Netting of Yellowstone Lake Trout is a Long-Term Venture

Lake trout suppression in Yellowstone National Park began in 1998

By Dillon Tabish

JACKSON, Wyo. — It will take another 14 years of intensive lake trout killing in Yellowstone Lake in order for native cutthroat trout to be re-established to the level sought by managers, new research predicts.

Montana State University student John Syslo modeled the effectiveness of Yellowstone’s lake trout netting and killing efforts for his doctoral dissertation, which was completed in April.

Syslo’s study also discovered that lake trout have shifted their diet away from cutthroat and now subsist primarily on a small type of crustacean called amphipods or scuds.

“The ratio of lake trout to cutthroat trout changed so much that prey just weren’t as available,” Syslo told the Jackson Hole News & Guide. “It seems like an availability issue.”

Diet analysis of lake trout in Yellowstone Lake from the late 1990s found that fish 5 years or older ate more than 41 cutthroat per year.

Taking stomach samples between 2011 and 2013, Syslo determined that lake trout 5 to 7 years old consumed about nine cutthroat trout a year and that lake trout older than 8 years ate between 20 and 32 cutthroat annually.

In addition to projecting future scenarios, Syslo’s 169-page thesis also summarized the efforts to remove lake trout from the 136-square-mile lake.

Lake trout suppression in Yellowstone National Park began in 1998, four years after they were first discovered there, and the program is now the longest running of its kind in the West. Scientists believe lake trout were illegally introduced in the mid- to late 1980s.

The presence of the exotic fish has had a ripple effect on the Yellowstone Lake ecosystem with some research tracing it to increases in grizzly bear predation on elk calves.

Estimates are that 126,000 lake trout lived in Yellowstone Lake in 1998 and that numbers rose to 746,000 in 2012 before dipping to 608,000 in 2013, Syslo’s study said.

The native strain of Yellowstone cutthroat, meanwhile, declined from 1.96 million in 1986 to 463,000 in 2000, before rebounding to 1.31 million by 2012, the study said.

Syslo’s modeling of the long-term netting effects on lake trout found that if lake trout are reduced to 97 percent of 2013 numbers, then there’s a 70 percent chance cutthroat will reach goals outlined in the park’s 2011 Native Fish Conservation Plan.

The findings in the diet analysis portion of the study were not anticipated, Syslo said. Because of the decline of cutthroat, he hypothesized that lake trout would shift their diet to other species of fish — either they’d cannibalize or eat more native dace or exotic suckers, shiners and chubs. Mature lake trout are believed to be piscivores, meaning they eat mostly other fish.

Instead, lake trout of all size classes relied more on scuds — tiny crustaceans — than anything else. Yellowstone Lake’s cutthroats of all sizes also ate mostly scuds, the study found.

“Not only do lake trout pose a threat to Yellowstone cutthroat trout through predation,” the study said, “but our results indicate competition could become an additional stressor for the Yellowstone cutthroat trout population if (scuds) are limiting.”

The study suggested that scuds are a keystone species in Yellowstone Lake, and hypothesized there was an increase in scud numbers as cutthroat declined.

Syslo also touched on the topic of accidental by-catch in the study, and he suggested that as cutthroat numbers rebound gill nets could impede native trout recovery. But ceasing netting had worse results for cutthroat, he said.

“Based on the modeling in my dissertation, it looks like the positive effects of removing lake trout outweigh the negative effects of by-catch of cutthroat trout,” Syslo said.

Syslo’s research provides a blueprint for fisheries managers grappling with booming populations of non-native predators such as lake trout.

“A lesson to draw from Yellowstone Lake and apply to other situations would be go in with a lot of fishing effort,” he said. “If you decide you’re going to suppress a population go in with as much effort as you can muster initially.”