Idaho officials are finding fewer invasive quagga mussels – and dead fish – in Snake River

TWIN FALLS (Idaho Capital Sun) — Officials in Idaho say they are seeing fewer quagga mussel larvae and fewer dead fish compared to last year as they finish the second week of administering a copper chemical in the Snake River in an attempt to eliminate the invasive mussels.

Quagga mussel larvae were first detected in state water samples in the Snake River in September 2023 and then again on Sept. 24 of this year.

Contractors and officials with the Idaho State Department of Agriculture have been applying a copper-based chemical called Natrix in concentrations of one part per million to the Snake River near Twin Falls since Oct. 8.

In addition to applying Natrix to the river, state officials have also doubled their water sampling and monitoring program, created a series of watercraft inspection and hot wash stations across the state and launched a public affairs campaign helping educate the public to clean, drain and dry all boats, kayaks, canoes, paddleboards, wading boots and anything else coming into contact with water that could spread the invasive mussels.

“We are using even more tools to detect mussels than last year,” Idaho State Department of Agriculture Director Chanel Tewalt said in a phone interview Tuesday. “We are working with more labs and environmental DNA analysis to give us the best possible look we can have at the spread of mussels in the Snake River.”

“With more tools, we have expanded a more aggressive treatment strategy,” Tewalt added.

How does the copper-based chemical Natrix work?

Natrix is designed to kill the mussel and the larvae – Tewalt said she wants to kill every single mussel in an attempt to keep them from spreading. 

Tewalt said the state is also preparing for another treatment in November targeting deep water pools. But she is confident the state is reducing the amount of mussels. 

“The one silver lining is it tells us there must not be a very big population – if there was a great presence of mussels they would have been easier to detect,” Tewalt said. “That gives us hope to implement a treatment like this.”

Quagga mussels are a nonnative invasive species that state officials discovered in water samples in Idaho for the first time in September 2023, the Idaho Capital Sun previously reported. In the water samples, officials discovered tiny quagga mussel larvae known as villagers that are not visible to the naked eye.

Tewalt told the Sun quagga mussels reproduce rapidly, and an invasion of the mussels could have huge environmental and economic costs to the state. The mussels are capable of reproducing and clustering so densely that they cover and clog pipes used for drinking water, irrigation or agriculture, Tewalt said. The mussels can cover boats and docks, and they filter vital nutrients from the water, crowding out native species. 

Idaho Fish and Game manager reports fewer dead fish in Snake River

While state officials are confident in Natrix’s ability to kill mussels, the chemical also kills fish.

Following the Natrix treatment program in 2023, Idaho Fish and Game staff removed six or seven tons of dead fish that had floated to the surface of the river, the Sun previously reported. 

Among the dead fish were 48 white sturgeon, the oldest of which was 35 years old and up to eight feet in length. 

Mike Peterson, regional fisheries manager for the Idaho Department of Fish and Game, said there have been far fewer fish killed this year during the treatment program. As of Wednesday, Peterson said he hasn’t seen any dead sturgeon this year. 

Some species of fish, like smallmouth bass, bluegill and green sunfish had little or no mortality in the treatment areas, Peterson said. Other species, including northern pikeminnow, largescale sucker and perch exhibited high levels of mortality.
“The mortality we experienced (this year) is about what we expected,” Peterson said. 

Peterson said the number and density of dead fish is down compared to last year. For example, when Idaho Department of Fish and Game staff ran a transect line across the river last year to measure mortality, it was common to net 40 to 50 dead fish. This year, Peterson said the highest number of dead fish netted on a transect line is seven.

That section of the Snake River is traditionally stocked with fish. But out of caution, Idaho Department of Fish and Game officials did not stock that section of the Snake River with fish again this year in case the state needed to administer another round of Natrix, Peterson said.

Peterson said Idaho Department of Fish and Game staff will work to recover the fish populations in the affected part of the Snake River after the state finishes the treatments designed to kill mussels. Work to restore the river could include relocating fish from other areas and resuming stocking the river, Peterson said. 

Tewalt said officials are continuously monitoring the river during the treatment phase to ensure they are using the correct concentration of Natrix. She said crews are also monitoring the copper content of the river to ensure copper is dissipating. 

“Dissipation is looking like what we expected and behaving similarly to dispensation last year, which is excellent,” Tewalt said. 

Peterson said he is not surprised the state is having to administer additional treatments to kill the mussels. Peterson compared the situation to having an outbreak of noxious weeds like puncturevine at your house.

“You can spray it and it will die, but it’s likely you will see it pop back up next year,” Peterson said. “With additional treatment, it typically goes away and I think that is where we are at here with quagga mussels. We had the initial treatment and the density ISDA reported was much lower in number than what they had found. I’m optimistic we are heading in the right direction.”