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PARADISE VALLEY — A previously unidentified strain of parasite has grown so prolific in the Yellowstone River that it is overwhelming whitefish, killing thousands and prompting the state last week to close more than 180 miles of the river to all recreation.

“The sheer parasite loading itself is almost shocking the fish, that’s what’s killing them,” said Eileen Ryce, Montana Fish, Wildlife and Parks’ hatchery section chief, during a Tuesday press conference along the banks of the Yellowstone River south of Livingston.

Eileen Ryce

Eileen Ryce, of Montana Fish, Wildlife and Parks, speaks at a press conference on the Yellowstone River at the Mallard's Rest fishing access.

Scientists like Ryce believe that the parasite bloom may be so large and unprecedented in part because the Yellowstone River is experiencing near-record low flows and high water temperatures.

“We could see varying responses (of the parasite) depending on which river it gets into,” said Travis Horton, Region 3 fisheries manager in Bozeman.

The nearby Madison River, where flows are dam controlled, may be more resilient to the parasite since the water’s temperature is cooler, coming from the bottom of Hebgen Lake, and the flows can be maintained at a steadier rate. Likewise, outbreaks of similar parasites in Idaho that killed whitefish in 2011 and 2012 may have been more limited because some of the streams are dam controlled, Horton said.

The FWP officials spoke to a crowd of about 50 people — anglers, local lawmakers, the media and business owners — at a fishing access site where the stench of dead whitefish rotting on the riverbank wafted through the air.

Press conference

Media, business owners, anglers and others gather for a press conference on the banks of the Yellowstone River.

Acknowledging the effect of the river’s closure to anglers, floaters and on all of the many businesses that service such recreationists, Gov. Steve Bullock said “the impact goes far beyond the water’s edge.” Yet he said “science needs to guide this.”

“We need to make sure we are not only protecting this watershed and this water but indeed our entire state,” he said, noting that the outdoor recreation industry creates about 64,000 jobs and $6 billion in consumer spending annually in Montana.

When Rep. Alan Redfield, R-Livingston, asked why certain portions of the river couldn’t be open to fishing to ease some of the impact on local businesses, Horton said FWP hopes to contain the parasite outbreak to the Yellowstone and make its duration short-lived.

“We want to try to minimize this,” Horton said. “We don’t want to make this situation worse by rushing ahead.”

Haze from regional fires

Haze from regional fires settles over the Absaroka Mountains as the Yellowstone River flows past the Mallard's Rest fishing access in Paradise Valley on Tuesday.

Horton added that when the Yellowstone River’s peak water temperatures drop below 55 degrees and stay there, FWP would consider lifting the river closure.

“We’re not looking at it being a permanent closure,” said Jeff Hagener, FWP director.

Travis Horton

Travis Horton, of Montana Fish, Wildlife and Parks, speaks at a press conference on the Yellowstone River.

He noted the rule to enforce the closure is a temporary emergency closure good for 120 days. After that time the department is forced to re-examine the issue to decide whether the closure should be extended. 

The fish kill has been documented by FWP crews from the Yellowstone National Park boundary about 100 miles downstream to just west of the town of Big Timber, Horton said. Other public reports of dead whitefish and suckers downstream from Big Timber to Laurel have not been confirmed by FWP.

The microscopic parasite that infects the fish needs two hosts to complete its life cycle, Ryce explained. It starts in a Bryozoan, a moss animal similar to a freshwater sponge. The Bryozoan releases the parasite that fish absorb through their gills. Similar parasites have caused internal bleeding and kidney failure in whitefish in other outbreaks, such as the ones in Idaho.

But the Yellowstone outbreak is different, Ryce said. There are so many spores being found inside the fish that the scientists are calling it a naive reaction.

“What we mean by that is that the immune response that the fish are expressing would suggest that they have not been exposed to this parasite previously,” she said. “That has concerned us since it would suggest it’s a new infection. At least we haven’t seen anything on this scale previously.”

“The other thing that’s concerning is from the histology results there were high numbers of parasites seen in multiple tissues. That suggests to us that the infective load that’s currently out in the river is very high. Why that’s a concern is that the sheer volume of parasites that’s out there makes it very easy for the parasite to be spread to other waters.”

That’s the reason FWP took the unusual step to close down much of the upper river to recreation, Horton said, calling it a “very dramatic step that we took.” It’s also why the agency is stressing that the best way to fight the spread of the parasite, along with other aquatic invasive species, is to clean, drain and dry all boats and fishing equipment.

FWP crews will continue to search for dead and dying fish in the Yellowstone River and its tributaries like the Stillwater and Boulder rivers to try to determine the extent of the outbreak.

“I hope we can continue to work together, recognizing that the long-term objective is to protect this valley and the river,” Bullock said.

Yet Ryce noted that FWP staff doesn’t expect the parasite to disappear, even with colder water temperatures and higher river flows. However, if the fish are given time to adapt they may build up some immunity to the parasite and recover.



Montana Untamed Editor

Montana Untamed editor for the Billings Gazette.