Glacier National Park’s Quartz Lake could be a model for combating invasive lake trout.

Carter Fredenberg, a fisheries biologist at the park, said the ongoing suppression project is yielding encouraging results, with lake trout populations down significantly after six years of targeted gill netting and angling.

Introduced into Flathead Lake a century ago, lake trout have colonized upstream lakes and displaced native fish populations, notably the federally protected bull trout. 

Within the park, 17 lakes west of the Continental Divide historically contained bull trout populations, and nine of them now host lake trout as well. Five lakes have downstream waterfalls or rapids that prevent upstream migration.

Fredenberg noted that Quartz Lake, in which lake trout were first documented in 2005, offered park fisheries biologists a chance to respond before the invasive fish population began out-competing bull trout.

“The probability of success in this system was relatively high,” he said. “There were no mysis shrimp, we got on it early and it was a small system.”

By 2009, the park cleared the necessary regulatory hurdles to begin the experimental project, but not without serious logistical challenges. 

Equipment and supplies have to be either hiked in or flown in by helicopter and the lake’s surface, at an elevation of 4,419 feet, often remains frozen until early summer. 

During a presentation hosted by the park last week, Fredenberg said his crew has to constantly thaw out gill nets and pull them up by hand — a process that could take up to two days when they become snagged on logs on the lake bed.

The researchers mapped out the lake trout’s spawning locations using “Judas fish,” individual lake trout tagged with radio transmitters and then tracked during the fall spawning season. Each of the tagged fish congregated in one of two spawning areas along the edge of the lake, where debris avalanches routinely deposit large logs and boulders that provide the ideal place for female lake trout to lay their eggs.

In addition to encouraging anglers to take lake trout, the team deployed gill nets at the spawning locations, netting 140 adults in the first year and between 40 and 50 each year afterward. Last year the adult harvest hit an all-time low, with about 30 spawning-age fish captured. On the other hand, a total of 5,119 juvenile fish were caught over the project’s first six years, reaching a peak of more than 2,000 in 2013 before dropping again last year.

Fredenburg estimates the lake trout population when work began numbered about 4,000. Based on catch rates, he estimates the population could hit the park’s goal of 500 lake trout between 2018 and 2028, depending on how conservatively they estimate the remaining population.

But since the driving reason for the project is bull trout protection, suppression efforts also need to avoid accidentally killing the threatened fish in the process. The first year, gill nets caught 19 lake trout for every bull trout caught. By 2014, that ratio had risen to 149 lake trout per bull trout, increasing each year as the researchers honed their understanding of the lake trout’s spawning habits. Of 58 bull trout caught last year, 11 died.

Fredenberg added that redd counts, in which fisheries managers use the total bull trout nesting sites as a proxy for the mature adult population, hit an all-time high last year and were more than double the average. But he said that wasn’t necessarily a meaningful statistic.

“There’s a lot of variability in redd counts — that’s just the way they work,” he said. “I think we would need multiple years of high counts to really be able to say, ‘Wow, this increase is due to us gill netting.’”

So will Quartz Lake ever be completely free of lake trout?

“Everyone likes to think about that, but the hard part is that as you get down to fewer and fewer individuals, you have to put more time and more effort in for fewer fish,” Fredenberg said.

A genetic analysis of lake trout in Swan Lake found that only two or three invading lake trout were responsible for the fish’s subsequent takeover of the lake, meaning if even a couple of invasive fish are able to elude suppression efforts, the cycle could start all over again.

And suppression isn’t cheap. 

Clint Muhlfeld is an aquatic ecologist with the U.S. Geological Survey, which is helping the park trim the lake trout population. He said the grant-funded project at Quartz Lake averages between $50,000 and $75,000 a year, but once target levels are met, the annual cost will drop below $50,000.

Despite the inherent obstacles at the geographically isolated lake, Muhlfeld said he believes Fredenberg’s work could hold the key to suppression efforts elsewhere.

“It hadn’t been shown that you could do this,” he said. “To my knowledge, this is the most successful lake trout suppression effort that has been done.”

The next step will be attempting to duplicate that success in nearby Logging Lake, where the lake trout population is far more established. The invasive fish were first documented there in 1984, and have all but eradicated the once-robust bull trout population. 

Muhlfeld said a two-pronged approach there will combine the suppression strategies pioneered in Quartz Lake while moving some of the remaining bull trout upstream to establish a population in Grace Lake.

“Conservation introductions are a new concept in the face of climate change,” Muhlfeld said. “There is no model for this.”

Fredenberg shares Muhlfeld’s excitement about the project, contrasting it with Quartz Lake, where the bull trout population had not noticeably declined when work started.

“The really neat thing about [Logging Lake] is that we have seen the declines in the bull trout population there, and so part of the way we’ll be able to judge success is the bull trout response.”

Reporter Samuel Wilson can be reached at 758-4407 or by email at swilson@dailyinterlake.com