State officials have confirmed the presence of nonnative brown trout in the Flathead River system.

An angler reportedly caught a singular brown trout upstream of Pressentine Fishing Access Site near Evergreen in early June, marking the first time the species has been observed in the Flathead River Drainage in more than two decades. Montana Fish, Wildlife and Parks identified the fish through a photo submitted by a local guide.  

Brown trout already reside in many of the state’s waterbodies, and officials even stock the prized sport fish in some lakes and rivers. But introducing brown trout to the Flathead River poses risks, said Kenny Briedinger, a fisheries biologist for the state wildlife agency. Brown trout often outcompete native fish for food and other resources, which could endanger some of the region’s more vulnerable species like westslope cutthroat trout and bull trout. 

“Here, if they become established, [brown trout] would be considered invasive,” said Briedinger. 

He said it is was “very likely” the brown trout was illegally introduced to the Flathead River. 

State officials are now investigating whether the species has made its way to other sites in the Flathead River system using a process called environmental DNA sampling. Researchers have collected water samples from dozens of sites on the Flathead River between Kalispell and Columbia Falls and are analyzing them for traces of genetic material from brown trout. 

The results will give biologists a good estimate of how far afield the brown trout may have spread, said Breidinger. 

Anglers can assist in the research efforts by submitting brown trout caught in the Flathead River or its tributaries to local wildlife officials. Researchers plan to analyze the samples to determine where the introduced brown trout originated from. 

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.

The river system already has significant problems with non-native rainbow trout, which have cross-bred with the native cutthroat trout.