Recent spawning surveys indicate most of Glacier National Park’s west-side bull trout populations are teetering toward extinction, but an aggressive netting project in one lake was successful in suppressing invading lake trout.

There are 17 lakes on the west side of the park that have historically supported bull trout — but 10 of those have been invaded by lake trout migrating upstream from the North Fork Flathead River and the Flathead Lake ecosystem.

“Nine of those have been overwhelmed [by lake trout] to a point where those bull trout populations are on the brink of extinction,” said Clint Muhlfeld, a U.S. Geological Survey fisheries biologist who is based in the park.

Surveys of bull trout “redds” or spawning beds have been concentrated on streams above lakes in the park. Not all known bull trout populations are monitored in the park, mainly because of adverse weather and the remoteness of backcountry spawning streams.

“Looking at the redd count data, things don’t look good for a number of them,” said Chris Downs, the park’s fisheries biologist. “That’s the trend that is unfortunately unfolding in the large, west-side lakes.”

Downs noted that redd counts in streams above most of the lakes didn’t start until 2002, and by that time the impacts of invading lake trout were well under way.

This year’s survey above Logging Lake detected no redds, compared to five last year. Logging Lake supported a productive bull trout population before lake trout were detected in 1984.

No redds have been counted upstream from Bowman Lake since  2007, and only one redd was found above Harrison Lake this year.

With five redds counted upstream, tiny Akokala Lake had the highest count on the west side this year with the exception of Quartz Lake, Downs said.

Although west-side spawning surveys are relatively new, previous gill-net sampling in the lakes has provided a picture of what has transpired over time.

In 1977, no lake trout were detected in any of the west-side lakes, Downs said.

Fast-forward to 2000 gill net sampling that found a ratio of two lake trout for every bull trout in Logging Lake, and a ratio of 6-to-1 in Bowman Lake.

That kind of turnover, occurring over less than 30 years, “is remarkably rapid” in ecological terms, Downs said.

Muhlfeld said losing individual bull trout populations in the west-side lakes has broader implications, in terms of genetic diversity, for the Flathead Basin’s bull trout metapopulation.

“The more populations you have, the greater chance you have that the metapopulation can survive over time,” Mulhfeld said.

The one bright spot for a Glacier lake that is directly connected to the Flathead river system is Quartz Lake, where lake trout were discovered only a few years ago. Muhlfeld said 48 bull trout redds were counted this year in stream sections above the lake.

Park officials approved a four-year experimental lake trout suppression project on Quartz Lake that began in early September. Muhlfeld led a team of several people in gill netting over a two month period, with successful results.

A boat was airlifted into the lake and netting initially was concentrated on juvenile lake trout. Netting later targeted adults as spawning got under way in October. Eleven adults that were previously tagged with radio transmitters revealed two major  spawning locations in the lake.

In the end, 350 juveniles and 130 adults were caught in gill net sets that ranged from 600 feet up to 1,800 feet, with the crew hauling up nets by hand. The by-catch of bull trout was “very low,” Muhlfeld said, about one for every 20 lake trout that were netted.

Remarkably, 10 of the 11 radio-tagged fish were netted. And net hauls gradually diminished, both indications that the netting was effective.

“I think we did a really good job of knocking them back,” Muhlfeld said.

Had the high number of juveniles been left unchecked, “it would be devastating to the bull trout population,” he added. “We really started this suppression program in the nick of time.”

There are similar lake trout suppression projects under way on waters such as Yellowstone Lake, Swan Lake and Idaho’s Lake Pend Oreille. Compared to those efforts, Muhlfeld believes the Quartz Lake project has the highest likelihood of success, partly because it is smaller, the invasion is in an early stage and a barrier has been installed downstream from the lake to isolate the current population.

“It’s like a bathtub,” Muhlfeld said. “There’s nowhere for these guys to hide.”

Muhlfeld explained several advantages that lake trout have over bull trout: Female lakers produce far more eggs, pound per pound. They spawn in the lakes and juveniles rear in the lakes, with abundant food sources.

Bull trout, meanwhile, spawn less frequently and they spawn up streams, where offspring grow for several years with less abundant food and conditions that can be difficult.

“I would guess that lake trout have a competitive growth advantage, and on top of that the lakers eat the bulls,” Muhlfeld said. “It’s just a numbers game.”

The picture is brighter for bull trout on the east side of the park, where lake trout do not have a presence.

Two St. Mary River tributaries have been surveyed for bull trout redds annually for the last 13 years. Boulder Creek continues to have the largest spawning group of bull trout, with this year’s redd count slightly above the annual average of 33 redds.

This year’s count of just four redds in the other stream, Kennedy Creek, was well below the annual average of about 20.

Park officials consider the ongoing expansion of lake trout to be the most critical challenge for native bull trout.

Reporter Jim Mann may be reached at 758-4407 or by e-mail at jmann@dailyinterlake.com