Montana Fish, Wildlife and Parks has released a report evaluating a multi-year experimental gill-netting project on Swan Lake that has resulted in the removal of 21,330 lake trout.

Within a month, the department plans to release an environmental assessment that proposes a five-year extension of the project, despite concerns about the “by-catch” mortality of bull trout, the threatened species the project is aimed at helping.

By-catch refers to bull trout inadvertently netted during efforts to catch and remove lake trout.

From 2009 through 2011, a total of 21,330 lake trout from 6 to 36 inches long were removed from the lake.

There were 5,213 caught in 2009, 10,021 in 2010 and 5,165 in 2011.

Last year’s declining catch, which included a greater number of young lake trout partly because of smaller net mesh, suggests that the spring and fall netting efforts were effective in reducing the abundance of lake trout, which first were detected in the lake in 1998.

Leo Rosenthal, a state biologist who is the leader of the multi-agency project, said continuing netting is necessary to determine its long-term effectiveness.

“Some of that has yet to be seen in how it translates to the lake trout population as a whole,” he said. “What we’ve seen so far is that the mortality rates on these juvenile fish are higher than the literature suggests are sustainable.”

In other words, effectively suppressing younger age classes of lake trout creates an unsustainable population.

Prior to the project getting under way in 2007, a bull trout risk analysis was conducted.

“At that time, we estimated an adult bull trout population of around 5,000 fish in the Swan Lake core area,” the report states. “We knew that from extensive surveys that bull trout spawned annually at detectable levels in at least 10 Swan Lake tributaries.”  

Surveys of those tributaries for bull trout spawning nests, called redds, haven been conducted since 1995. The highest aggregate count was 833 redds in 1997, with relatively stable results up until 2007, when 762 redds were counted.

But every year since then, there has been a substantial decline, to the lowest-ever count of 312 in fall 2011.

That prompted Montana Fish, Wildlife and Parks in March to go to catch-and-release-only fishing for bull trout on Swan Lake. Before that happened, Swan Lake was the only place in Western Montana where anglers could keep bull trout (the limit was one per day).

While gill netting started in 2007 and continued in 2008, a “comprehensive experimental lake trout removal effort” has been conducted on Swan Lake from 2009 through 2011.

Over the five years of netting, a total of 1,523 bull trout were netted, with more smaller fish rather than spawners being captured over time.

About 40 percent of those fish died as a result of entanglement in gill nets. Netting crews used special techniques for reviving netted bull trout and developed a method for estimating the probability of revived fish surviving once they were released.

Applying that method, the calculated estimate of total bull trout gill net mortality was 792 fish since 2007.

“By-catch of non-targeted fish species other than lake trout was relatively low throughout the project,” the report states. “However, despite efforts to minimize by-catch mortality (the timing netting, location of nets, reviving captured fish), the inadvertent catch and associated mortality of bull trout remains a concern. This concern is compounded by an observed decreasing trend of adult bull trout abundance (reflected by redd counts) in recent years in the Swan drainage.”

“It’s obviously something we’re keeping close tabs on,” Rosenthal said. “It’s a research project, so there are risks we are taking with this project.”

When the environmental assessment proposing a five-year extension of the project comes out within the next month, there will be a public comment period.

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