The Big Hole River in Montana is one of fly fishing’s most cherished rivers. That made the news of tanking trout populations in recent years a notable headline, even for those who don’t live near the river. Understanding the significance of the fishery both to the local economy in southwest Montana, and to Montana as a whole, recent work has been undertaken to understand why trout are suffering throughout the Big Hole and other Jefferson River Basin streams.
Part of that work is consistent population sampling through electrofishing, which Montana Fish, Wildlife, and Parks (MFWP) is doing in partnership with Montana State University. Their spring sampling is complete enough for the Big Hole that they’ve released early data that show trout populations trending upwards for most of the river. Some stretches still show trout populations well below historical figures, though.
“Trout abundances continue to improve in the Jerry Creek section upstream of Divide, with brown trout numbers exceeding the long-term average for the second consecutive year,” the report states. “Rainbow trout abundances in that section are about 27 percent below the long-term average, though this year’s estimate shows an increase of about 47 percent from 2025.”
Despite the improving numbers, trout populations are still in danger, and there’s not a concrete reason for the sudden drop-off. A potential option for management in the near term could be the continuation of a complete closure on the Big Hole from October 1 through March 31 each year.
In addition to the plunging populations, MFWP also noted that some brown trout and whitefish were observed with white lesions near their heads. The cause of those lesions is still undetermined, and MFWP plans to hire a fisheries epidemiologist to determine what pathogen might be causing those lesions. Less than 1% of all caught fish have been observed with those lesions, however, so the potential health problem isn’t widespread.
Low flows are likely the largest culprit in limiting the trout population in the Big Hole, although this research is continuing so that fisheries managers can make decisions with a complete picture at hand.