In September 2023, the Idaho Department of Agriculture confirmed the presence of quagga mussel larvae in the Snake River near Twin Falls. Quagga mussels pose a major threat to Idaho’s water systems, native species, agriculture and utilities. (Mia Maldonado/Idaho Capital Sun)
The Idaho State Department of Agriculture is planning a second round of treatments in the Snake River in November, targeting invasive quagga mussels.
This second round of treatment comes just a month after the department treated 8 miles of the river between Oct. 8 and 19 with Natrix, a copper-based pesticide registered by the Environmental Protection Agency and used to kill aquatic invasive species.
The treatments are part of an effort to eradicate quagga mussels that were first discovered in the Snake River in September 2023, and again on September 24 of this year, the Idaho Capital Sun previously reported.
Officials have said the invasive mussels pose a major threat to Idaho because they can spread and clog pipes that provide water for drinking, energy, agriculture and recreation. The mussels are a highly competitive species that can also disrupt fish populations and wildlife habitat.
The second round of this year’s Natrix treatment aims to eradicate the mussels in standing water and deep pools of the river. The exact dates for the treatment in November will be determined at a later date, according to a press release from the department.
Copper levels are resolving as expected and the river is expected to return to normal levels by early November, the ministry said in the news release.
The current closure of river access from the Hansen Bridge to Yingst Grade remains in effect until further notice.
What did this year’s treatment plan look like?
On Oct. 8, Idaho State Department of Agriculture staff and partners applied Natrix to the river at a rate of one part per million copper concentration for a total of 200 consecutive hours, according to the department’s release.
The treatment area was expanded upstream this year, just downstream from Hansen Bridge to Centennial Waterfront Park. The dissipation area of ​​the treatment extends from the Hansen Bridge to the Kanaka Rapids.
The copper concentration applied to the river is the maximum amount stated on the product label to kill mussels, while also being below the drinking water standard for humans.
Department staff and partners monitored copper levels 24 hours a day at 51 river sampling locations in the dissipation area throughout the treatment. Deep pools, waterfalls and hydroelectric power stations caused problems with treatment.
Staff also monitored the presence of mussels in the water entering the river through irrigation and well systems through traditional mussel sampling and eDNA. This included extensive sampling upstream above the treatment area.
The river experienced color and clarity changes due to the impact the copper content had on organic matter, algae and aquatic plants in the river.
The department reported limited fish kills. Sentinel species, including non-native Asian mussels, were planted in strategic locations during the treatment to assess treatment effectiveness. The department continues to evaluate the impact of treatment on the sentinel species.
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