Ben Eisinger loves fishing for bull trout, an activity he looks forward to every season.

Bull trout are difficult to catch, said Eisinger, owner of True Water Fly Shop in Kalispell. The fish are large, native predators favored by anglers. But there are few places left to legally fish for the trout as they face steep population declines.

“When you look at a bull trout you see this big beast of a fish who just fought harder than any fish you’ve ever caught,” Eisinger said. “It’s probably my favorite fishing in Montana.”

Eisinger said he has not seen a decrease of bull trout in the Flathead’s waters, but biologists at Montana Fish, Wildlife, and Parks recently raised the alarm about declines in the number of bull trout spawning nests, called redds, across western Montana.

According to a study, trends show redd declines in around 50% of observed streams and tributaries. While the amount of redds in the other 50% is seemingly stable for now, the species’ overall numbers have decreased.

“They ain’t gonna be what they used to be,” lamented Tom Weaver, a fisheries biologist at Montana Fish, Wildlife, and Parks.

Weaver spearheaded the first efforts of tracking bull trout redd numbers in the Flathead Basin. His team began the work in 1978, publishing data sets since 1980. Bull trout were listed as a threatened species in 1999 under the Endangered Species Act.

Why have bull trout numbers decreased overall? Experts attribute it to habitat loss, invasive species and, more recently, climate change.

According to Clint Muhlfield, a research aquatic ecologist for the U.S. Geological Survey, bull trout are an excellent indicator of watershed health. The fish — though a top predator — are sensitive. They require clean, cold, connected and complex waters.

The species has adapted to these waters for thousands of years and their lives are reliant on years of evolution in the area that is embedded into their genetic makeup.

“As bull trout have declined, you reduce their genetic diversity and abundance throughout the landscape, reducing their ability to persist,” Muhlfield said.

Every fall, the fish migrate north to spawn in tributaries home to cold groundwater. Females lay their eggs in the clean gravel and, around 225 days later, a new generation of fish swim into the streams where they stay for up to three years before traveling to larger, more bountiful lakes.

In disconnected waters, the trout can’t migrate to their spawning sites. In dirty or warming waters, the fish can’t spawn.

The more pressing issue, according to Weaver, is the presence of invasive species — specifically mysis shrimp and lake trout. Since the early 1900s, non-native fish have been stocked in the Flathead Valley, disrupting the food chain.

The addition of mysis shrimp in the mid-90s, and their subsequent entrance into Flathead Lake in the ʼ80s, resulted in a rise of lake trout numbers. But it came at a cost. By 1991, according to Weaver, the effect on bull trout was obvious.

Lake trout, who stay in deeper waters, feed on the mysis shrimp and bull trout themselves — outcompeting the native species. If lake trout invade a system that was previously dominated by bull trout, the bull trout decline, a trend Muhlfield has seen in the Flathead.

“It is alarming how quickly things can change when invasive lake trout are established,” he said.

According to the biologist responsible for redd counts in the West Valley, Kenny Breidinger, the Flathead population has stabilized over the past 20 years. Per basinwide data, the population peaked in 1982 with a total of 1,156 redds counted. In 2008 there were 503 and in 2018 there were 615 total. Last year, biologists counted 493 redds.

Basin wide counts in the West Valley, since 2000, average around 400 to 600 redds, showing some stability. The area of most concern, Weaver said, is Swan Lake, which continues to decline exponentially.

Despite the decline in the native fish’s population, their value has increased — hence why anglers like Ben Eisinger look forward to catching, and releasing, them.

“They’re Montana’s true natives,” Muhlfield said. “They have enormous cultural, socioeconomic and ecological value in the state.”

Nowadays, anglers can legally fish for them in Lake Koocanusa and the South Fork of the Flathead River. Harvest of the fish, while heavily limited, is still allowed in the Hungry Horse Reservoir. Anglers must have a catch card in order to fish for bull trout.

In the meantime, anglers and biologists are looking for ways to mitigate the impacts of the population decrease. Efforts to restore habitats, relocate and actively fish for invasive species help in the long run.

“The system is impacted by humans,” said Jason Blakney, a fisheries biologist with Montana Fish, Wildlife and Parks. “So it makes sense that bull trout now need a hand from us to continue on their life cycle.”

Reporter Kate Heston can be reached at kheston@dailyinterlake.com or 758-4459.