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BIO-WEST scientists, along with American Southwest Ichthyological researchers and federal cooperators, have documented a native fish community that is regaining dominance over nonnative fishes in the Colorado River in Grand Canyon National Park.
This is unheard of in the mainstem of the Colorado River, and the location in the Grand Canyon where this fish community was found is also unexpected due to the effects of the Glen Canyon Dam on water flow and temperature.
Historically, the Colorado River basin held 35 fish species within the mainstem river, and along the Grand Canyon reach of the river there were eight species of fish commonly found. These fish included Humpback Chub, Bonytail, Roundtail Chub, Colorado Pikeminnow, Speckled Dace, Flannelmouth Sucker, Bluehead Sucker, and Razorback Sucker.
The fish community in the Colorado River started to shift in the early 1900s following the introduction of nonnative fish such as Rainbow Trout and Brown Trout, which were introduced as sportfish.
After the Hoover Dam and Lake Mead were completed in 1935, other nonnative fishes were added to the reservoir for sport bait, and forage.
The introduction of these nonnative fishes led to drastic changes in the Grand Canyon fish community as the introduced species made their way upstream.
By the 1990s, there were 18 nonnative fish species documented in the Grand Canyon.
The study began in 2014 and continues today with funding from the US Bureau of Reclamation. The goal is to get an accurate picture of the Grand Canyon fish community as well as document the current native fish distribution along the Colorado River in a section from near Bright Angel Creek through Pearce Ferry.
Results from the study show the native fish community being dominated by four of the original eight species that were found in the area in the 1800s.
A fifth species, Razorback Sucker, was also found through a separate larval fish survey which shows the endangered species is once more breeding along their historical Colorado River habitats within the Grand Canyon.
It was also documented in another study that one of the native fish species, Humpback Chub, appears to be expanding their range and their population in the Grand Canyon is the largest known in the Colorado River basin.
It is believed that cold water released from the Glen Canyon Dam lowered the maximum water temperature of the Colorado River, which negatively affected the native fish population’s ability to reproduce, grow, and survive. Several of the nonnative fish species, on the other hand, preferred the colder temperatures.
Regulating the flow of water through the dam is also believed to have hurt the Grand Canyon fish community, especially for young fish. Releasing cold water from Glen Canyon Dam has led to lower levels of turbidity in the river, which made it harder for native fish to hide from nonnative predators, like trout, who rely on visibility to see their food.
In the late 1990s and early 2000s, Lake Mead water levels began to drop, which added about 125km of flowing river to the Colorado River. Also, the Pearce Ferry Rapid formed near the inflow area to Lake Mead.
These changes in the river conditions appear to have helped improve the ability of native fish to thrive in the Colorado River. The faster water flow, increasing temperature, and resulting higher level of turbidity are all factors that are believed to play a role in helping the native fish populations increase and the nonnative fish populations decrease in the area.
This study provides a contemporary example of an area where native fish have regained dominance over nonnative invaders.
Additional studies and monitoring could help provide more information on how the current conditions of the Colorado River are affecting the native fish community. Better understanding of these processes would help inform recovery and conservation efforts for these species in other locations.
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