KALISPELL - Montana Fish, Wildlife and Parks netted two walleye in Swan Lake in October 2015, which had never previously been discovered in the lake.
The discovery led to an investigation to find who had illegally introduced the fish and how.
Montana is home to some of the finest sport fisheries in the world and they generate millions of dollars in revenue each year.
But when someone moves live fish from one body of water into another, they put the future of those fisheries in jeopardy.
"They can really harm fisheries," FWP Region One Education Officer Dillon Tabish said. "They can hurt recreational opportunities and they can cause the crash of fishery populations."
Around 600 illegal introductions have been confirmed by FWP in more than 250 water bodies in Montana.
Tabish said after two years of investigating, biologists had a breakthrough in the Swan Lake case by studying the ear bones of the walleye.
"We have a biologist here who was studying something called an otolith, which is a calcium substance in the ear bones of a fish," Tabish said. "And so he studies those walleye's otoliths and that was able to trace where this fish had come from."
Biologists determined that the two walleye were originally from Lake Helena.
Tabish said learning they weren't born in Swan Lake was good news.
"That shows that these two fish, at least, were not born in Swan Lake, which hopefully means a population has not been established yet," he said. "It also helps us gain new insight and clues into where these fish came from which will hopefully lead to finding the individual or individuals who did this introduction."
Tabish added there are different reasons for why someone would introduce fish illegally.
"Sometimes it's because they're trying to establish a new type of sport fishery. So potentially this was someone who wanted to be able to go fish for walleye on Swan Lake and there is a lot of great walleye fisheries across the region and we would like folks to stick to those ones, not creating new ones," Tabish said.
A reward up to $35,250 is available for information leading to the conviction of whomever is responsible for the illegal introduction of walleye into Swan Lake.
Anyone with information is encouraged to call 1-800-TIP MONT. Callers do not need to identify themselves and may be eligible for the cash reward.
The state of Montana also offers rewards up to $15,250 for information about illegal fish introductions and Montana Trout Unlimited has pledged a reward of $20,000 for a tip that results in the conviction of the individual or individuals responsible for illegally introducing the walleye into Swan Lake.
The Fish and Wildlife Commission approved a mandatory kill regulation for walleye in Swan Lake, Swan River, and their tributaries. All walleye caught by anglers must be kept and immediately killed.
Walleye must be reported to FWP within 24 hours and arrangements made for the entire fish to be turned into FWP within 10 days.