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• UCREFRP Catalog Number: 9300
• Author: Osmundson, D. B.
• Title: Importance of the 15-Mile Reach to Colorado River Populations of Endangered Fish.
• USFW Year: 1996.
• USFW - Doc Type: Grand Junction, CO.
• Copyright Material: NO

and periphyton (algae that grows on rock surfaces) in
support the native bluehead sucker (Catostomus disco
(Catostomus latipinnis). Upper reaches generally hav
downstream reaches; this serves to keep gravel and cc
Many aquatic insects require silt-free crevices around
support the native roundtail chub (Gila robusta), anot
Upstream reaches not only have higher food product:
the most suitable sites for spawning. As eggs deposi
drift with the current downstream. In the Colorado l
downstream of Moab, Utah having traveled a distanc
out and deposits of silt and sand provide many warm
of young, developing Colorado squawfish. During tl
backwaters and feed upon the zooplankton and chiro
become piscivorous, an abundance of small fishes coi
iese upper reaches; these algal types
As) and flannelmouth sucker
higher gradients than do more
ble substrates more free of silt and sand.
>cks for spaces to five; these insects in turn
,r forage fish of the Colorado squawfish.
n, but the clean cobble bars there provide
d in these areas hatch, the resulting larvae
ver, most of these larvae end up
of 60-180 miles. There the river flattens
sites ideally suited to the rearing
first year of fife, squawfish seek out these
iid larvae found there. Later, as they
ue to provide them with ample forage.
As Colorado squawfish become larger and approach ulthood, larger forage fish are required to
maintain growth rates. However, larger-sized forage sh are scarce in these downstream reaches
and maturing Colorado squawfish begin to move up d down the river seeking better feeding
grounds (Fig. 3). Eventually, the abundant supplies o native fish in the upper river are discovered
and adult squawfish remain in these upper reaches the remainder of their lives. As a result, the
average size of Colorado squawfish increases as one moves up river, with the greatest numbers of
large fish found above Westwater Canyon (Fig. 4). Even above Westwater, the differences in
average size among the three river strata are striking. Most Colorado squawfish in the Grand
Valley are clearly larger than those in the Loma-to-W stwater reach, and within the Grand Valley,
the 15-mile reach contains the greatest proportion of 1 ge, adult fish.
The complete life cycle.of the razorback sucker, in 1
largely unknown but may be similar to that of Cola
concentrated in upstream regions of the Green and
sites in the upper Green River have also been found
evidence suggests that bottomlands up and down be
for larvae when these habitats flooded during the fo
as of its river-wide habitat use pattern, is
squawfish because adults are generally
orado rivers and larvae from spawning
drift long distances down- river. Other
rivers historically served as nursery areas
;rly higher spring runoff period.
To understand the habitat needs of these large-river
takes into account all phases in their respective life c
particular species, population dynamics occur at the
importance of a particular area, like the 15-mile reac
must be viewed in the context of how it fits into the
habitat cannot support a viable population just as a i
cannot. Without taking the above discussion into ac
erroneous view of the importance of the 15-mile rea
the 185 miles of the Colorado River (above the Gre,
important in helping maintain populations of Colora
I, an ecological perspective is required that
-les and a recognition that for these
ndscape scale. Thus, the relative
cannot be viewed in isolation; instead, it
rger picture. A river without good adult
er with poor spawning or nursery habitat
)unt, one might arrive at a simplistic and
G that is, because it constitutes only 81/6 of
River confluence), it is probably not terribly
squawfish and razorback sucker.
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