State wildlife officials plan to revamp invasive fish suppression efforts on the East Fork Bull River to combat a so-called “hotspot of hybridization” threatening native westslope cutthroat and bull trout populations.

Starting in July, fisheries technicians with Montana Fish, Wildlife and Parks will use electrofishing backpacks and weir traps to remove brown, brook and rainbow trout from the East Fork Bull River. All three species are non-native to western Montana and “seriously threaten the persistence of [native westslope cutthroat trout and bull trout] through hybridization and competition for resources,” according to an environmental assessment released by the state wildlife agency April 1. 

Westslope cutthroat trout and bull trout are listed as Species of Special Concern by the state, and bull trout are listed as threatened under the Endangered Species Act.  

The Bull River drainage has provided a rare refuge for both native species, thanks in part to state conservation efforts. Wildlife officials used electrofishing and fish traps to remove 3,500 brown trout, 1,900 brook trout and 60 rainbow trout from the East Fork Bull River between 2007 and 2009. Populations of brown and brook trout remained low for more than a decade after the initial removal.  

But more recent studies, completed in 2025 and 2026, suggest that non-native fish populations are on the rise in the lower reaches of the East Fork Bull River. Left unchecked, officials worry the non-native fish could spread upstream, jeopardizing the genetic integrity and long-term viability of westslope cutthroat trout and bull trout throughout the tributary. 

“Fish, Wildlife and Parks acknowledges that hybridization and non-native trout will always exist within the Bull River drainage,” reads the environmental assessment. “However, Fish, Wildlife and Parks believes that reducing non-native competition ... and slowing the spread of hybridization ... is a realistic and important goal in the long-term effort to protect these native species.” 

If the project receives approval, officials will begin electrofishing in July. Fish removal efforts will continue through November, though no electrofishing will occur during bull trout spawning periods. Captured non-native fish that measure greater than 250 millimeters will be relocated to Triangle Pond, a nearby community fishing site. 

The state wildlife agency is accepting comments on the project’s environmental assessment through 5 p.m., April 15. Comments may be emailed to Travis Rehm at [email protected]. 

Reporter Hailey Smalley can be reached at 406-758-4433 or [email protected]. If you value local journalism, pledge your support at dailyinterlake.com/support.