MISSOULA – A targeted attack on invasive lake trout appears to be succeeding in the Swan Valley, and could hold some answers for how best to reduce the non-native fish in nearby Flathead Lake.
“We’re pretty excited about what we’re accomplishing on Swan Lake,” said Wade Fredenberg. “From what we’ve seen, we think we’re being very successful in reducing the lake trout numbers.”
Fredenberg works for the U.S. Fish and Wildlife Service, where he is in charge of recovering Montana’s endangered bull trout populations. He’s killing lake trout by the thousands because lake trout kill bull trout.
The invaders tend to outcompete the natives, he said, and often make a meal of bull trout. In Glacier National Park waterways, biologists have found that, once introduced, lake trout will completely replace bull trout in about 30 years.
In 1998, anglers discovered non-native lake trout in Swan Lake. That was the same year native bull trout were given protections under the Endangered Species Act.
Since then, many millions of dollars have been spent improving bull trout habitat, and securing their headwater spawning grounds. But until now, Fredenberg said, little has been done to address the lake trout invasion, “and lake trout are, without a doubt, the limiting factor to bull trout recovery.”
Last summer was the first in a three-year program to hit Swan Lake’s invasives hard, and Fredenberg — along with colleagues at the state Department of Fish, Wildlife and Parks — recently released a report on their progress. The early news, they wrote, is promising.
The Swan’s explosive lake trout population, they estimated, was perhaps 9,000 strong coming into the 2009 summer, and “is a growing threat to bull trout populations in Swan Lake, the Swan River system, and interconnected Lindbergh and Holland lakes upstream.”
If left unchecked, Fredenberg said, the lake trout will simply take over.
And so a coalition of federal, state, tribal and non-governmental biologists began collecting data several years ago, identifying lake trout spawning areas, learning how and when to best to catch the fish. They netted the lake, then implanted tracking devices into individual trout.
Fish movements indicated that spawning sites were highly concentrated, covering just five acres on the 2,700-acre lake.
“It allowed us to really focus our attentions,” Fredenberg said.
Scientists also learned when the bull trout left the lake to spawn upstream. That allowed the scientists to reduce the number of natives accidentally caught in their nets. They learned what seasons were best, and what times of day, and at what depths they could best catch lake trout.
“It’s just fishing,” Fredenberg said, “and the more we keep at it, the better we get at catching what we want to catch.”
Last summer’s efforts were two-pronged: contract netting aimed at juvenile and subadult lake trout, and spawner netting aimed at adult fish.
They had hoped to catch about 4,800 lake trout during the late-summer contract netting, but eventually hauled in a whopping 5,213. Most were small, as targeted, and went to area food banks.
The nets also snagged 238 bull trout, which was not nearly as many as had been accidentally caught in previous years. More than half of those natives were released alive.
Then, in October, netters returned to Swan Lake, focusing on the adult spawners. In 16 days, they nabbed 239 lake trout at the spawning grounds. They were mostly male, and mostly big.
Combined, the operations netted more than half the estimated population, “which is important,” Fredenberg said, “because the bottom line is, we’ve had a target of taking out 50 percent of the lake trout. If we can hit 50 percent annually, we can start to push the population down.”
A bulk of the overall lake trout population isn’t even counted, he said, because they’re small — 3 years old or younger — and so slip through both the monitoring and the catch nets. That’s why catching at least half of the known fish is so important to actually reducing population numbers.
“Right now,” Fredenberg said, “we think we’re on the right track.”
Fredenberg has plans for fine-tuning the tools he has in 2010, and for finding new tools, as well.
He’ll tweak net placements and timing, and he’ll focus even more accurately on spawning lake trout. And he’s working on a machine that could hit invasives “at the spigot” — which is to say at the egg nest.
The Swan, he says, still has one of Montana’s most stable bull trout populations, and is one of the few places where anglers can still drop a line for the protected fish.
“I believe we can’t afford to lose that,” he said. “That’s Montana’s native heritage.”