Fish survey finds growth in trout population on Big Hole River since 2023

The effort to try to restore trout populations in the Big Hole River after historic low numbers were recorded last year received preliminary good news that initial counts this spring along two sections of the river show increases from 2023, especially among brown trout.

The population growth is “encouraging,” Montana Fish, Wildlife and Parks Big Hole River fisheries biologist Jim Olsen said Tuesday, but the brown trout and rainbow trout populations are still only about one-half to three-quarters of the long-term average population sizes in the Big Hole.

It is one of four rivers in southwest Montana where FWP, Montana State University, and local fisheries groups are studying what has caused trout population declines over the past decade and mysterious illnesses in some fish.

In the Melrose section of the Big Hole River, which is below the Salmon Fly Fishing Access Site, crews this spring found an estimated 192 rainbow trout and 506 brown trout per mile – up from 161 rainbows and 324 brown trout per mile in last year’s estimates. But the long-term averages of 488 rainbow trout and 908 brown trout per mile in that section still far outpace current estimates.

In the Hogback section of the river, which sits between the Glen Fishing Access Site and the Tony Schoonen Fishing Access Site, crews found slightly fewer rainbow trout this year (263) than they did last year (288). But they counted a large increase in brown trout, finding an estimated 722 this year compared to 289 last year. Historically, crews have found about 473 rainbow trout and 920 brown trout in that section of the river.

Biologists also did population estimates for the Jerry Creek section near Wise River and the Pennington section upstream of Twin Bridges to sample upper and lower sections of the river, but the estimates have not been finalized.

Olsen said the results so far from the two sections have been good, and that it’s likely that better water levels last year helped improve the survival of juvenile fish because many of the fish counted this year were 2-year-old fish.

Save Wild Trout, another group working on studying fish declines on the Big Hole, acknowledged the survey results and said in a social media post that “a good water year & hard work can make a difference.”

“The forecast & nutrient pollution mean lots more work to do. The breakdown – young, juvenile, adults – is an important indicator of overall health,” the group said.

The Big Hole, Beaverhead, Ruby and Madison rivers are among those in southwest Montana that have seen decreased streamflows compared to historical averages over the past decade, and those lower water levels are among the factors biologists are looking into as to why trout populations have declined.

But if a better water year in 2023 was a boon for the trout populations on the Big Hole, water supply forecasts so far this year could spell trouble.

The May water supply forecast released this week by the Natural Resources Conservation Service said snowpack in the Jefferson River basin is currently at 69% of normal, compared to 127% this time last year. And April brought only 70% of the monthly average precipitation to the basin.

Those conditions have the NRCS forecasting water supply in the Jefferson and Beaverhead rivers to be just 35% to 50% of normal for May through July and about the same through September. The Big Hole at Glen, for instance, was running at 1,260 cubic feet per second on Tuesday – close to the 25th percentile for May 7 and nearly 900 cfs below the mean flow for the date compared to the past 26 years of records.

The NRCS said precipitation forecast this week could help streamflows but even a wet spring would likely not boost rivers back to normal levels.

“It likely won’t be enough to compensate for the lack of precipitation earlier this season. Many rivers and streams will still likely see below normal runoff volumes this season, however with additional precipitation in the coming weeks runoff might end up closer to the 30% exceedance forecasts,” the May water supply forecast report says.

FWP and MSU researchers have started tagging trout as part of their study in the Jefferson and Madison basins and are asking anglers to report when they catch tagged fish to help them understand population sizes, locations and health. Certain tags will make anglers eligible for awards.

They also are conducting a fish mortality study, a juvenile fish study and a fish health study as part of the project. They also hope to conduct molecular testing for fungi, bacteria and viruses in the Big Hole and Beaverhead rivers to help better understand what is happening to the fisheries.