WSP05499

• File Number: 8200.766
• Description: Gunnison River General
• State: CO
• Basin: Gunnison
• Water Division: 4
• Date: 4/1/1997
• Author: Mueller and Hiebert
• Title: Assessment of Kokanee Salmon Transport through Blue Mesa Powerplant - Colorado - During the Spring and Summer of 1994-1996
• Water Supply Pro - Doc Type: Report/Study

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We attempted to look for trends in water releases from Blue Mesa
Reservoir that might help explain a possible decline in the
reservoir's fishery. We went on the basic assumption that water
releases during April through July would directly effected reservoir
survival. Years with high release rates would transport a greater
percentage of young fish, thus influencing the harvest of adults in 2
to 3 years. Harvest numbers (255,000) and epUE (2.69 fish/trip) were
first collected in 1993 and since that time have declined. Release
rates during April through July 1991, averaged 1,598 cfs (45.3 m'/s)
which is below the average (2,089 cfs) for the past 6 years. Release
rates during this same period of time in 1992 dropped to 1,113 cfs
(31.5 m'/s) , a 30 percent reduction in releases. However, both the
harvest numbers and epUE in 1994 declined rather than increased, which
should have happened if survival was a primary function of reservoir
releases.

Martinez (1994, 1995) pointed out a growing amount of evidence that
suggests at least some instability in this fishery. Concerns have
been raised regarding the security of the Blue Mesa Kokanee egg supply
and the growing predation threat to kokanee and other young fish by
lake trout. Bioenergetic models have suggested that possibly 50
percent or 22,680 kg of the fish lost to lake trout during 1994 were
consumed by fish protected by regulation (Martinez 1994, 1995). (To
provide some degree of comparison, less than 2,000 kg of kokanee are
stock from Roaring Judy Hatchery each year). These findings have led
to changes in angling regulations to encourage lake trout harvest.
Slot-lengths have been dropped and bag limits have been increased in
an effort to reduce predation.

Test flow criteria for the Gunnison River had little, if any influence
on the volume or timing of releases from Blue Mesa Reservoir during
the study. While high flows in 1995 and 1996 undoubtedly contributed
to kokanee losses, these flows cannot account for the rainbow trout
losses. Less than 1 percent of the fish collected in the sieve net
were rainbow trout. This suggests that other unrelated factors such
as lake trout predation could have had a far greater influence on the
survival of juvenile trout and salmon. Factors such as bird
predation, natural morality, the recent outbreak of whirling disease,
and changes in angler effort could also affect the reservoir
population.

Kokanee entrainment studies at Dworshak Reservoir in Idaho (Maiolie,
pers. com.) suggests that high fish losses may occur due to high
discharges in early spring. This again is at a time when kokanee
densities are the highest in the deeper portions of reservoirs which
are normally near powerplant or dam release intakes. Early spring
monitoring of fish populations in the intake area at Blue Mesa and

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