Fall gill netting for lake trout on Swan Lake wrapped up in late October and upstream bull trout spawning surveys revealed a fourth year of below-average results.

The surveys for bull trout spawning beds called redds were conducted in Elk, Goat, Squeezer and Lion creeks upstream from Swan Lake. 

There were 335 redds detected , similar to counts from the last three years and about 40 percent below a 19-year average of about 555 redds.

“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 is a factor,” according to a Montana Fish, Wildlife and Parks press release. 

Biologists started an experimental lake trout removal project on Swan Lake in 2009 using gill nets. The netting work has also led to bull trout “by-catch” mortality.

“The bull trout by-catch associated with this ongoing lake trout removal likely is also reducing bull trout numbers,” the press release states.

Swan Lake netting removed 6,988 lake trout ranging from 5 to 36 inches when juvenile fish were being targeted between Aug. 12 and 30, a period when most bull trout are in the river system on their spawning run.

The bull trout by-catch was 200 fish, with 64 percent surviving.

Netting conducted in October, targeting adult spawners, resulted in 210 lake trout being removed and a bull trout by-catch of 135 fish, with 67 percent surviving and 44 actual mortalities. Netting killed a total of 106 bull trout this year.

Asked how much by-catch is too much, U.S. Fish and Wildlife Service bull trout recovery coordinator Wade Fredenberg said, “There is no magic number.”

What counts, he said, is whether bull trout reproduction remains sufficient to maintain a viable population of juvenile fish.

“As long as we’re seeing redd count numbers that we’re seeing, we’re not concerned that the juvenile bull trout population is going to collapse,” Fredenberg said.

And while redd counts are down the last few years, the number of redds — 335 — is still strong compared even to the larger Flathead Basin, he added.

By-catch, however, is an issue.

“Yes, we are concerned, and we are monitoring that by-catch and we are doing everything we can do to minimize it,” he said.

Another way to look at the Swan bull trout fishery is to wonder what it would look like if there was no netting effort.

“We know what would happen if we didn’t do anything, and that is that in all likelihood bull trout redd counts would be crashing,” Fredenberg said.

The netting effort has removed 35,000 lake trout since 2009, and it is likely the population would have grown exponentially over the last few years without netting, to the detriment of bull trout, he said.

Reporter Jim Mann may be reached at 758-4407 or by email at jmann@dailyinterlake.com.