KALISPELL - An annual stream survey of native bull trout in the Swan River drainage revealed a continued decline in reproduction, while the number of spawning trout in the North Fork Flathead Basin also fell below average.
The survey showed improved spawning in the Flathead's Middle and South Fork drainages, but biologists say the slumping population in Swan Lake is troubling, particularly given the basin's history as a regional stronghold for bull trout. Even more disturbing is that the principal cause of the decline, invasive lake trout, is affecting numerous other lakes and streams in the Flathead Basin.
The trend is unlikely to reverse itself unless effective conservation and management programs are established.
Field crews with Montana Fish, Wildlife and Parks conducted the 2011 bull trout spawning surveys between Sept. 26 and Oct. 28 in ideal water flow conditions. The agency's long-running counts of "redds," or streambed spawning nests, serve as an index of how many adult bull trout successfully spawn each year.
This was the 30th year of redd counts in the Swan River drainage, with the annual index comprising four spawning streams. Since 1995, assistance from the U.S. Forest Service has allowed FWP to conduct basinwide counts, with an annual average of 651 redds recorded during that 16-year period. The 2011 basinwide count of 312 redds is 52 percent below the average.
"What we saw this year followed the trend that we have seen since 2007, which was the last good year for bull trout in the Swan," said Leo Rosenthal, a fisheries biologist for FWP. "Since then we have seen a steady decline in redds, both in the index reaches and the basinwide count."
After peaking at more than 800 redds in 1998 - the same year that lake trout were first detected in Swan Lake - bull trout redd numbers in the Swan River drainage plummeted, dropping by about 30 percent between 1999 and 2005. They rebounded in 2006 and 2007, but have steadily declined since.
Most biologists agree that the decline is due in large part to predation by non-native lake trout, which exploded in numbers beginning in 1992 and have dramatically changed the dynamic of the aquatic food web throughout the entire Flathead basin.
Rosenthal said lake trout have only increased in abundance since they were first detected in Swan Lake.
Some biologists believe an experimental gill-netting program to suppress the invasion of lake trout is important to conserving the remaining bull trout population, which was once robust in the Swan River drainage. However, the project, which just finished its third year, has also resulted in accidental bull trout kills.
"We know that some of those fish are caught in the nets, but we are keeping tight tabs on those numbers," said Rosenthal, one of the suppression program's leaders. "It is difficult to tease out exactly what is causing the decline in redds, but it is probably a combination of stressors, including lake trout and by-catch mortalities."
Beginning this year, FWP officials removed one sure stressor by changing angling regulations. For years, Swan Lake has been managed with a one-per-day bull trout fishing regulation, with annual harvests averaging 180 fish. A shift to catch-and-release regulations was approved by the FWP Commission on Nov. 10.
"Removing the one-a-day harvest in Swan Lake is the one thing that we can control," Rosenthal said. "If this lake trout removal project is worth continuing, the one thing we can control is anglers."
Researchers will spend the winter crunching data gathered from the three-year removal project to determine if it is effective. They'll also consider whether the benefits of the suppression project outweigh the number of bull trout killed as by-catch mortalities. Rosenthal said crews removed more than 5,000 juvenile lake trout this year, and about 280 adults - roughly half of what they caught last year.
"It's still too early to tell whether this is effective," he said.
Aquatics ecologist Clint Muhlfeld, whose research at the U.S. Geological Survey's Glacier National Park field station is meant to inform conservation and management programs, said suppression programs are critical to protecting "the best of the last" native fish species in Montana.
Unfortunately, native bull trout are already on the brink of catastrophe in many lakes infested by lake trout, including a system of mountain lakes upstream from Flathead Lake in Glacier National Park.
Of the 12 lakes west of the Continental Divide in Glacier National Park that are connected to Flathead Lake - all of them home to previously healthy populations of bull trout - 10 have been invaded by lake trout. Bull trout are now functionally extinct in eight of them, Muhlfeld said.
"The trend we are seeing in Swan Lake is very consistent with what we are seeing in the rest of the system," Muhlfeld said. "These lake trout have radiated out from Flathead Lake and infested other lakes, which is causing the dramatic declines we're seeing."
Lake trout were first planted in Flathead Lake a century ago, and for decades the population remained relatively small, co-existing with native bull trout and westslope cutthroat trout. But in the 1980s, when mysis shrimp were introduced, the historic food web began to unravel.
Lake trout suddenly had an unlimited protein source at hand and the population exploded, with the non-native species outcompeting bull trout and native westslope cutthroat trout at an alarming rate.
Bull trout were listed as "threatened" under the Endangered Species Act in 1998, but the Flathead Lake aquatic community hasn't been the same since.
In the past 12 years, management agencies have tried to steady the native bull trout populations in the reaches of Flathead Lake's eight tributaries, some of which have remained relatively stable. Other local populations have shrunk so quickly that they may not be viable much longer.
Redd counts in the index reaches of the North Fork Flathead River were 54 in 2010 and 65 in 2011, considerably below the previous 12-year average of 107 redds. Researchers counted just eight redds in Trail Creek this year, and only six in Coal Creek.
"We need to protect what we have left, because if we don't the writing is on the wall," Muhlfeld said. "There is not one situation I'm aware of in which lake trout have invaded and become established and we haven't seen the bull trout numbers plummet."
Muhlfeld has been part of a program to remove lake trout from Glacier National Park's Quartz Lake, which, like Swan Lake, is still in the early stages of invasion. He said experimental suppression programs are critical to controlling the invasion of lake trout, and should be implemented before it's too late.
"We are writing the manual for how to deal with this invasive species, but we have to do something fast," he said. "Because unless effective conservation and management programs are conducted immediately, we are likely to lose bull trout populations that we have had for 14,000 years. Bull trout are part of Montana's heritage, and we want them around for future generations to enjoy."
Reporter Tristan Scott can be reached at (406) 730-1067 or at firstname.lastname@example.org.