State biologists have found no further evidence of invasive brown trout in the Flathead River after a fishy photo surfaced last summer.

A local guide submitted a photo depicting a brown trout purportedly caught near the Pessentine Fishing Access Site in Evergreen to Montana Fish, Wildlife and Parks in June 2025. The report alarmed state biologists, who said the introduction of nonnative brown trout to the Flathead River Drainage could have disastrous effects on native populations of westslope cutthroat trout and bull trout. 

State biologists took water samples from 13 sites between Old Steel Bridge and Teakettle Bridge in July to ascertain the extent of the brown trout invasion. The samples were shipped to a laboratory and analyzed for trace amounts of genetic material from brown trout. All tested negative for the invasive fish. 

That doesn’t necessary mean there are no brown trout in the Flathead River, said Kenny Briedinger, a fisheries biologist with Montana Fish, Wildlife and Parks. He compared finding tidbits of genetic material in the river to locating a needle in a haystack. 

“There’s just a lot of water in the Flathead River, and we’re only able to filter a little bit with each sample,” said Breidinger.   

The odds of finding environmental DNA in a sample increase with the population size of the target species, so Breidinger said the negative tests at least provide a good indication that there are very few brown trout in the Flathead River. 

Under the circumstances, he is optimistic that the negative DNA tests do indicate that brown trout remain absent from the Flathead River.  

The photo that alerted officials to a possible introduction was angled toward the bottom of a boat, with no visible landmarks, and Breidinger speculated that the pictured fish may not have been caught in the Flathead River, as was initially reported. 

“We never were able to confirm this was a legitimate report,” he said. “At this point, we’re hopeful that there’s not a brown trout population in the river.” 

Montana Fish, Wildlife and Parks continues to ask anglers to submit any brown trout caught in the Flathead River or its tributaries to the regional wildlife office for sampling.  

Brown trout can be distinguished from other trout species by their golden brown or yellow-brown color. Adults typically have dark spots surrounded by lighter halos. Spots vary in color and may be black, red or orange.   

Reporter Hailey Smalley can be reached at 758-4433 or [email protected].