Glacier sees progress in battle against lake trout

Biologists in Glacier National Park say they’re making progress in the fight against non-native lake trout.

Last fall U.S. Geological Survey scientists and a crew of biologists started suppression efforts against lake trout in Quartz Lake. The idyllic lake, tucked up in the high mountains of the North Fork just south of Bowman Lake, is one of Glacier’s last bastions for embattled bull trout.

Bull trout are listed as a threatened species under the Endangered Species Act. The trout face a host of threats, including habitat degradation, global climate change and competition from non-native species. In Glacier, the greatest threat to bull trout has been from non-native lake trout.

Lake trout were introduced into Flathead Lake decades ago and slowly, but surely, they have invaded any Park lake that is connected to the Flathead River system.

When lake trout invade a lake, they take it over, gobbling up native cutthroat trout and out-competing bull trout for habitat.

Logging Lake, for example, used to be a prime bull trout fishery, but lake trout invaded and the bull trout fishery has all but crashed.

Gill netting and redd surveys this year turned up either no bull trout or just a few fish. Redds are bull trout spawning beds. When a fish spawns, it excavates gravel from the stream bed and lays its eggs in the resulting nest of gravel. Biologists count those spawning beds to get a handle on the fish population.

Quartz Lake is still an excellent bull trout fishery, but a few years ago, biologists found lake trout had almost made their way into the lake. An aggressive effort to net and kill lake trout in Quartz was begun last fall.

Headed up by U.S. Geological Survey biologist Clint Muhlfeld, a team of biologists has been netting both juvenile lake trout and adult lake trout out of Quartz.

Juvenile fish are netted in the spring and adults in the fall. Muhlfeld said last week the effort was promising.

But the work isn’t easy; a team of biologists working under Muhlfeld go out on the lake at night in a small motorboat, pulling gill nets by hand. They have to work at night because that’s when lake trout spawn.

They also implanted some adult lake trout with radio transmitters. By tracking the fish, they were able to pinpoint the lake trout spawning areas in avalanche chutes that enter the lake, making it easier to net the fish.

Last year they caught and killed more than 500 lake trout — adults and juveniles — from Quartz. This year the catch was lower, with 224 juvenile fish caught and 64 adults.

The netting effort appears to be working, Muhlfeld said. For one, the ratio of the males to females has increased. Initially there were about two males for every female. Now there are three to four males for every female.

There’s also a significant age gap in the population. The oldest fish they caught was about 35 inches long and was 24 years old — likely one of the first lake trout in the lake. But the majority of the fish are far younger — an indication that lake trout haven’t been able to get a stronghold in Quartz.

Muhlfeld is working on innovative tactics to improve the catch rate. One method he’s considering is to use a breeding female as a lure. It would work like this: Biologists catch a female that’s ready to spawn, sew up her vent, which stops her from laying eggs, and then put her in a cage back into the water. Males that try to mate with her are netted and killed.

Dead fish have their air bladders cut and the carcasses are sunk in the lake.

This effort will continue for at least two more years and Muhlfeld is looking for additional funding.

The proof appears to be in the bull trout population, which continues to be robust.

“The bull trout appear to be holding their own,” Muhlfeld noted.

Another tactic biologists are considering is moving bull trout farther up drainages. Save for Akokala Lake, the Park’s secure bull trout fisheries are above barriers — waterfalls that keep non-native lake trout from migrating into them.

A study is underway by a graduate student to examine other lakes that could support bull trout in a drainage. Take Logging Lake for example. Upstream from Logging is Grace Lake. Grace Lake currently has Yellowstone cutthroat trout, but no bull trout in it. Biologists are considering moving bulls into Grace, which is protected by a waterfall. That could preserve the bull trout in the drainage. Research is also being done on Pocket Lake, which is in the headwaters of Bowman Lake.

That feasibility study will take a couple more years.

Meanwhile, biologists have been busy in the field re-sampling lakes for bull trout populations, said Chris Downs, Glacier Park’s fishery biologist. This winter, Downs will crunch the data, with a report this spring. The numbers aren’t encouraging he cautioned.

Biologists fighting to save Glacier’s bull trout have a tough road ahead.

“They’re on the brink of extirpation unless we do something about it,” Muhlfeld said.