2025 Bighorn Trout Population Estimates

By Demi Blythe

Fisheries Biologist, FWP

Every spring and fall, Montana Fish, Wildlife & Parks (FWP) hits the water to check-in on our Bighorn River wild trout. These surveys are done just below Three Mile Access (Upper Section) in the spring and down by Mallard’s Landing (Lower Section) in the fall and have been part of the Bighorn River sampling since 1992. The goal? To keep tabs on trends and overall health of our brown and rainbow trout populations.

"In 2025, surveys from both Upper and Lower river sections showed encouraging results."

In the Upper Section, we saw increases in both brown and rainbow trout, estimating about 2,446 trout per mile (1,245 brown trout and 1,222 rainbows) longer than seven inches (Figure 1). Both species looked great, with excellent body condition and the kind of healthy weight that makes anglers grin. It’s always good to see those wild trout holding steady, especially after a few variable water years.

Three Mile Access (Upper Section)

In the Lower Section, we estimated 2,187 trout per mile (531 brown trout and a whopping 1,642 rainbows) longer than seven inches (Figure 2). Brown trout numbers were right in line with recent years, but the rainbow population jumped noticeably compared to the past five. That’s big news for anyone who loves chasing the hard-fighting Bighorn ‘bows. FWP crews also encountered plenty of smaller fish in the mix including juveniles under six inches and a cohort of both species between 8 and 12 inches. It appears that both species are successfully recruiting the next generation.

Mallard's Landing (Lower Section)

Although river flows were lower than average in 2025, the river stayed relatively steady throughout the runoff period. This kind of stability is beneficial for rainbows as it supports favorable conditions during their spawning period. Lower flows can also impact productivity in the form of algae and bugs. Sunlight reaches more of the riverbed, boosting (pesky) algae and aquatic insect production. That means more midges, scuds, and mayflies, which is the steak and eggs of a rainbow’s diet. Low, steady flows tend to concentrate food sources in the main channel. Rainbows, which actively feed in the drift, take advantage of that constant conveyor belt of tasty bugs. It is one of the reasons why, even in tougher water years, the Bighorn keeps producing nice hogs of fish.

Also, recent BHRA side-channel reconnection work seems to be paying off. These reconnected side channels provide critical rearing habitat for juvenile trout and refuge areas from predators like brown trout and high flows. Even during drier years, the side channels stay connected longer and help young fish survive to adulthood and could be contributing to the healthy recruitment we saw during 2025 surveys.

This year also marked the first time FWP used PIT (Passive Integrated Transponders)

tags during both surveys. These tiny tags, about the size of a grain of rice, will allow biologists to track individual fish over time. When a PIT tagged trout is recaptured, FWP can track details about its growth and movement. Over time, these data will paint a clearer picture of how trout use different sections of the river and better refine population estimates. 

As we close out 2025, both brown and rainbow trout are doing well, recruitment looks strong, and the river is producing healthy, well-fed fish across multiple year-classes. After a low but stable water year and promising survey results, there’s plenty to be optimistic about fishing the Bighorn River as we head into 2026.