In the coming weeks, Montana Fish, Wildlife and Parks will propose a management plan for walleyes in Swan Lake after two of the non-native fish were discovered to have been illegally introduced into the popular sport fishery.

Wedged between the Swan Range and the northern foothills of the Mission Mountains, Swan Lake is no stranger to non-native species, and state fisheries managers have for the past six years implemented an aggressive lake trout gill-netting program to support the lake’s struggling populations of kokanee salmon and native bull trout.

The prospect of a reproducing walleye population in the lake, however, is a whole different can of worms.

The predatory fish have a history of pushing out existing fisheries and taking over lakes where they have been introduced.

Mark Deleray, the state’s regional fisheries manager, is nervously awaiting a lab analysis of the two walleyes’ otoliths — inner ear bones that can reveal whether the fish were recently transported from another lake or are the progeny of an existing population.

“Fish lay down layers of bone as they grow, kind of like the rings on a tree,” Deleray explained. “If the chemical composition of the outer layers is different from the inner layers, it indicates it came from a different body of water.”

Each water body has its own chemical signature, so the agency has also sent samples of Swan Lake’s water to a lab in Massachusetts, one of only a couple of facilities in the country capable of performing the analysis.

If it turns out the fish were recently introduced, it could mean the fishermen caught the invaders before a reproducing population established itself. If not, contending with the new species could change Swan Lake forever.

Walleye introductions have been a mixed bag in Northwest Montana. They’ve flourished for decades in Noxon Rapids Reservoir, but were unable to successfully reproduce in Lake Five near Glacier National Park.

Much depends on whether the fish’s preferred spawning habitat is present, as they prefer rocky, gravel-strewn shorelines where the fertilized eggs can drop into protective cracks and fissures.

One of the classic examples of walleye introduction played out during the 1990s in the Canyon Ferry Reservoir near Helena, where trophy-worthy perch and rainbow trout had historically attracted sport fishermen from around the country.

Fisheries biologist Eric Roberts said the reservoir and connected waterways have consistently been responsible for about 10 percent of all angling in Montana.

“You talk to old-timers around here that have fished Canyon Ferry forever, so many of them talk about back in the old days when they’d go out and catch five-gallon buckets of perch every day,” Roberts said. “Those days have long since passed.”

An illegal introduction of walleye in the late ’80s saw its population spike during the following decade, and with a corresponding drop in rainbow and perch during the mid-’90s.

The ensuing battle over whether to eradicate or manage the new walleye population pitted subsects of the fishing community against one another. A short-lived gill netting project had the side effect of revealing that a new population of trophy-sized walleye had taken hold, and walleye anglers lined up in opposition to removal efforts.

In 2000, the agency adopted a new management plan for the system, which Roberts said “committed Fish, Wildlife and Parks to maintaining a quality, multispecies fishery that included walleye as part of the mix.”

However, the Canyon Ferry perch population remains at an all-time low, and the state’s costs for stocking rainbow in the system have increased seven-fold, as the fish now must be kept until they are large enough to be able to evade the predatory walleye. Montana Fish, Wildlife and Parks now spends $150,000 per year to maintain the Canyon Ferry rainbow fishery.

That said, Roberts noted the overall use has remained consistent, with a shift from a perch-dominated angler base to one with a yen for walleyes.

Tasked with managing the fisheries in Region 1, Deleray is well aware of Canyon Ferry’s history, and is hoping to avoid a repeat in Swan Lake.

While “bucket biology” has entered the popular lexicon to refer to the scientifically clumsy strategy of dumping non-native fish into a water body, Deleray said that term is too soft.

“It’s ecosystem vandalism,” he said. “‘Bucket biology’ kind of insinuates that there’s some kind of science involved. It’s not cute, it’s an illegal act and I think some people don’t take it seriously.”

Bruce Farling, executive director of Montana Trout Unlimited, also favors a change in terminology.

“These self-styled, Johnnie Appleseed fish-spreaders are out there, and they’re wreaking havoc,” Farling said. “It’s damaging to fishermen in the state, and it’s damaging to the economy that’s based on fishing.”

His organization is chipping in more than $10,000 of the $15,000 reward currently on the table for information leading to the conviction of those responsible for the walleye in Swan Lake — and he expects that amount will soon increase.

Both Deleray and Farling also worry about the potential downstream impacts from a breeding walleye population in Swan Lake. The Swan River winds north to the Bigfork Dam before dumping into Flathead Lake, but Farling doesn’t think the structure ultimately will hold the fish out.

“If they successfully reproduce in that lake, they will move downstream,” he said. “Somebody put them in Noxon Reservoir, and they’ve moved down the Clark Fork system and they’re in Pend Oreille right now. People in Idaho are not happy about that.”

The state wildlife agency is currently reviewing a range of possible management options, should the testing reveal that walleye have been reproducing in Swan Lake.

There are currently no regulations for walleye west of the divide, meaning that fishing for the sport fish is essentially unlimited. That could change, however, as fisheries managers look to disincentivize future introductions.

“There’s quite a bit of interest from portions of the angling public that desire these species,” Deleray said. “You don’t want to reward the person who’s doing the illegal introductions.”

On one end of the management spectrum, limitless walleye fishing could remove enough of the population — if they are reproducing — to nip the problem in the bud and extirpate the species from the lake. But it could also be a win for whoever dumped walleye into Swan Lake.

Taking the opposite tact, the agency also could choose to make an example of the “ecosystem vandals,” short of catching them: shut down the fishery altogether.

Bob Gilbert, executive director of Montana Walleyes Unlimited, objected to that approach.

“Our group is adamantly opposed to illegal introductions, and we want to control them, but we don’t want to punish the whole public for something that two or three people had done by shutting down a whole body of water,” he said. “We need to make sure that we use methods that are effective, that will work, but we don’t want to get into draconian efforts.”

He added that his group strives to dissuade its membership from illegal introductions and is kicking in a $1,000 reward for information leading to a conviction in the case, but admitted there isn’t a simple solution to the problem. He stressed public education and, like Farling, wants to see the state wildlife agency increase fines and put as much effort into prosecuting illegal fish introduction as it does poaching.

“I guarantee once you start catching people, it’s going to be in the minds of the people doing it. They’re going to be nervous.”

For now, Northwest Montana’s fisheries are unique in a state where most management focuses on maintaining world-class sport fishing. Many of the lakes and streams throughout the rest of the Treasure State boast renowned populations of introduced species, while Region 1 managers spend a majority of time and money maintaining one of the last strongholds for native bull and cutthroat trout.

“We’re very lucky because we have so many bodies of water here,” Deleray said. “We have so much diversity and quality spread throughout the region, and our concern is that we’re going to lose some of that.”

Reporter Sam Wilson can be reached at 758-4407 or by email at swilson@dailyinterlake.com.