National Park Service to poison bass upstream of Grand Canyon to save native fish

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LEES FERRY – The National Park Service will poison a Colorado River side channel below Glen Canyon Dam to remove fish that threaten native species, the agency announced.

Meanwhile, biologists warn, rising temperatures and declining oxygen levels in water the dam releases into the river’s Lees Ferry stretch are stressing the popular trout fishery there.

Nonnative smallmouth bass are known to have passed through the dam’s hydropower-generating turbines for several years to reach the Lees Ferry area, but their successful breeding at that stretch was not documented until this year. If it continues, bass could establish themselves far downstream and eat humpback chubs, which are protected as a threatened species.

Immature fish that are native to the Colorado River in the Grand Canyon such as flannelmouth suckers, bluehead suckers, speckled dace and humpback chub, swim in a backwater eddy near river mile 185 of the Colorado River in Grand Canyon National Park. These fish like to congregate in these backwater eddies because it provides protection from the main river channel and the water is a little warmer than the water flowing in the main channel.
Amaury De Labbey (Boston) fishes for rainbow trout in the Colorado River, Sept. 9, 2022, at Lees Ferry, Arizona.
Utah State University lab technician Justin Furby weighs a smallmouth bass June 7, 2022. Smallmouth bass feast on humpback chub in the river's upper section, where agencies spend millions of dollars annually to keep the intruders in check.
Humpback chub and other native Colorado River fish are seen in a hoop net near the confluence of the Colorado River in Grand Canyon National Park. U.S. Fish and Wildlife was catching, tagging, and measuring native fish in the Little Colorado River, including the endangered humpback chub. In the past, the humpback chub has struggled in the Colorado River as water temperatures have decreased with colder water passing through Glen Canyon Dam from deep in Lake Powell. The humpback chub still does well in the side tributaries including the Little Colorado River.

“Threats to the native fish are increasing due to the warmer temperatures of water passing through the dam and related increased river temperatures below the dam,” according to a statement released by Glen Canyon National Recreation Area, which includes the river segment.

Warmer water aided bass in spawning below the dam.

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Fish stocked in the river are now a potential threat

Beyond bass and other introduced sportfish that swim just above the dam and could push through, brown trout that the Park Service itself stocked for anglers in the river during the 20th century are a potential threat. For that reason, anglers now can claim a bounty for browns that they kill in the river.

But worsening river conditions are now threatening the rainbow trout fishery, which is considered less dangerous to native fish and supports several fly-fishing guide services.

In this photo provided by Terry Gunn, an angler displays a rainbow trout caught at Lees Ferry near Marble Canyon, Ariz., May 10, 2010. From prized rainbow trout to protected native fish, declining reservoirs are threatening the existence of these creatures, and also increasing the cost of keeping them alive.
Bathtub rings show how low Lake Powell levels have declines June 8, 2022, in Page, Ariz.

The U.S. Bureau of Reclamation, which operates the dam, said this month that the same drought conditions that have plunged Lake Powell’s water levels into crisis are now leaving Lees Ferry water oxygen-starved. Warmer water has less dissolved oxygen, and the reservoir’s sinking surface elevation is causing flows through the dam’s tunnels to warm.

Like many issues tied to the Colorado’s shrinking water supplies, this one is accelerating.

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Low oxygen levels can stress and kill trout or force them to move downstream past where riffles add oxygen back into the flowing water. For now, trout remain evident in abundance at Lees Ferry.

Fishing remains good, though tailwater temperatures that should be around 50 degrees in summer are instead roughly 70, said Wendy Gunn, who owns and operates Lees Ferry Anglers.

“The food base is still good,” Gunn said, “so we’re still seeing healthy fish, but we’re starting to see fish struggle and we are seeing some dead fish.”

Wendy Gunn talks about trout fishing, September 9, 2022, at Lees Ferry Anglers, Marble Canyon, Arizona.

The reservoir’s continued decline could warm the river further next summer, worsening the oxygen depletion.

What is being used to kill invasive fish, why do some local tribes object?

The agency  treated the slough with the fish killer rotenone on Saturday and Sunday. A second treatment may be necessary within two months if more invasive fish are found to have hatched or hidden in the area’s dense vegetation.

While rotenone is a plant-derived chemical commonly used by fish managers, and the Park Service said it will carefully protect the environment and human health by isolating it in the slough, at least one Native American tribe with cultural links to the river and Grand Canyon has objected.

The Zuni Pueblo of New Mexico provided The Arizona Republic a Sept. 8 letter that Zuni Gov. Val Panteah Sr. sent to Glen Canyon’s acting superintendent.



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