A three-week netting project on Montana's Swan Lake is complete, turning up more than 3,700 lake trout and giving biologists the ability to estimate the overall population of the invasive species in the lake.

“We believe that roughly represents about half the population that was in the lake,” said Wade Fredenberg, a U.S. Fish and Wildlife Service fisheries biologist involved with the project.

“The jury is still out” on the population estimate, said Travis Horton, native species coordinator for Montana Fish, Wildlife and Parks. “We’ll see what the data shows.”

A firm population estimate, Fredenberg cautioned, will be generated through a “depletion population estimate,” formulas applied to the declining catch rate as the netting progressed.

Harbor Fisheries, a Great Lakes fishing contractor from Wisconsin, was called in to carry out the work of setting two miles of gill net every morning and evening for three five-day “passes” through the lake.

The first pass brought in about 1,500 lake trout; the second pass, about 1,200; and the final pass netted about 1,000.

“The bottom line is in these three weekly passes through the lake, we wanted to see a declining catch,” Fredenberg said.

All lake trout that were caught were killed, most of them being delivered to food banks, and some being delivered to a raptor rehabilitation center.

“It was a lot of fish,” Fredenberg said. “I’m sure we delivered somewhere in the neighborhood of 1,500 pounds of fish.”

A Montana State University graduate student, Ben Cox, established effective netting locations.

And the contractor, Horton said, was “just incredible at setting up nets and fishing that way.”

“We targeted all portions of the lake that were more than 60 feet deep,” Fredenberg said, explaining the goal of avoiding shorelines and other shallow areas that are inhabited by species other than lake trout. “There are fewer lake trout in those areas and there’s a lot of other species in those areas we wanted to avoid.”

Particularly, threatened bull trout. The project was intentionally carried out at a time when a large portion of the adult bull trout population was spawning in tributaries to the Swan River, which flows into Swan Lake.

As a result, the “by-catch” of bull trout amounted to just over 200 fish, and many of them survived.

“It’s incredible how low the by-catch was,” Horton said. “That was one thing I was concerned about initially ... The skill of the crew in setting the nets is part of that and picking the nets. They are effective in getting the fish out of the nets alive when they can.”

The netting was also carried out at a time when lake trout were beginning to spawn in deeper waters of the lake.

“When you get down in that deep water there’s nothing there but lake trout and a few bull trout,” Fredenberg said, adding that fine mesh nets were used, enabling the catch of lake trout as small as six inches long.

Lake trout, first detected in Swan Lake in 1998, are considered an imminent threat to the lake’s bull trout and kokanee salmon populations.

The netting project could pave the way for any future work aimed at suppressing the lake trout population.

Fredenberg noted that the project involved getting spatial netting coverage across the lake, in order to get a valid sampling for the population estimate. There were locations, he said, where concentrated netting efforts would have yielded much higher catches.

“We generated good quality data, but we did not maximize our abilities in terms of removing fish,” he said.

“That’s one of the big differences in trying to get the data first rather than jumping right into suppression,” Horton said. “We did a little of both this year ... We obviously removed quite a lot of lake trout but we also got good data out of it.”

Horton said a suppression effort would involve an environmental assessment, with public comment.

“The expectation is that we’ll move into a suppression mode, next year, if everything works out,” he said.

The project resulted from a collaborative effort between the U.S. Fish and Wildlife Service, Montana Fish, Wildlife and Parks, the U.S. Forest Service, the Confederated Salish-Kootenai Tribes, and the contributions of Trout Unlimited volunteers.