughout the year (Osmundson and
Kaeding 1989, Osmundson et al. 1998), and
spawning there has been verified by the
collection of larvae in 1982 and 1995
(McAda and Kaeding 1991; Anderson
1999).
SYNTHESIS
Distribution of Endangered Fish in the
Upper Colorado River
Though more is known about the life
history of Colorado pikeminnow than of
razorback sucker, both species evidently
require large stretches of river to fulfill their
respective life history needs. As individuals
progress through various life stages, their
habitat requirements change. For instance,
young Colorado pikeminnow rear in reach-
es where nursery habitat is most suitable,
whereas larger and older individuals move
to more upstream habitats where food for
adults is more abundant (Osmundson et al.
1998). This distribution pattern appears to
be consistent in both Colorado and Green
river systems. Colorado pikeminnow are
the top predator in the riverine food chain.
They subsist almost exclusively on a diet of
fishes and prefer areas where prey fish are
abundant. In the Colorado River, the great-
est fish production and consequently the
greatest number of adult Colorado pikemin-
now occur upstream of Westwater Canyon.
During recent studies (Osmundson 1999a),
electrofishing catch rates of forage-size fish
(mostly native suckers) were 3-4 times
higher in reaches upstream of Westwater
Canyon than in reaches downstream (see
Fig. 2). In the Green River, the majority of
adult Colorado pikeminnow are also found
in upper reaches as well as in two major
tributaries, the Yampa and White rivers
(McAda et al. 1994).
Upstream reaches generally have great-
er numbers of forage fish because food for
these fishes (algae and aquatic insects) is
more abundant than in downstream reaches
(Lamarra 1999). Higher water clarity, and
therefore deeper light penetration, allow
greater production of phytoplankton (free-
floating algae) and periphyton (algae that
grows on rock surfaces). These algae are
food for various species of aquatic inverte-
brates. Also, upper river reaches generally
have steeper gradients than the more down-
stream reaches, causing water to flow more
swiftly over the substrate. This serves to
clean silt and sand from the upper layer of
gravel and cobble. Silt-free crevices around
rocks provide important attachment sites and
living spaces for aquatic invertebrates
(Osmundson and Scheer 1998). Algae and
aquatic invertebrates living in these clean
substrates are food for native fishes such as
bluehead sucker Catostomus discobolus,
flannelmouth sucker Catostomus latipinnis
and roundtail chub Gila robusta which are
potential prey for Colorado pikeminnow
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