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Lake Trout Netting in Swan Lake and Swan Valley Bull Trout Redd Counts Completed For 2013

Fishing - Region 1

Wed Nov 06 15:43:00 MST 2013

Lake trout netting: The fifth year of targeted gillnetting to remove non-native lake trout in Swan Lake wrapped up in late October. A total of 6,988 lake trout ranging in size from 5 to 36 inches were removed by gill nets during the juvenile netting portion of the project which was conducted from August 12 to August 30, 2013. This time period was chosen because most mature bull trout are on their spawning run and are absent from the lake. Bull trout bycatch was 200 fish. Similar to previous years, 64 percent of these bull trout were released alive (72 actual mortalities). Kokanee salmon are also caught during netting, but their numbers appear to be stable based on consistent catch rates since 2009.

All lake trout less than 22”in length were cleaned, packed on ice, and sent to local area food banks for distribution. Fish greater than 22” were not retained for food bank distribution because of human consumption guidelines related to mercury content. Those fish were either given to local wildlife rehabilitation centers or used to create lure for wildlife research projects.

Netting conducted along the lake trout spawning area later in the fall (October 7-25) resulted in catching an additional 210 adult lake trout. Spawning lake trout ranged in size from 20 to 36 inches. Bull trout bycatch was consistent with previous years with an additional 135 bull trout captured during fall netting (67 percent of these bull trout were released alive with 44 actual mortalities).

Non-native lake trout were first detected in the Swan River system in 1998, causing concern because of potential impacts to the threatened bull trout population, as well as the popular kokanee salmon fishery. The experimental removal project was initiated in 2009 as a feasibility study to determine if gillnetting is an effective tool to remove lake trout and improve conditions for bull trout and kokanee. Results of the experimental removal project will assist fisheries managers with future management of the lake trout population in Swan Lake.

Swan Lake bull trout redd counts: This is the 32nd year of redd counts in the Swan Drainage (Table 1). Although Elk, Goat, Squeezer and Lion creeks comprise our annual index, assistance from the USFS and Plum Creek has allowed us to complete total basin-wide counts annually since 1995. The basin-wide redd count has averaged 555 during this 19 year period (1995-2013). The 2013 basin-wide count of 335 is approximately 40% below this 19 year average.

Redd numbers in the Swan Drainage increased steadily through the 80’s and 90’s, peaking in 1997 and 1998 at over 800 redds. From 1999 to 2001 numbers declined by about 15%. We observed another 15% decline during the four year period from 2002 to 2005. Redd numbers rebounded in 2006 and 2007, but then steadily declined to the lower numbers observed over the past four years. Mysis shrimp became well established in the 80’s and along with kokanee and more conservative fishing regulations helped fuel the observed increase in the bull trout population.

Lake trout were first detected in the Swan River drainage in 1998 and juvenile fish were first captured in gillnets in 2003, indicating that reproduction was occurring. A graduate study initiated in 2006 revealed that lake trout up to 16 years old were present in Swan Lake, and confirmed that the population has been steadily increasing since the early 1990’s. Although the exact mechanism causing the recent decline in bull trout redd numbers is unknown, it is likely that 20 years of increasing lake trout numbers could be a factor. Lake trout have led to declines in bull trout populations in other waters similar to Swan Lake across the region. Biologists began an experimental lake trout removal in 2009 using gill nets as the primary method. The bull trout bycatch associated with this ongoing lake trout removal could also affect bull trout numbers. However, after several years of declining redd numbers, FWP made a regulation change in 2012 to allow only catch and release fishing for bull trout in Swan Lake. A creel survey in 2009 revealed that anglers were harvesting approximately 180 bull trout annually. Bull trout bycatch mortality in the gillnetting project has been less than 180 fish so the regulation change should offset the additive mortality associated with the experimental removal. Data being collected as part of the removal experiment will be used to evaluate the potential effects of bycatch. Biologists will continue to monitor bull trout redd numbers to determine the effectiveness of potential changes in fisheries management.



Contact: Leo Rosenthal, Fisheries Biologist, 751-4548.

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