Idaho officials find less invasive quagga mussels — and dead fish — in Snake River • Idaho Capital Sun

In addition to applying Natrix to the river, state officials have also doubled their water sampling and monitoring program, established a series of watercraft inspection and hot wash stations throughout the state, and launched a public affairs campaign to inform the public about cleaning, dewatering and drying Educate all boats, kayaks, canoes, paddleboards, wading boots and anything else that comes into contact with water and could spread the invasive mussels.

“We are using even more mussel detection tools than we did last year,” said Chanel Tewalt, director of the Idaho Department of Agriculture, in a telephone interview Tuesday. “We are collaborating with more laboratories and environmental DNA analysis to give us the best possible insight into the spread of mussels in the Snake River.”

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

How does the copper-based chemical Natrix work?

Natrix is ​​supposed to kill the mussel and its larvae – Tewalt said she wants to kill every single mussel to prevent their spread.

Tewalt said the state is also preparing for another treatment in November that will target deep water bodies. However, she is confident the state will reduce the amount of clams.

“The only silver lining is that it tells us that there must not be a very large population – if there had been a large presence of mussels, they would have been easier to detect,” Tewalt said. “This gives us hope to implement such a treatment.”

Quagga mussels are a non-native invasive species that state officials first discovered in water samples in Idaho in September 2023, the Idaho Capital Sun previously reported. In the water samples, officials discovered tiny quagga mussel larvae, called villagers, which are not visible to the naked eye.

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

Idaho Fish and Game Manager reports fewer dead fish in the Snake River

While state officials believe Natrix can kill mussels, the chemical also kills fish.

After the Natrix treatment program in 2023, Idaho Fish and Game staff removed six or seven tons of dead fish that had been floating on the river’s surface. 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 long.

Mike Peterson, regional fisheries manager for the Idaho Department of Fish and Game, said far fewer fish were killed during the treatment program this year. As of Wednesday, Peterson said he had not seen a dead sturgeon this year.

Some fish species, such as smallmouth bass, bluegill and greengill, experienced little or no mortality in the treatment areas, Peterson said. Other species, including pike minnow, large fish and perch, had high mortality rates.
“The mortality we have experienced (this year) is about what we expected,” Peterson said.

Peterson said the number and density of dead fish has decreased 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 catch 40 to 50 dead fish. Peterson said seven dead fish were caught on one transect line this year.

This section of the Snake River is traditionally stocked with fish. But out of an abundance of caution, Idaho Department of Fish and Game officials have not restocked that stretch of the Snake River 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 restore fish populations in the affected portion of the Snake River after the state completes mussel killing treatments. Work to restore the river could include relocating fish from other areas and re-occupying the river, Peterson said.

Tewalt said officials continuously monitor the flow during the treatment phase to ensure they are using the correct concentration of Natrix. She said crews are also monitoring the river’s copper levels to ensure the copper is dissolving.

“The dissipation looks as we expected and behaves similarly to last year’s dissipation, which is excellent,” Tewalt said.

Peterson said he wasn’t surprised the state had to do additional treatments to kill the mussels. Peterson likened the situation to an outbreak of noxious weeds like stingwort in your home.

“You can spray it and it will die, but it’s likely it will come back next year,” Peterson said. “With additional treatment it usually goes away, and I think that’s where we’re at with quagga mussels. We had the first treatment and the density reported by ISDA was much lower than what they found. I am optimistic that we are going in the right direction.”