9628

• UCREFRP Catalog Number: 9628
• Author: Gloss, S. P., J. E. Lovich and T. S. Melis.
• Title: The State of the Colorado River Ecosystem in Grand Canyon - A Report of the Grand Canyon Monitoring and Research Center 1991-2004.
• USFW Year: 2005.
• USFW - Doc Type: \
• Copyright Material: NO

38 The State of the Colorado River Ecosystem in Grand Canyon

rainbow trout fishery gained a reputation by the mid-
1970s as a world class, blue ribbon fishery famous for its
scenic grandeur and large, trophy-sized trout. Monitor-
ing in this reach is primarily done through electrofish-
ing and surveys of anglers by the Arizona Game and
Fish Department in cooperation with the USGS Grand
Canyon Monitoring and Research Center. The fishery
was initiated with stocking efforts and was maintained
primarily by stocking until the late 1990s.' Since closure
of Glen Canyon Dam in 1963, however, this fishery has
experienced variable success rates by anglers, and the
trout populations have changed in response to stocking,
dam releases, and food availability (McKinney and oth-
ers, 1999, 2001).
Recently, more stable river flows, which are the
result of the interim flows in 1991 and subsequent
implementation of the MLFF alternative in 1996, have
encouraged natural reproduction and made stocking
unnecessalY Stable flows and increased natural repro-
duction resulted in an expanding number of fish (fig.
2), but the larger number of fish was offset by smaller
average size and decreasing condition (plumpness) of the
fish (fig. 3). Because the overall carrying capacity of the
river remains relatively constant, the Lees Ferry reach
is able to produce a smaller number of large fish or a
greater number of small fish, a principle that is known as
conservation of biomass. As early as 1996, the Arizona
Game and Fish Department recognized the declining
size of trout in this fishery and recommended changes in
angling regulations to increase the size of fish; however,
anglers appeared unwilling to accept lower catch rates
of larger fish (Niccum and others, 1998). Average fish
condition continued to decline for several more years but
finally rebounded in 2002 (fig. 3).
As part of the Glen Canyon Dam Adaptive
Management Program, fluctuating nonnative fish sup-
pression flows were initiated beginning in 2003 and
continued through 2005 in an effort to reduce the
number of trout and increase their average size. The
experimental flow treatment involved increased diur-
nal flow fluctuations of 5,000 to 20,000 cubic feet per
second (cfs) from January through March of each year.
Overall, these fluctuating flows were intended to disrupt
spawning activity, to reduce egg survival, and to disad-
vantage young-of-year (YOY) trout that did survive. Early

I Stocking of fingerling rainbow trout was reduced in the mid-
1990s to about 20,000 (ish per ycar and ended completely in 1999
when it was apparent that natural reproduction under the modified
low fluctuating flow alternative was producing more than enough
recruitment to sustain the fishery (\Villiam R. Persons, Arizona Game
and Fish Department, oral commun., 2005).

6
~ 4
:>
c:
.E 3
--
..c:
B
'" 2
u
0
1990 1992 1994 1996 1998 2000 2002 2004

Figure 2. The average number of rainbow trout caught by using
electrofishing at several fixed sampling locations in the Lees Ferry
reach of the Colorado River from 1991 to 2003. Increasing catch-
per-unit effort is thoughtto be indicative of an increasing number
of fish in the population (Arizona Game and Fish Department and
U.S. Geological Survey, unpub. data, 2005).

90
88
o 86
....
u 84
~
c: 82
o
:.e 80
-c
5 78
u 76

74
72

1992

1994

2002

1996

1998

2000

Figure 3. Condition factor, or relative weight, of Lees Ferry
trout from 1991 to 2003. Condition factor expresses the length-
to-weight relationship and is an attribute that reflects the health
of individual fish as well as affects angler satisfaction. Relative
weight declined with the increase in fish density in the late 1990s
but increased in 2002-03. Present condition seems acceptable to
anglers (Arizona Game and Fish Department and U.S. Geological
Survey, unpub. data, 2005).

indications suggest that these experimental flows have
had only minimal effects on the recruitment dynamics of
rainbow trout. The total egg deposition loss because of
Glen Canyon Dam operations in 2003 ranged from 30%
to 40% in the Lees Ferry reach, with about half of this
mortality being a direct consequence of the enhanced
fluctuating flows in January through March (Korman
and others, 2005); however, clectrofishing catch rates
began to increase in 2003 (fig. 2). There also appears to
be a corresponding increase in angler use associated with