BOISE, ID — Mountain whitefish populations across much of Idaho remain stable or increasing, though recent studies identified declines in certain rivers and stream sizes, according to Idaho Fish and Game.

The department recently completed two studies examining mountain whitefish trends. While biologists routinely monitor trout populations, whitefish have historically received less focused monitoring. That changed as fisheries staff analyzed long-term snorkel survey data and conducted updated field surveys in multiple regions of Idaho.

The first study reviewed decades of snorkel-count data collected in the Salmon and Clearwater river basins of central Idaho. Biologists found that most mountain whitefish populations in those basins have remained stable or increased since the mid-1980s. An exception was the South Fork of the Clearwater River, where declines were observed.

The second study focused on southern Idaho, covering the upper Snake River basin and areas near the Oregon, Nevada and Wyoming borders. Dozens of streams previously surveyed in the late 1990s and early 2000s were re-surveyed between 2020 and 2024.

In many locations, whitefish remain abundant, with densities reaching as high as 8,000 fish per mile in some rivers. However, biologists also documented declines in certain areas.

In smaller streams — generally those less than about 60 feet wide — mountain whitefish are now often absent where they were present two decades ago. Declines were also recorded in larger rivers, with every surveyed location wider than 100 feet showing reductions in whitefish numbers.

To better understand those patterns, Fish and Game is funding a University of Idaho graduate student to investigate possible causes of the declines, including changes in distribution and abundance.

Despite the localized declines, the department said it is not concerned that mountain whitefish are broadly at risk in Idaho. Officials describe the species as likely the state’s most prolific salmonid and plan to increase monitoring efforts moving forward to track future changes.

Mountain whitefish are native to Idaho and are commonly encountered by anglers fishing for trout in rivers throughout the state.