Tentative results from Swan Lake gillnetting reported

Montana Fish, Wildlife and Parks personnel have wrapped up gillnetting on Swan Lake for the year with some new discoveries to report. The project began in 2009 in an attempt to reduce the non-native lake trout population in the lake.

While data is still being analyzed and compiled, state fisheries biologist Leo Rosenthal said catch numbers appeared to be similar to last year. Biologists discovered a new spawning area on Swan Lake this year. They previously knew of only one spawning area in the lake.

Netting takes place twice a year for about three weeks. Juvenile lake trout are targeted in August and netting targets spawning adults in October. The goal is to learn if this level of gillnetting is effective at controlling lake trout populations and to establish a plan to protect the native bull trout and kokanee salmon.

Lake trout were first discovered in Swan Lake in 1998 — the same year bull trout were classified as a threatened species under the Endangered Species Act. Swan Lake has been home to one of the strongest bull trout populations in Montana and the Columbia River Basin.

The discovery of lake trout in the lake caused alarm because of their reputation for rapidly expanding and dominating fish communities. Concern increased in 2003 when biologists gillnetted juvenile lake trout in Swan Lake during low-intensity sampling. The discovery of young fish indicated wild reproduction was occurring. Seven more juvenile lake trout were caught the next year and 28 more in 2005.

Biologists continued to monitor fish populations in the lake. In a 2006 Montana State University survey, 194 lake trout and 110 bull trout were caught. Further research was conducted over the next two years to more closely determine the number of lake trout in the lake. Biologists came up with about 8,800.

FWP initiated a three-year gillnetting experiment in 2009 to see if that method would be effective in controlling lake trout populations in a way that benefited bull trout and kokanee. About 20,000 lake trout were removed from Swan Lake between 2009 and 2011.

However, it was still unknown how the netting affected the lake trout population. In May 2012, FWP proposed extending the removal project for another five years to further evaluate the effectiveness.

While similar netting experiments have been conducted on other lakes with no definitive success, Swan Lake is different because of the early stage of lake trout establishment. There is also significant data recorded from before lake trout were discovered, which will help determine the effectiveness of the netting efforts, Rosenthal said.

“This is early enough in the establishment of lake trout that it provides some of the better opportunities to do something like this,” Rosenthal said. “It’s a test. Every one of those lakes is different from each other. It’s really a case by case basis.”

The project costs approximately $90,000 a year and is funded by FWP and the Swan Valley Bull Trout Working Group, which comprises five government agencies and Trout Unlimited. FWP also tracks the population of bull trout and kokanee every year.

Kokanee salmon, while not native, are an important fish from a recreational perspective. While bull trout had seen a bit of a decline those fish populations seem to have stabilized, Rosenthal said. The goal of the netting project isn’t to get rid of lake trout, but to control the population before they change the dynamic of Swan Lake.

The project will be completed in 2016, at which time biologist hope to be able to create a management plan based on what they’ve learned during the project.

“I’m hoping the data will speak for itself,” Rosenthal said.