Yellowstone River

The Yellowstone River is seen on Sept. 23, 2016, at the Emigrant Rest Area.

Biologists saw declines in large trout on the upper Yellowstone River this spring, the first time biologists have checked in on fish populations since last summer’s parasite-caused fish kill.

Scott Opitz, Montana Fish, Wildlife and Parks’ upper Yellowstone River biologist, said sampling runs in two four-mile river stretches turned up significantly fewer brown trout between 12 inches and 20 inches long this spring. He said the decline in fish in those sizes “looks like it’s maybe around 50 percent.”

“It’s not definitive at this point, but it looks like it’s in that ballpark,” Opitz said.

He couldn’t say definitively whether the parasite that caused the fish kill was to blame, saying he would caution against jumping to any conclusions. He said the harsh winter could have played a role, or some of the fish could have simply moved. He also said the overall abundance of brown trout is within a normal range.

“There still are brown trout there,” he said.

Biologists are still analyzing the results.

Population estimates on the Yellowstone River are under a greater spotlight this year following the parasite outbreak last summer that killed tens of thousands of whitefish. FWP closed 183 miles of the river plus its tributaries to all recreation in response, hoping the move might give fish a chance to survive and stem the spread of the parasite.

The kill mainly affected whitefish, though dead trout were found. A large portion of the fish mortality occurred on the stretch of river that runs through the heart of the Paradise Valley, near Mill Creek.

Early sampling runs showed a decline in whitefish, which was something FWP expected to see. The decline in large trout wasn’t something they expected.

Opitz said the brown trout declines were documented in the Mill Creek stretch and upstream near Corwin Springs. Biologists expect to see some turnover in a fish population every year, but a 50 percent decline in larger adult fish is beyond what they would consider normal.

But on the same stretches, they also saw an increase in smaller brown trout, meaning the composition of fish age classes has shifted in those river sections.

“There’s a lot less of those adult brown trout and a lot of juveniles coming in behind them,” he said.

He said they saw a similar trend with rainbow trout, though not as significant a decline in larger trout. They didn’t see a significant change in the Yellowstone cutthroat population.

The parasite that caused the fish kill is still in the river. In other places where it has occurred, kills have been known to happen annually. But snowpack and runoff conditions have biologists feeling optimistic about this summer.

The parasite that caused the fish kill relies on bryozoans, freshwater sponges, to complete their life cycle. Bryozoans do well during low flows and warm water temperatures, and the high flows could reduce their density.

Opitz said the high water of this spring “certainly should be pushing back numbers of that host for the parasite.”

Michael Wright can be reached at mwright@dailychronicle.com or at 582-2638. Follow him on Twitter @mj_wright1.

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