Native Fish in the Grand Canyon

The Colorado 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.       

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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.

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 "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 Flaming Gorge.

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.