Biologists report that the interagency project to sample and begin to reduce the lake trout population last fall in Swan Lake came off successfully. The fish sampling and removal took place over about a three-week period in September. The sampling enabled biologists to determine how many lake trout lived in Swan Lake by using a "depletion estimate" technique. By applying a formula to the number of lake trout from each sampling period, biologists calculated the size of the original lake trout population that existed before sampling began. This technique yielded a population estimate of about 8,800 lake trout from 7-36 inches in length.
In the 2008 sampling, FWP and others contracted with professional fishery consultants to conduct gill netting, using over two linear miles of nets at each setting. Netting was systematically deployed for 2-3 hours and retrieved, eventually blanketing the two major areas of the lake greater than 60 feet deep over 15 days.
In total, 3,784 lake trout in the 7-36-inch size range were removed during the sampling. Of these, approximately 70 adult fish ranging from 20-36 inches were caught and removed during the netting. By-catch of other fish species was relatively low throughout the netting effort. Bull trout by-catch was lower than expected, with about 240 being caught, and half of those being safely released. Very few other species of fish were captured during the netting due to the deep-water habitat the nets were set in.
In another result from the project, biologists have implanted sonic tags in 18 adult lake trout so their movements could be followed. Biologists tracked these larger fish and identified two major spawning locations along the east shore of the lake, where eggs were also located on the rocky substrate.
Based on the success of sampling over the past two years, agencies propose to move forward with an experimental 3-year plan to further reduce the number of predatory lake trout in Swan Lake. According to FWP Fisheries Biologist Leo Rosenthal, this plan will be detailed in an Environmental Assessment to be released this spring. The project is motivated by the fact that nonnative, predatory lake trout threaten the bull trout and kokanee populations and fisheries of Swan Lake.
The project is expected to cost about $65,000 per year, with most of the expense going to the contract netting effort. Information obtained from the proposed action will aid in understanding the impacts of lake trout on the aquatic ecosystem in Swan Lake, and will determine feasibility and effectiveness of alternatives for managing the lake trout population in Swan Lake. It will also result in a reduction of the lake trout population. All salvageable-size lake trout sampled during the project not used for scientific investigations will continue to be culled and donated to food banks or other facilities.
Lake trout were first noted in the Swan drainage in 1998 and evidence has indicated the population is growing rapidly. Sampling has confirmed this. Swan Lake is presently one of the few places in the nation where anglers can keep one bull trout per day. Anglers should carefully identify any trout caught and take care in releasing bull trout. Anglers are encouraged to keep lake trout they catch if they can positively identify them. The daily limit on lake trout in Swan Lake is 10 fish. Anglers are asked to report to FWP any large lake trout caught which contains a sonic tag, resembling a lipstick tube, implanted in the body cavity.
The lake trout project on Swan Lake is being conducted through a collaborative interagency effort. Cooperators include: the U.S. Fish and Wildlife Service, the Montana Cooperative Fishery Research Unit from Montana State University, Montana Fish, Wildlife & Parks, U.S. Forest Service, Montana and Flathead Chapter Trout Unlimited, the Confederated Salish and Kootenai Tribes, Montana Department of Natural Resources and Conservation, and Bonneville Power Administration.
Recent News
