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Swan Lake 2009 Update

There’s good news and not-so-good news from this year’s lake trout netting on Swan Lake. Netting this year resulted in removal of over 5,500 invasive lake trout from Swan Lake. Agencies report that this represents about 68% of the remaining lake trout population and about 1/3 of the spawners. Approximately 44% of the population was removed in 2008. They estimated that they need to remove at least 50% of the lake trout population each year to suppress the population enough to give bull trout the opportunity to respond. That goal was easily accomplished this year.

The lake trout population in the Swan drainage has virtually exploded since they were first confirmed in Swan Lake by anglers in 1998. The population is now estimated to be equal to, if not greater than that of native bull trout. Lake trout are voracious, heavy predators on native fish. Redd counts of both bull trout and kokanee in the Swan drainage were down this year, although no estimate was made on how much of the decline was due to lake trout. The goal of the three-year project, carried out by a professional contracting consultant, is to “to determine the feasibility of using gill nets as a management tool for reducing the population of lake trout in Swan Lake“. Netting efforts this year seem to confirm the aim of the experimental project. Specifically, this year’s netting confirmed that it is possible to narrowly target the invasive lake trout population, using radio tags and timing of the netting, with little to no effect on native fish populations. Only 238 bull trout were taken in this years netting and only 205 kokanee. About half of the bycatch of native fish survive netting. All moderate-sized lake trout captured were donated to area food banks where they proved to be a popular item. In the case of larger fish which contain higher levels of environmental mercury, fish were donated to the Montana Raptor Institute in Polson.

Netting, over the three year project lifetime, is expected to reduce the lake trout population in Swan Lake to around 1,000 fish. Now the bad news; Lake trout reached nowhere near their carrying capacity in Swan Lake. Without the suppression caused by netting, the lake trout population is likely to double every two years. Lake trout have now spread throughout the drainage. They have been confirmed in Lindbergh Lake where they have established a reproducing population and it is likely that there is also a population in Holland Lake, although that has not been confirmed. The Swan Valley Bull Trout Working Group will use the results of the current experimental netting project to determine what kind of effort will be necessary to keep the population of invaders at bay into the future. A smaller netting operation every two to three years may be needed to keep the lake trout population from rebounding too quickly. The problem, of course, is funding. The current effort requires about $60,000 per year for the netting operations and all of that funding has not been secured yet. Whether funding for future efforts can be obtained will have to be worked out. For now, the group estimates that this project will gain the native fish in Swan Lake about a 10-year reprieve.

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