8158

• UCREFRP Catalog Number: 8158
• Author: Osmundson, D. B.
• Title: Longitudinal Variation in Fish Community Structure and Water Temperature in the Upper Colorado River; Final Report.
• USFW Year: 1999.
• USFW - Doc Type: Grand Junction, Colorado.
• Copyright Material: NO

EXECUTIVE SUMMARY
The size of the adult Colorado pikeminnow (Ptychocheilus lucius) population in the upper
Colorado River is currently well below the estimated minimum size needed to assure long-
term viability. If recovery activities are successful in increasing the frequency of strong year
classes, there must be sufficient adult habitat available to support an expanding population.
To date, no estimates have been made of how many adult Colorado pikeminnow the river can
support in good condition. The primary focus of the field effort of this study was to
document the longitudinal distribution of fish that might serve as forage for adult Colorado
pikeminnow, and to determine the limitations that declining upstream temperatures impose on
the suitability of habitat and thereby delimit the Colorado pikeminnow's current and potential
range. This information was then used to estimate the extent to which we can expect passage
facilities on the Gunnison and mainstem Colorado rivers to increase the number of miles of
suitable adult habitat. Integrating data from other studies, a preliminary estimate of carrying
capacity is also provided, both on a per-mile basis and a river-wide basis assuming different
management scenarios.
Most fish electrofished in both the Colorado and Gunnison rivers were native species,
primarily bluehead suckers Catostomus discobolus and flannelmouth suckers C. latipinnis.
Densities of fish that could serve as forage for adult Colorado pikeminnow (soft-rayed fish <
300 mm long) were found to be about three times higher in river strata upstream of
Westwater Canyon than in more downstream strata. Within the Colorado River, densities of
these fish did not systematically increase in an upstream direction; instead, densities were
similar among upstream strata and similar among downstream strata, with the exception of
Stratum 1 (Moab, Utah to the Green River confluence), which had much lower densities of
fish than other strata downstream of Westwater Canyon. In these downstream strata, most
forage-size fish were young bluehead suckers which were almost exclusively associated with
riffle habitats. In contrast, in upstream strata, several species comprised the forage-size fish
and these were found in both run and riffle habitats.
Body condition of bluehead and flannelmouth suckers and roundtail chubs progressively
declined in a downstream direction, and may have been reflective of a downstream decline of
primary and secondary productivity. Diet of suckers largely consisted of organic debris and
invertebrates; for roundtail chubs, diets consisted of organic debris, fish and invertebrates.
Chironomids made up much of the invertebrate portion of the sucker diet; and in upstream
strata, Ephemeroptera and Trichoptera were also commonly eaten.
Main-channel temperatures declined in an upstream direction. Annual thermal units (ATU)
for Colorado pikeminnow growth near Moab, Utah (Stratum 1) were double what they were
near Cameo, Colorado (Stratum 10), some 234 km upstream. Based on distributions of adult
Colorado pikeminnow in two rivers with unrestricted access to upstream coolwater reaches
(Yampa and Gunnison rivers), a minimum of about 40 ATU appears to be needed for
ix