SWAN LAKE – The sign on the Highway 83 overlook touts the “large population of bull trout” in this scenic mountain lake.
The 30-foot fishing boat hauling up more than a mile of gill net argues otherwise.
Aboard the R.V. Finn, a crew of five men will spend much of October trying to catch as many invasive lake trout as they can. The non-native predator spread from its stronghold in Flathead Lake into Swan Lake around 1998. The smaller lake’s bull trout and kokanee salmon populations saw immediate declines.
Gill-netting is the Montana Department of Fish, Wildlife and Park’s latest plot twist in a fish story that spans a century of human interventions and unintended consequences.
It began around 1905, when the federal U.S. Fish Commission introduced lake trout from the Great Lakes into Flathead Lake to enhance recreational fishing there.
For decades, lake trout played a minor role in an ecosystem that also featured a huge non-native kokanee salmon population, as well as native bull trout, cutthroat trout, mountain whitefish and various suckers, sculpins, minnows and shiners.
In the 1970s, state fisheries workers added mysis shrimp from Alberta’s Waterton Lake to several small western Montana lakes, including Swan, Whitefish, and Holland lakes. They hoped the freshwater shrimp, each about the size of a macaroni noodle, would boost the kokanee schools like they had in Canada.
It didn’t work that way. Mysis drifted down the Swan River into Flathead Lake, where they became food for lake trout. Juvenile lake trout live in deep water, safe from most other predators until they grow large enough to raid the shallows.
Over the next 25 years, lake trout boomed. Then, between 1985 and 1988, the kokanee population crashed. Spawning runs to Glacier National Park’s Lake McDonald that used to draw hundreds of bald eagles suddenly ceased.
“Kokanee never recovered, bull trout declined, and lake trout came to be the dominant top predator,” a 2011 research paper from the Flathead Lake Biological Station reported. “Lake trout increased 19-fold and lake whitefish 5-fold, largely at the expense of bull trout. ... (K)okanee clearly were very abundant before mysis and completely disappeared subsequently. Kokanee represented 92 percent of the angler catch in 1981, but none of the catch by 1992.”
In 1998, the U.S. Fish and Wildlife Service designated the bull trout a threatened species. And the hunt began for ways to undo the damage.
On a drizzly October morning, Leo Rosenthal was looking forward to fishing from a bigger boat.
Not that the Fish, Wildlife and Parks Swan Lake fisheries management biologist had a bad ride. His 18-foot Boston Whaler with a 90-horsepower outboard motor could get him where he needed to go. It just couldn’t bring in enough lake trout.
“There’s no way I could do this with the equipment I have,” Rosenthal said as he cruised toward the Finn, from Bailey’s Harbor, Wisc. “These guys are the experts in what they do. They’ve also done some of the work on Lake Pend Oreille and Yellowstone Lake.”
Hickey Brothers Fisheries Boat captain Jack Tong stood between the boat’s small cabin and its long steel processing table, inching the boat along a 6,300-foot set of gill nets deployed below Highway 83.
A rotating drum winch with nubby teeth hauled the net out of the water (in his smaller boat, Rosenthal had to raise nets by hand). Tong waited by the gunwale with a gaff and catchnet, watching for fish that might break free as the gill net rose out of the water. Next to him, FWP biologist Jim Duralo coiled the net as it passed by, readying it for its next setting.
On the other side of the table, FWP researchers Brian Vanee of New York and Ben Triano of New Jersey untangled, sorted and processed the fish. They had just a few hours each morning to pull up the night’s catch before the daytime-preferring bull trout and kokanee became by-catch.
The 5- to 12-pound lake trout almost lounge in the net, like career criminals relaxing on their way back to jail. Except they’re going into a cooler, where they’ll become bait for troublesome grizzly bears or food for injured bald eagles. The big spawners have bio-accumulated too much mercury to be safe for human consumption.
Next most common are the suckers – big, geeky fish with snouts like vacuum cleaners and scales like chain mail. No matter how tangled they get, they keep muscling for freedom.
“We very seldom get a dead sucker in the net,” Vanee said as he freed a sucker as big around as his jacketed arm. “You get them untangled and they just swim away.”
Often maligned as “trash fish,” long-nosed suckers and hulking pike minnows are allies in the fight against lake trout. They’ve come to snuffle up every egg they can find on the lake bottom.
While bulls and cutthroats build spawning redds in gravelly creek bottoms, lake trout dump their eggs and sperm between rocks in between 5 and 40 feet of lake water. In another unintended consequence, the boulder-strewn slope supporting the highway above the lake makes a perfect breeding ground for the lake trout.
The Finn waits for them coming and going. Tong and crew set their nets along the highway just before sunset as the lakers come to spawn, removing them after dark. Then they lay the curtain again in the morning to catch the trout as they head back to the depths.
“The goal is to improve or keep things good for Swan Lake,” Rosenthal said. “We’re trying to see if we can remove enough lake trout using gill nets, and then see what’s the response of the kokanee and bull trout. This is still an experiment.”
For about every 20 lake trout, the crew catches a bull trout in the net. Long and lean, the bulls have pinkish-red spots on their green-gray backs. Almost half of them come up dead.
“When they get caught, they go berserk,” Rosenthal said of the bulls. “They thrash around and the net pins their gills shut. Then they don’t last five minutes.”
Any netted bull trout showing signs of life gets placed in a holding bucket and sometimes given a shot of oxygen to help its recovery. The survivors get a tiny acoustic transmitter injected under the skin by their dorsal fin. When close to a receiver box, the transmitter reveals a tracking number that helps biologists determine what the fish has been up to.
Female lake trout that haven’t laid their eggs yet get placed in a separate cooler with a constant oxygen feed. Vanee and Triano use their eggs to test methods of zapping spawning grounds with enough electricity to kill the invasive lake trout eggs without hurting other fish in the water. Their work has been underway for two years.
A phonebook’s worth of agencies and groups has a stake in the bull trout’s defense. The Swan Valley Bull Trout Working Group includes FWP, the U.S. Forest Service and Fish and Wildlife Service, Trout Unlimited, the Confederated Salish and Kootenai Tribes, Montana Department of Natural Resources and Conservation and Montana State University. They’ve been busy since 2008, and have three more years of funding to go.
“Based on what’s happened in other areas, the writing’s kind of on the wall,” Rosenthal said of rebalancing the fish populations. “In the Flathead, it’s too late to save the kokanee. But here, the numbers are still relatively good.”