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Montana Redd Counts: Invasive Lake Trout Increase Leading To ESA-Listed Bull Trout Declines
Posted on Friday, November 18, 2011 (PST)

Recent stream surveys have revealed an alarming trend in declining bull trout reproduction in Montana's Swan River drainage, along with troubling numbers in the North Fork Flathead Basin and improving spawning in the Flathead’s Middle and South Fork drainages.

 

Montana Fish, Wildlife and Parks continued long-running counts of “redds,” or distinctive streambed spawning nests, between Sept. 26 and Oct. 28 in water and weather conditions that were optimal for the surveys.

 

This was the 30th year for redd counts in the Swan drainage, focusing on index sections in four spawning streams. The basinwide count has averaged 651 redds over the last 16 years and this year’s count of 312 is 52 percent below that average.

 

The drainage’s counts peaked at more than 800 redds in 1998, but there have been substantial declines in years since then.

 

Non-native lake trout, which are bull trout competitors and predators, first were detected in Swan Lake in 1998.

 

“Although the exact mechanism causing the recent decline in bull trout redd numbers is unknown, it is likely that the increasing lake trout population could be a factor,” according to a press release explaining the recent stream surveys. “Lake trout have led to declines in bull trout populations in other waters similar to Swan Lake across the region.”

 

Fisheries biologist Clint Muhlfeld said the decline in bull trout is following a familiar pattern.

 

“The Swan counts are low because lake trout have invaded and become well established in that system,” said Muhlfeld, a fisheries biologist with the U.S. Geological Survey who is based in Glacier National Park. “That’s the classic patterns we’ve seen in all these lakes.”

 

Montana Fish, Wildlife and Parks is three years into an experimental gill netting program aimed at suppressing lake trout numbers in Swan Lake, and that effort is important to conserving the remaining bull trout, Muhlfeld maintains.

 

After bull trout were listed as a threatened species in 1998, Swan Lake was one of few remaining places where anglers could catch and keep a bull trout.

 

Recently, however, the Montana Fish, Wildlife and Parks Commission shifted back to catch-and-release regulations for bull trout on Swan Lake, partly because of below-average redd counts in the drainage.

 

Elsewhere, the combined counts of redds in the Middle and North Fork Flathead River drainages account for bull trout spawners that migrate from Flathead Lake. Combined, this year’s count of 189 redds in eight stream index sections in the two drainages is about average for the past decade.

 

The Middle Fork index counts of 136 last year and 124 this year are considerably higher than the annual average of 86 redds over the previous 12 years, but counts in the North Fork index reaches of 54 and 65 redds last year and this year are well below the 12-year average of 107 redds.

 

“The bottom line is the North Fork counts aren’t that great,” Muhlfeld said, pointing to counts in Big, Coal and Trail creeks that have numbered fewer than a dozen over the last couple years, just a fraction of the numbers found in the 1980s and 1990s.

 

This year’s counts of 65 redds in the North Fork streams are nearly at the record low counts of 52 in 1997 and 44 in 1998.

 

Muhlfeld said it’s important for people not to over-generalize the Flathead Basin’s overall redd count numbers, which can appear somewhat stable.

 

“If we talk total numbers things look OK, but if you look at the local populations, the different pieces of the puzzle, those local populations dictate the viability of the bull trout metapopulation,” Muhlfeld said.

 

Of particular concern are the lakes on the west side of Glacier Park that have been invaded by lake trout from the North Fork Flathead River.

 

There are 12 lakes in the park connected to the Flathead Lake and river system, and all those lakes historically have had bull trout populations. Of the 12, nine have been invaded by lake trout and “eight of which the numbers [of bull trout] are so low they are on the brink of extinction,” Muhlfeld said.

 

Muhlfeld has led redd counts in streams above the Glacier lakes and has found minimal evidence of spawning activity.

 

Three redds were counted above Logging Lake this year and in previous years, none were found. There have been similar results for most of the other park lakes that have been invaded by lake trout.

 

The bright spot in this year’s stream surveys came from the South Fork Flathead drainage above Hungry Horse Dam, which has functioned as a barrier to lake trout invasion.

 

The count of 124 redds in South Fork spawning streams feeding into Hungry Horse Reservoir is about 46 percent above the 18-year average of 85 redds for the same stream sections. The 2011 South Fork basinwide total, including spawning streams in the Bob Marshall Wilderness, numbered 610 redds, the highest count on record.

 

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