The following are excerpts from:
River through the Grand Canyon: Natural History and Human Change, 1991,
Coruthers and Brown.
Why are half (4 out of 8) of the native fish species on the Grand Canyon threatened or
endangered when other species brought in from elsewhere proliferate?
A portion of the answer lies in the fact that few native species existed in the ancient
Colorado River. The low diversity was due in part to the river's limited
geographical area and isolation from other bodies of water.
The pre-dam Colorado was not only a relatively limited, closed system, but one marked
by enormous variability as well. Wild extremes in river flows, water temperature and
sediment load limited the diversity of species that could survive and flourish.
Consequently, few fish species evolved in the environment. Before the early 1900's,
the dominant fish were probably Colorado Sqawfish, one of three Chub species, Flannelmouth
Suckers and Razorback Suckers.
Given this low diversity, only one native predatory fish evolved, the Colorado
Squawfish. The other native fish evolved predator-avoidance mechanisms
to survive in the presence of this single predator fish.
However, this all changed when exotic predators were introduced. Channel catfish
and Carp were introduced in the late 1800's to the Colorado River drainage resulting in a
rapid shift in species dominance. Catfish voraciously prey on razorbacks and the
chubs. Carp eat fish eggs and disturb breeding areas.
As late as the mid-1960s little concern was shown for the potential impact on these
exotic introductions. By 1920, the National Park Service implemented a program to
introduce rainbow trout and brown trout in the Grand Canyon tributaries. By the time
the program was discontinued in 1964, 1.8 million eggs and fingerlings had been
introduced. Warm water sport fish were introduced to the lower reservoirs.
Later, Striped Bass (another veracious predator) was introduced into Lake
Mead. Lake Mead, positioned at the lower end of the Grand Canyon, provides a
reservoir of exotic fish that can easily move upstream in the Grand Canyon.
In summary, the fate and decline of native fish in the Grand Canyon can be compared to
the many isolated or "island" faunas when they were subjected to the sudden
introduction on non-native predators.
To add insult to injury, a common fisheries management practice there years ago was to
eliminate the native fish species of a river before introducing exotic gamefish.
1962, attempted to eradicate by poisoning all native fishes in 500 miles of the Green
River and its tributaries. Twenty thousand gallons of emulsified rotenone
preparation was dumped into the River. The impact of this action on the downstream
fishery is unknown but there was probably a substantial indirect influence since the
reservoir of native fish that regularly migrated downstream in the Grand Canyon must have
been severely reduced.
In 1963, the gates of Glen Canyon Dam were closed which effected the downstream water
temperature, turbidity and discharge.
The fishery in its present state has a naturalized mixture of native and exotic
species. Four native species: the Humpback Chub, Speckled Dace, Bluehead
Sucker and Fannelmouth Suckers are still present in viable numbers.
Fish thought to be no longer present in the Grand Canyon include the Bonytail Chub, the
Roundtail Chub, the Colorado Sqawfish and the Razorback Sucker. The last Razorback
Sucker was caught in 1984 below Bass Rapid. The last Squawfish seen was in 1972 by
Havasu Creek. Limited numbers of small squawfish persist in the upper basin of
Colorado River primarily in the Green, Yampa and San Juan Rivers.
One fact is certain---there is no turning back. Changes in the Grand Canyon
ecosystem are so complete that restoration of the native fishery is impossible.
What is left is a mix of native and naturalized non-native species which seems to be
adjusting to a new equilibrium of its own. The National Park Service, the U.S. Fish
and Wildlife and the Arizona Game and Fish biologists and administrators all agree that
the only realistic management plan is to attempt to maintain the status quo.
End of Excerpts.
Many biologist do not believe that draining Lake Powell will restore
native fish populations.
Lake Powell's cooler water have reduced Channel Catfish and Carp
populations and keep the predatory Striped Bass from moving up from Lake Mead. If
Lake Powell were drained...these predatory fish could threaten the remaining native fish
populations. Thus any advantage that increasing turbidity and temperatures would
have would more than likely by offset by increased predatation from exotic species.
******************** Quotes ***********************
"Removal of the dam would have negative results [on native fish
populations]. With restoration the river would run warmer and the population of
non-native fish would increase. This would not be promising for the native fish, as
most of the non-native fish are predatory species."
Mike Douglas, Dept. of Biology, Museum of Northern Arizona
Glen Canyon might not be the best place [for preserving] native fish
because of many factors. It's likely that pre-dam conditions were not very favorable
for native fish. If I had to choose a reservoir for native fish, I would choose
I don't think restoration [of Glen Canyon] is a good idea. I don't
think we are going to restore it back to 1958 [conditions]. Non-native fish would be
a problem. It would become a great non-native fish place. I think that
sediment will [also] be a problem."
Paul Holden, Bio/West, Inc.