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Mountain Outlaw Look Back: ‘Yellowstone’s War on Lake Trout’

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An NPS Hammerhead crewmember displays a lake trout caught using a gill net. PHOTO BY NEAL HERBERT/NPS

As the Outlaw Partners editorial department nears the release of the Winter 2020 edition of Mountain Outlaw magazine, we at EBS look to share some of the best stories from that cherished sister publication as it heads into a celebratory phase—10 years running, and strong. Enjoy.

– EBS STAFF

Yellowstone’s War on Lake Trout

Is aggressive gillnetting finally paying off?

By Christine Gianas Weinheimer

It’s rare for gillnetting boat crews to feel gratified when their nets yield fewer fish. But that’s what happened on Yellowstone Lake last season when crews saw a 25 percent reduction in the number of invasive lake trout they caught over the previous season. It signaled a job well done.

Gillnetting boats, staffed by National Park Service and private-sector crews, work the 132 square miles of Yellowstone Lake annually from May to October. They represent the linchpin in a race against time to remove non-native lake trout before they consume the park’s native Yellowstone cutthroat trout.

NPS is attempting to solve a problem that dates back more than 100 years. In 1890, the U.S. Fish Commission introduced lake trout into Yellowstone’s Lewis Lake for sport-fishing purposes, and eventually the fish found their way to Yellowstone Lake where, a century later, they began wreaking havoc.

One mature lake trout can eat approximately 41 Yellowstone cutthroat per year, and the large, deep-swimming fish have no natural predators. Further tipping the scales in their favor, lake trout can live more than 40 years, whereas the life span of a cutthroat is typically 10-12 years.

Yellowstone Lake’s cutthroat population was once estimated at 3.5-4 million fish. By the mid- to late-2000s, however, that number had plummeted to roughly 500,000.

Fast forward to 2018, when boat crews removed 297,000 lake trout as compared to 400,000 in 2017, a 25-percent decrease over just one year despite an increase of 8 percent in the number of gill-net units deployed. It likely indicates that the lake trout population is declining.

The outlook didn’t always look so bright. Gillnetting started in 1995, but in 2012 park officials began fearing it was a losing battle. That’s when Yellowstone Forever — the park’s official nonprofit partner — committed to help Yellowstone double down on its gillnetting efforts. Yellowstone Forever spearheaded a fundraising effort, the Native Fish Conservation Program, with an annual $1 million donation matched dollar for dollar by federal funds. The resources help Yellowstone implement its management plan focused on aggressive lake trout removal, and the investment is paying off.

“Since 1994, the boats have removed more than 3 million lake trout, of which roughly 2 million have been removed during the past six seasons since Yellowstone Forever started supporting increased netting,” said Jeff Augustin, senior director of park projects for Yellowstone Forever.

The goal of the park’s sustained efforts is to recover Yellowstone cutthroat trout to at least mid-1990s levels, when the prized fly-fishing catch was still abundant in the lake. But, as Augustin emphasizes, it’s not just about the fish.

“The size and health of Yellowstone’s native trout population has a direct impact throughout the entire food chain,” Augustin said. “Raptors, grizzlies, otters and other species rely on cutthroat as a vital food source. The loss of native trout would be devastating for the ecosystem.”

Native Yellowstone cutthroat trout spawn in the Lamar Valley.
PHOTO BY JAY FLEMING/NPS

According to a report published in March by the journal Science Advances, some park predators that have historically fed on Yellowstone cutthroat have been displaced from the ecosystem or switched to alternative prey. Dr. Todd Koel, head of the park’s Native Fish Conservation Program and the report’s lead author, says ospreys, for example, which only eat fish, were nearly displaced from riparian habitats around Yellowstone Lake. And bald eagles, in the absence of cutthroat trout, have shifted to scavenging carcasses and preying on common loon chicks and trumpeter swan cygnets — two waterfowl species that have declined in the park in recent years.

“This study demonstrates how the addition of an exotic species such as lake trout can change ecosystems,” said Koel. “Yellowstone is responding to the stressor of losing a native food source.”

Fortunately, progress has been measured in other areas. Gillnetting crews are seeing fewer small lake trout, indicating a lower birth rate, while higher numbers of cutthroat trout are being seen within the lake and spawning streams, and netting crews are catching more cutthroat in their hauls.

While the data is trending in the right direction, Koel says it’s not time to slow efforts. “We have no intention of letting off on the netting pressure.”

Koel says the team will also expand their attack on lake trout eggs. They strategically place dead lake trout on spawning areas to manipulate the water quality, which has proven effective in killing the eggs. Mature lake trout they have implanted with hydro-acoustic telemetry tags lead them to the spawning sites.

While Koel says he would like to “put the nail in the coffin” of these lake trout, he admits that may never happen. “Yellowstone plans to continue the program in one manner or another, indefinitely, as we will never capture the last lake trout in Yellowstone Lake.”

But he believes that complete annihilation of lake trout won’t be necessary for the native fish to fully recover. “We plan for cutthroat trout to regain their rightful role in the ecosystem.”

Learn more about the Native Fish Conservation Program at Yellowstone.org/native-fish.

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