Lake Trout Invasion Continues to Expand

Nonnative lake trout were first discovered in Swan Lake in 1998 and the rapidly expanding population jeopardized both the ESA-listed native bull trout population and the popular kokanee salmon fishery. Bull trout and kokanee numbers plumetted as lake trout boomed. In 2005, an intensive lake trout suppression program was begun on Swan Lake to reduce the numbers of lake trout and provide some relief to other fish species in the lake.

In the last couple of years, we have begun to see some positive results from the lake trout suppression effort on Swan Lake with increased numbers of juvenile bull trout, but we are now realizing how acute the problem really is. Lake trout have now invaded the entire Swan River drainage and presently endanger an expanding number of sport fish populations. A couple of years ago, lake trout were found in Lindbergh Lake in the upper basin and recently, we have learned that lake trout have infested Holland Lake. Both of these lakes support small and fragile populations of native bull trout which are now at risk of extirpation due to lake trout predation. The Holland Lake bull trout population numbers as few as 50 adult fish. Due to the small numbers of native fish in both of these lakes, netting suppression efforts are likely not possible due to the hazard to the threatened bull trout from bycatch.

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Based on the size of the captured fish and our movement results, it is likely that there are reproducing populations of lake trout in both Holland and Lindbergh lakes. Further, it appears that lake trout are likely more abundant and established in Lindbergh Lake compared to Holland Lake (as of 2013) because daily catch rates were much higher in Lindbergh Lake and a number of sub-adults were captured. Only adult lake trout were captured in Holland Lake suggesting that invasion may be relatively recent.

Results so far are preliminary, but as more data are collected on the movement of the invasive lake trout, scenarios for possible suppression efforts may be developed that will help us to keep at least some remnant of our native fish populations in the upper Swan River drainage.