National Park Service plans predator kill to aid Colorado River fish - The Arizona Republic

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National Park Service biologists planned to close off and poison a slough connected to the Colorado River upstream of the Grand Canyon to kill young, non-native bass this weekend, the agency said.

It's the second time that officials have used rotenone, a fish-killing agent, as an emergency measure to slow a mushrooming smallmouth bass invasion from Lake Powell that threatens native humpback chubs that swim the Colorado farther downstream.

This time they're seeking hundreds of young bass, instead of the handful first detected in the slough between Glen Canyon Dam and Lees Ferry last year.

"This is a very short-term kind of solution" while partners seeking to protect Grand Canyon fish plan bigger fixes such as a screen to keep more bass from slipping through the dam, said Jim Strogen, who represents Trout Unlimited in a working group advising the government. Another idea is to pulse cold water through the dam's bypass tubes to disrupt bass spawning

"We're struggling with things that will help us get a handle on keeping the population as low as possible," Strogen said.

Bass that were originally planted in Lake Powell as a popular sportfish have been spotted downstream from the dam for years, but had not been known to spawn there until last year. They're aggressive predators who eat rare native fish upstream of Lake Powell and are now considered a threat to humpback chubs — officially a threatened species — downstream.

This year's surveys of the slough — a warm backwater connected to the river but protected from its current by a sandbar — found 667 young-of-the-year bass, according to information that biologists presented at a meeting of the Glen Canyon Dam Adaptive Management Program's work group this month.

Surveyors also found some 1-year-olds swimming, most of them near the dam and none beyond where the Paria River joins the Colorado at Lees Ferry. Humpback chubs are farther downstream.

Park service says the poison is no threat beyond the treated area

Bass have long lived in Lake Powell but were not considered a significant threat to the river below until two decades of drought dropped the reservoir's surface perilously close to the dam's hydropower intakes. Bass prefer the zone near the surface, which is warmed by the sun. So the surface's decline both put them in a better position to slip through and brought warmer water for them to enjoy in the river.

A National Park Service news release said employees would first seal off the connection between the slough and river using an impermeable fabric barrier. Then, on Saturday, they would treat the slough with rotenone, a plant-derived fish poison that fisheries biologists have used for decades, including in the Colorado River drainage.

They planned to close the slough to ensure public safety while allowing boaters to pass by on the river. After removing the dead fish, they planned to neutralize the rotenone with potassium permanganate, a chemical used to purify drinking water. Should any rotenone enter the Colorado, the agency said, the river would immediately dilute it to "concentrations that are insignificant to wildlife or humans."

Some Native American tribes with links to the Grand Canyon and the Colorado River in the past objected to killing fish in the river, while others have tentatively supported it as a stopgap measure before more humane protections for native fish can be put in place.

Before last year's rotenone treatment, Zuni Gov. Val Panteah, Sr., wrote to Glen Canyon officials saying that the tribal council unanimously opposed it because all creatures, even non-native, have "the right to life, liberty and procreation within their natural environments."

Tribal Councilman Edward Wemytewa told The Arizona Republic that the Zuni position has not changed, but that "our conversations with the agencies continue as we try to navigate through complex issues."

The Center for Biological Diversity criticized the U.S. Bureau of Reclamation, which manages the dam, for what it alleges is a "chronic failure to act" on preventing more bass from getting through the dam or becoming established downstream. The agency has long discussed altering flows or creating a barrier, CBD Southwest Director Taylor McKinnon said, or opening the slough so regular river flows pass through it to make a less-favorable bass nursery.

"It's an emergency situation," McKinnon told The Republic in an email. "Stop-gap piscicide treatments won't keep pace absent dam screens and flows to prevent nonnative fish in the first place."

Asked for comment this week, Reclamation officials said they could not respond by deadline. Glen Canyon National Recreation Area officials also declined to comment beyond their press release about the weekend's plan, saying that their experts were on the river and unavailable.

Options remain to keep the bass in Lake Powell

In previous meetings with the advisory group that Reclamation consults for dam management, agency officials have said both the need for environmental review and the likely difficulty of engineering a working barrier above the dam have caused delays. A net that can stop bass, for instance, would have to be more than 100 feet deep, anchored firmly on sides and at the bottom, and would require maintenance to ensure the non-native quagga mussels that infest Lake Powell wouldn't clog it.

Kelly Burke, a work group member who represents the Grand Canyon Wildlands Council, said Reclamation officials this month reiterated to the group their commitment to pressing ahead with those prevention measures that may be possible before constructing a barrier. For instance, they hope to complete a study of the option to route colder water from lower in Lake Powell through the bypass tunnels next year, disrupting bass spawning downstream.

"It was quite clear that (Reclamation) is not reducing its commitment to the barrier, but that they are accelerating the options we can employ sooner," Burke said.

Strogen said he remains hopeful that Reclamation can change dam operations or engineer a barrier in time to keep bass from overwhelming the Grand Canyon. Ultimately, though, nature and all Colorado River water users may have the final say. Lake Powell rebounded this spring and summer after an unusually snowy winter, but Strogen fears its downward trend may resume. That could put more bass in the river, and create warmer conditions for them to thrive there.

"My long-term fear is that the West is drying up and there's not enough immediate conservation efforts to have the depth at Lake Powell that's going to help us get through this situation," he said.

Brandon Loomis covers environmental and climate issues for The Arizona Republic and azcentral.com. Reach him at brandon.loomis@arizonarepublic.com or follow him on X @brandonloomis.

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