GLACIER NATIONAL PARK — In civilization, invaders change the language, diet and customs of the places they conquer. Invasive fish don’t ride on chariots or tanks, but their disruption leaves almost warlike marks on the ecology.
That contest plays out right now between Montana’s native bull trout and invasive lake trout in the Flathead River Basin. New research indicates that while the lakers have run like Genghis Khan, the bulls might hang on if they get help.
A new study from the University of Montana’s Flathead Lake Biological Station shows just how big and permanent an impact invasive species have had on regional waterways. But it also gives wildlife managers a much better tool to tell if something like the Mack Days fishing derby or full-scale gillnetting will actually make a difference.
“Once we get to a tipping point, things go bad quickly for bull trout,” said Shawn Devlin, an aquatic ecologist at the Flathead Biological Station and co-author of the study.
“This work has showed if you give bulls a chance before that tipping point — before they’re in a spiral they’ll never come back from — they can be managed for conservation,” he added. “And the good news is, these lakes were invaded a lot longer than anyone realized but it took longer than expected for the effects to take hold. It was a neat finding. That gives hope to managers, that there’s more time below that tipping point than we realized.”
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Bull trout in lakes play the same ecological role as grizzly bears on land — the No. 1 predator in their native habitat. They eat other fish, grow large and reproduce slowly.
Lake trout fill a similar niche in their home waters of the Great Lakes and Midwest rivers. But they have a crucial spawning advantage.
Bull trout live a salmon-like life cycle of hatching in small creeks before reaching maturity in big rivers and lakes and then returning to spawn in that same creek they were born in. That makes them vulnerable to lots of other predators when young, as well as human threats like river dams, irrigation systems, and sedimentation from logging or road-building.
Lake trout spawn on deep-water rock outcrops. While grizzlies and eagles can harry bull trout in their shallow spawning streams, few competitors reach the lake trout egg deposits. And when they grow up, the lakers eat the same fish bull trout target.
Since they were artificially introduced into Flathead Lake in the early 20th century, lake trout have become a popular game fish because of their capacity to reach lunker size. Then a separate effort to enhance Swan Lake’s artificial Kokanee salmon population by adding mysis shrimp had an unintended consequence. The tiny shrimp flowed down the Swan River into Flathead Lake, where they became a new food source for the lake trout. The laker population quickly expanded, sending ripples through the ecology of every other fish species in the system.
Such transformations are called “trophic cascades.” In Flathead Lake’s case, young lake trout outcompeted the Kokanee for zooplankton and other tiny organisms, while mature lakers ate the schools of Kokanee out of existence. They also preyed on the native cutthroat and bull trout, depleting both their populations and their food supplies.
And then the lake trout started spreading through the Flathead River network, invading the bull trout strongholds of Glacier National Park’s west and south sides. Hungry Horse Dam prevented them from getting far into the Bob Marshall Wilderness Complex to the south. But McDonald, Logging, Quartz, Bowman and Kintla lakes all saw their bull trout populations crash.
Study lead author Charles Wainright of the U.S. Geological Survey spent 49 days prowling 10 remote Montana lakes. That included Glacier Park battlegrounds like Quartz and Logging and Arrow lakes, as well as sites in the Bob Marshall Wilderness Complex that still have undamaged bull trout habitat, such as Big Salmon Lake.
The bodies of water in the park comprise almost a third of the entire bull trout habitat in the Lower 48 States. Bull trout have “threatened” status under the federal Endangered Species Act. Preserving them has inspired several responses, from the Confederated Salish and Kootenai Tribes’ Mack Days fishing derby to multi-agency efforts to transplant bull trout into remote Glacier lakes that lake trout can’t reach.
“If we lose bull trout out of these lakes, the system will never shift back to what it looked like,” Devlin said. What the study found was that one species doesn’t just over-eat the other. Everything around them gets affected.
“The whole lake is important, not just the traditional food path of small things to big things,” Devlin said. “Bull trout are not good at finding other food. When they can’t get the large fish they used to eat in middle of the lake, they’re forced into the shallows and littoral zones with sub-optimal food. Then their growth rate gets stifled. Meanwhile, lake trout are growing like gangbusters.”
That change also affects everything around the two trout species: the phytoplankton, insects, frogs, spiders and everything else that feeds from the lake or falls into it. As bull trout shift from eating other fish to eating bugs, that affects bug populations as well as other trout like cutthroat and rainbow that hunt bugs. The entire food web gets frazzled, and can fray apart.
Which brings up the other important finding of the study: the time factor.
“It’s like if you thought you were late to a party, and you get there and you’re right on time — then, great,” Devlin said. By looking at both the ratios of invader fish to native fish, and what everything was eating, the study gave ways to gauge how far along — how close to permanent — an invasion had become. And it turned out, the process takes longer than most researchers expected.
That gives wildlife managers more options. Late-stage interventions might have to be as complicated as Glacier Park’s effort to create bull trout sanctuaries while gillnetting infested lakes. An early invasion might fall to simple fishing regulations, like no-limit takes on lake trout in protected waters. Flathead Lake has passed that point, prompting bigger efforts like commercial fishing and the Mack Days derby.
This marks the final week of the 2021 Mack Days, which brought in 12,939 lake trout. Private anglers competed for cash prizes in a variety of formats, from the heaviest daily catch to hooking tagged lake trout worth up to $10,000.
“Flathead Lake historically was filled with native bull trout and westslope cutthroat trout,” CSKT spokeswoman Stephanie Gillin said. “People tell stories of watching the ‘flats’ rising in the evenings in Big Arm Bay and what a beautiful sight it was. Elders have told about ice fishing out by Wild Horse Island and having to make larger holes in the ice to get the bull trout out. Anglers out in boats caught bull trout that were large and fought valiant fights at the ends of twisting lines.”