With native fish declining in Swan Lake, wildlife officials target invasive trout

Hoping to preserve dwindling native bull trout populations, state wildlife officials are looking to remove lake trout from Swan Lake.

“There’s a sense of urgency to do something before it’s too late,” said Leo Rosenthal, a fisheries management biologist with Montana Fish, Wildlife and Parks in Region 1.

Bull trout, which are native to the valley, have struggled since the introduction of invasive species like lake trout, rainbow trout and mysis shrimp in the region. The decision to develop a plan to remove lake trout came after state biologists observed a historically low number of bull trout redds, or spawning nests, in the Swan’s watershed system in 2023. 

The first step, which residents can comment on until Jan. 23, requires seeking the Fish and Wildlife Commission’s permission to initiate a public MEPA — Montana Environmental Policy Act — process where the removal of lake trout from Swan Lake will be evaluated for its capacity to avoid, minimize and mitigate damage to the environment. 

Details about the removal operation are scarce because the commission has yet to give the state wildlife agency the authority to create a plan. If the commission gives it the green light in February, then more details about the fish removal effort will be released for another round of public consideration. 

Swan Lake’s declining bull trout numbers continue to be an issue and have been for decades, according to Rosenthal. This proposal is not the first of its kind, he said.

From 2009 to 2016, state and federal officials conducted an experimental lake trout removal project using gillnetting, studying to see if it was an effective way to remove the invasive species in Swan Lake. Results from the study showed that gillnetting increased mortality of some lake trout populations. 

Gillnetting is likely a tool that the state wildlife agency will employ if the removal proposal is approved, officials from Montana Fish, Wildlife and Parks said. 

Since 2016, bull trout numbers have steadily declined, as have numbers of kokanee abundances, another fish that struggles to cohabitate with lake trout. Lake trout numbers increased considerably in that time, the state agency found. 

“With this, we are taking everything that we learned from that first project … and being more efficient with our netting process out there now,” Rosenthal said. 

Scientists have also developed an independent lake trout monitoring survey since 2016, Rosenthal said, which will allow the state wildlife agency to better monitor the effort’s effectiveness. 

Lake trout have been a part of the Swan Lake system since the ʼ80s, Rosenthal said. The fish were found in the lake around the same time that the species’ population exploded in Flathead Lake. 

Lake trout most likely gained access to Swan Lake by way of a fish ladder on the Bigfork Dam, Rosenthal said. In 1992, the ladder was declared inoperable, but the dissemination of the species already occurred. Tests completed by the state agency confirmed that lake trout had been in Swan Lake since the late 1980s. 

“There’s a sense of urgency now, in this last year of doing our redd counts, the number of bull trout redds for the Swan system has reached an all time low,” Rosenthal said. 

The removal of lake trout in Swan Lake could have a two-pronged benefit, according to Dillon Tabish, the information and education program manager for Montana Fish, Wildlife and Parks Region 1. While the focus remains on the prolonged success of bull trout populations, the effort could aid the kokanee abundance. 

“We do see the benefit of this proposal benefiting not only bull trout, but this will also benefit kokanee, which is another important fish,” Tabish said. 

The dual mandate of protecting and promoting fishing opportunities while protecting native species poses a challenge, Tabish said. But the situation in Swan Lake is dire for bull trout. 

“We want to do something to help address that situation, but the tools in the toolbox are pretty limited when it comes to bull trout. One of the tools, however, is removing a threatening species like lake trout that does have an effect on their numbers,” Tabish said. 

The ongoing public comment period is the first such period in the process. If the Fish and Wildlife Commission approves the initial proposal at their Feb. 16 meeting, Montana Fish, Wildlife and Parks will move forward with an Environmental Assessment, which would go out for further public comment. 

Actual efforts on the lake, if approved, are expected to begin in 2025. 

Public comments can be submitted until Jan. 23 at https://fwp.mt.gov/aboutfwp/commission/february-2024-meeting.