Over the past five years, wild trout populations in Montana’s Bighorn River have experienced a strong and encouraging rebound. After reaching a low point in 2020, fish-per-mile estimates for both brown and rainbow trout have steadily climbed, with 2025 representing one of the strongest population years observed in recent history. (Scan the QR code to view the full 2025 population estimates for all trout.)

Within this overall recovery, the resurgence of rainbow trout has been especially striking. Through recent sampling of two sections of the Bighorn River, Montana Fish, Wildlife & Parks (FWP), found that rainbow trout numbers in the Upper River increased from a concerning 642 fish per mile in 2020 to 1,222 fish per mile in 2025. In the Lower River, the rebound has been even more dramatic - rising from just 109 rainbow trout per mile in 2020 to 1,642 fish per mile in 2025, representing more than a tenfold increase.

Dynamic Flow = A Stronger Fishery
Understanding why rainbow trout have rebounded requires looking closely at one of the most important drivers of trout recruitment on the Bighorn River: river flows. Although flows on the Bighorn River are regulated by Yellowtail Dam, discharge still varies from year to year - and rainbow trout respond closely to those changes.
Following several consecutive years of prolonged high runoff during June from 2017 through 2019, rainbow trout numbers declined to near-historic lows by 2020. In contrast, the more moderate and stable flows observed during 2020, 2021, and 2022 created favorable conditions for spawning adults and redds, allowing the population to begin rebuilding.

Another high runoff year occurred in 2023. However, unlike the earlier sequence of high-water years, it was followed by more moderate—though relatively low—flows in 2024, conditions that appear to have supported successful recruitment reflected in the strong population observed in 2025.
Together, these patterns reinforce an important lesson about the Bighorn River: variability in streamflow is both natural and beneficial. It is when extreme runoff occurs in consecutive years - particularly during the spawning season - that rainbow trout recruitment can struggle. 

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Monitoring Critical Life Stages
Understanding exactly when rainbow trout spawn is critical to evaluating how flow conditions influence recruitment success. In response to the population decline observed in 2020, BHRA launched a drone monitoring program to better define the timing, duration, and peak of spawning on the Bighorn River. While textbook estimates place rainbow trout spawning between early March and mid-June, BHRA’s 2024 (31 flights) and 2025 (33 flights) surveys revealed a more site-specific pattern: spawning activity begins to rise in May, peaks in early June, and declines by early July - slightly later than generic literature estimates.

With this refined timeline, BHRA is now better positioned to inform water and fisheries managers on how changes in flow stability, discharge levels, and ramping rates may support—or disrupt—peak spawning conditions.