Lake trout proliferate in Swan Lake

Netting turns up invaders by the thousands

The numbers are in, and the invasion of Swan Lake is well under way.

An unprecedented netting project on the lake from Sept. 17 through Oct. 4 turned up 2,174 lake trout, a non-native species that was first detected in the lake through an angler's report in 1998.

"There are probably tens of thousands of lake trout in Swan Lake," said Wade Fredenberg, fisheries biologist with the U.S. Fish and Wildlife Service. "The fact that we caught 2,100 without even coming close to covering the whole lake says something."

The netting was carried out by commercial fishing contractors from Wisconsin.

More than 26 miles of gill nets were set at various locations around Swan Lake where lake trout were thought to be concentrated. The fish ranged from 22 pounds down to the smallest fish the nets could snare.

"As you went down the [size] pyramid, the numbers got higher and higher," said Fredenberg, who is based in Creston. "That's an indication the population is healthy and has been reproducing for quite a few years now."

For Fredenberg, the results of the netting are disturbing but somewhat predictable.

"This is totally consistent with what we've seen in other places," he said. "By the time we detect these invasions through angler reports, it's already a lot further along than we thought it was."

The findings are disturbing because Swan Lake is just the latest bull trout stronghold to be invaded in the region.

"In just the last 10 years, we've lost a huge proportion of our secure bull trout areas in Northwest Montana," said Fredenberg, who heads the Swan Valley Bull Trout Working Group.

Most recently, lake trout invasions have been detected in Glacier National Park's Quartz, Logging and Harrison lakes. Lake trout already were well established in Bowman and Kintla lakes, as well as Lake McDonald and other large lakes in the region, including Flathead and Whitefish lakes and Priest Lake and Lake Pend Oreille in Idaho.

In every case, the nature of the fisheries have been altered dramatically.

Lake trout tend to dominate and displace the native bull trout, and they have direct impacts on other fish. Similar impacts can be expected on Swan Lake, Fredenberg said.

"First, kokanee [salmon] will disappear," he said. "Then you'll see a big drop in bull trout and other species, and then lake trout growth rates will decline."

The lake will become of limited interest to anglers, Fredenberg predicts, pointing to the low angler numbers on Whitefish Lake and Lake McDonald, where lake trout now dominate.

"You don't see anglers flocking to Whitefish Lake," Fredenberg said.

While many Flathead Valley residents may be accustomed to the lake domination of the Flathead River system, there are anglers who remember when the bull trout was king.

The basin's bull trout population has dwindled from hundreds of thousands to just thousands and a designation as a threatened species protected under the Endangered Species Act.

"If you go back 100 years, the numbers of bull trout in the system were probably the same as the number of lake trout now," Fredenberg said. "The lake trout have essentially replaced them … Instead of being a mainstay, bull trout have become a novelty, really."

That's what is at stake with Swan Lake, and potentially, other lakes in the Swan River drainage. Lake trout can easily swim upriver from Swan Lake, eventually reaching waters such as Holland and Lindbergh lakes, Fredenberg warned.

"I think it's pretty clear that if we can't do something, we're going to end up with another system that's similar to the others in the valley," Fredenberg said.

While it might appear to be a lost cause, various methods of suppressing the lake trout population will be discussed by biologists and fisheries managers over the winter.

"The numbers of large spawning fish are still relatively low," Fredenberg said. "We still have a strong, healthy bull trout population and we still have kokanee. From those standpoints, we do have opportunities in Swan Lake that we don't have in other systems."

But any suppression effort - which could involve various forms of targeted netting - largely will depend on an analysis of the recent netting results, along with economics and public support, Fredenberg said.

"From talking to some of the anglers and folks who live there, there is a strong interest in trying to keep what they have rather than letting it turn into another Flathead Lake," Fredenberg said.

The netting project produced a huge amount of information that will be evaluated in the coming months.

Of the 2,174 lake trout that were netted, 1,391 were fitted with small tags and released to generate a population estimate based on a "mark-recapture" statistical analysis. Just more than 780 lake trout died during the netting operation or were kept for sampling purposes.

The sample fish will, for instance, be used to establish age classes in the lake. A tiny ear bone, called an otolith, can be examined to determine age in much the same way as trees can be aged by growth rings.

Thirty large lake trout, some weighing up to 22 pounds, were fitted with sonic tags to track their movements. A Montana State University graduate student, Ben Cox, is getting locations on spawning activity that is now under way.

The netting was conducted at a time when adult bull trout were out of the lake and spawning in Swan River tributaries. But 378 bull trout still were caught and 237 were released alive.

The netting turned up fewer non-trout species than expected: 203 pike minnow, 32 mountain whitefish and 39 kokanee salmon.

The netting project was sponsored by Montana Fish, Wildlife and Parks, the U.S. Fish and Wildlife Service, the U.S. Geological Survey, the Confederated Salish-Kootenai Tribes, the Bonneville Power Administration, the U.S. Forest Service, Montana State University and Montana Trout Unlimited.

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