A long-held fear for those who care about the Snake River came to pass in mid-September when quagga mussels were found in the river near Twin Falls during routine invasive species monitoring by the Idaho State Department of Agriculture (ISDA). The discovery of this invasive, non-native species in Idaho is very troubling and the potential spread to the entire Snake and Columbia River systems would be catastrophic, with the potential to significantly disrupt aquatic ecosystems and trigger widespread economic impacts.  

Aquatic pesticide treatment

In response to the recent detection of quagga larvae (known as veligers) and at least one adult mussel, ISDA has coordinated a rapid response in coordination with other state and federal agencies. Without any viable alternatives to eradicate the invasive mussels before they got a foothold in the Snake River, ISDA decided to proceed with treatment using a copper-based pesticide product called Natrix. According to the Natrix label, “this copper product is toxic to fish and aquatic organisms.” During the Twin Falls town hall meeting on Oct. 1, Director Chewalt noted that the planned treatment represented “the most aggressive approach of any effort in the U.S.”

The initial treatment phase occurred from October 3-13, using approximately 40,000 gallons of Natrix in total to achieve the desired target concentration in the river. Unsurprisingly, the initial reports have indicated that there has been high mortality of fish, plankton, shellfish and vegetation in the 6-mile treatment area, with some additional downstream mortality as a result of the oxygen depletion in the water as the material decomposes. In a recent interview, IDFG director Jim Fredericks said, “Not all fish are dead in there, but the majority likely have been killed. That’s thousands of fish. It is a large number.” IDFG will assess the total damage to the fishery by comparing electrofishing surveys conducted before and after the treatment. Because the copper pesticide passes through the gills of fish and does not accumulate in the tissue, it does not appear to threaten wildlife or birds that may consume the dead fish. 

In the below photos from the Snake River downstream of Twin Falls, you can clearly see the fish mortality and disintegration of aquatic plants in the river, both of which were to be expected with this treatment:

What’s next?

ISDA is currently evaluating the efficacy of the treatment and are actively monitoring for veliger presence in the river. The one adult quagga mussel that was identified before the treatment died as a result of the treatment, providing some reason for optimism. Depending on the outcome of this initial treatment, ISDA may pursue secondary treatment. Furthermore, this section of the Snake River will likely need to be intensely monitored for the next several years to check for signs of quaggas.

Regardless of whether the treatment is effective, it should be evident to all Idahoans that we need to redouble our efforts to monitor for the potential introduction of mussels into Idaho’s waters. The location and effectiveness of boat check and decontamination stations should be closely evaluated by ISDA, the Governor, and the Idaho Legislature. 

Furthermore, ICL remains concerned about the precedent this treatment effort may set. While we are hopeful that the aggressive treatment will be successful, in the event that it is not, we have significant reservations about expanding or repeating this treatment in the Mid-Snake or beyond, and emphasize that each occurrence of invasive mussels warrants site-specific consideration. ICL also continues to seek information on potential secondary mussel control efforts in the river, including the use of niclosamide (another pesticide) and pseudomonas (a biological control).

In the mid-Snake River, additional investments will be needed to restore habitat impacted by the treatments, and to promote recovery. In addition, concerns with water quality in the mid-Snake River have long been a focus of ICL’s, and the discovery of these invasive quagga mussels in this section of the river provides an impetus to redouble efforts to restore the Snake to a healthy, ecologically functioning river.

Learn more about the latest updates on the quagga mussel issue at our upcoming webinar – November 1st at 7 pm.

For more information and ongoing updates regarding quagga mussels in the Snake River, visit ISDA’s official site at https://agri.idaho.gov/main/plants/snake-river-quagga-mussel-veligers/.

To stay updated on this issue and other issues facing the Mid-Snake River, sign up for our Snake River Campaign email updates.