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

(Osmundson 1999a).
Clean cobble and gravel substrates of
upper reaches also provide suitable spawn-
ing sites for Colorado pikeminnow. As
eggs deposited in these areas hatch, emerg-
ing larvae drift downstream with the cur-
rent. In the Colorado River, most pikemin-
now larvae end up downstream of Moab,
Utah where the gradient flattens and the
river slows (Trammell and Chart 1999).
There, silt and sand is deposited creating
bars and associated backwaters. Backwat-
ers are warm, productive, still-water habi-
tats well suited for rearing of young Colo-
rado pikeminnow. During their first year of
life, Colorado pikeminnow concentrate in
backwaters and feed on zooplankton and
chironomid larvae (Muth and Snyder
1995). Later, as Colorado pikeminnow
become piscivorous, an abundance of small
prey fish continues to provide ample forage
(Osmundson et al. 1998).
As Colorado pikeminnow reach adult-
hood, larger forage fish are presumably
required to maintain body condition and
growth rates. However, such forage fish
are scarce in downstream reaches. As
young adults, Colorado pikeminnow begin
to disperse up and down the river (Fig. 3),
either in search of spawning areas or better
feeding grounds. Eventually, the more
abundant supplies of native fish in the
upper river are discovered and adult
pikeminnow remain there and establish
home ranges (Osmundson et al. 1998).
This dispersal pattern results in somewhat
segregated life stages with adult densities
clumped near the upstream margins of their
range. Currently, the Grand Valley con-
tains the greatest densities of adult Colo-
rado pikeminnow in the Colorado River
(Fig. 4). Also, this distribution pattern results
in the average size of Colorado pikeminnow
to progressively increase in an upstream
direction, with the greatest average size
occurring in the 15-mile reach (Fig. 5). Thus,
the importance of this reach is in providing
habitat for adults.
Compared to Colorado pikeminnow,
specific attributes of the life cycle of the
razorback sucker, in terms of its use of the
entire river, is largely unknown. However,
there are some similarities between the two
species: like Colorado pikeminnow, razor-
back adults are generally concentrated in
upstream regions of the Green and Colorado
rivers, they exhibit long-range spawning
migrations (Modde and Irving 1998), and
larvae drift downstream from spawning sites
in the middle and upper Green River (Chart
et al. 1999). Other evidence suggests that
flooded bottomlands along the Green and
Colorado rivers historically served as nursery
areas for larvae. Historically, bottomlands
were inundated more frequently and for a
greater duration each spring than occurs
under current river regulation practices
(Osmundson and Kaeding 1991; Modde
1996; Modde et al. 1996).
The 15-mile Reach in Perspective
Contemporary river ecology emphasizes
the propensity for biodiversity and bio-
production to be largely controlled by habitat
maintenance processes such as cut and fill
alleviation mediated by river discharge.
Along a river's length, environmental hetero-
geneity maximizes in alluvial reaches, and
floodplains appear to function as centers of
biophysical organization (Stanford et al..
1996). In addition, core populations (rela-
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