Two walleyes caught in Swan Lake by state wildlife officials last year were introduced into the lake, according to a recently completed scientific analysis of the fish.

After months of waiting, Montana Fish, Wildlife and Parks received test results Friday that established the walleyes were not born in the popular fishing lake and were illegally introduced after being caught elsewhere.

The state wildlife agency caught the walleyes in Swan Lake in October 2015 while netting lake trout as part of its lake trout removal program.

It was the first time walleyes had been found in Swan Lake and agency officials are still concerned that if a breeding population establishes itself, the predatory fish could damage the lake’s population of kokanee salmon and federally threatened bull trout.

To determine whether the walleyes had been recently introduced or were the result of an already-breeding population, state biologists sent samples of the fish’s pea-sized inner ear bones — called otoliths — to a specialized laboratory in Woods Hole, Massachusetts.

Sam Bourret, a fisheries biologist with the agency, said the otoliths establish a timeline of when fish existed in a particular body of water. Each day, the fish create a fresh layer of bone on their two otoliths, building them up like the rings on a tree.

Every new layer of bone contains the chemical fingerprint of the lake, the result of the types of minerals absorbed into the water from the underlying geology.

The Massachusetts lab — one of just a handful nationwide — then analyzes chemical isotopes in the rings to determine whether the fish at some point moved from one body of water to another.

More recent rings matched the chemical signatures from additional otoliths taken from lake trout in Swan Lake.

By looking at how the chemicals in the rings changed over time, Bourret estimated the walleyes were likely introduced sometime in 2015.

Additional walleye otolith samples from Lake Francis and Noxon Reservoir also were analyzed, but the results showed the fish were not taken from either of those lakes.

Bourret said the analysis took about three weeks for the lab to complete and costs about $1,100 per day.

The highly specialized machine uses a laser to score the otolith samples, which state biologists prepared by sanding them down until the core is exposed. After the laser removes about one micrometer of material, a spectrometer then determines the chemical contents.

Although he’s only worked for the state agency for two months, Bourett is especially equipped for the job of analyzing the otolith test results.

As a graduate student at the University of Idaho, he worked in a similar otolith lab run by Washington State University. His research mostly focused on Chinook salmon, but he said all species create otolith layers in the same fashion and will record the same chemical signatures if they are in the same body of water.

“It adds another tool in the toolbox of a diverse group of people that have a lot of expertise in different areas,” he said. “In the future there will be a lot of opportunities to use geochemical analysis to understand a lot of fish life history and the native fish life in the Flathead.”

And while two popular walleye fisheries have been ruled out as the source of introduction, he said he hopes to someday have a database of those chemical signatures from bodies of water throughout the state.

“There’s a good chance that we will try to develop a geochemical map for the state, so we can figure out, first off, where these walleyes are from and if there’s any future illegal introductions, that will make it easier to figure out where they came from,” he said. “That’s sort of my vision, though. How realistic that is, I’m not as sure.”

Bourett said the test results don’t preclude the possibility that other fish remain in Swan Lake, which winds for about 10 miles between the Swan Range and the northern foothills of the Mission Mountains.

“It indicates there isn’t already a self-sustaining population,” he said. “There could be other fish, but we caught fish that were introduced.”

In December, the state fish and wildlife commission passed a new fishing rule requiring anglers to kill any walleyes caught in Swan Lake and turn them in to the state wildlife agency. Anglers must report their catch within 24 hours and have 10 days to turn the fish over to the agency.

Regional state fisheries manager Mark Deleray said Tuesday the news likely wouldn’t change the agency’s regulatory response to the walleye discovery.

The mandatory kill rule is on the books through February 2017 and will remain so unless a future change to the fishing regulations reverses it.

State wildlife groups and Fish, Wildlife and Parks are offering a reward of nearly $30,000 for any information leading to the conviction of those responsible for the illegal introduction.

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