With the spooky season upon us, let’s talk about the invasive species that is threatening to haunt our freshwater ecosystems here in Idaho – quagga mussels. As we have written about many times on this blog, the potential spread of quaggas to the entire Snake and Columbia River systems would be catastrophic, with a high likelihood of significantly disrupting aquatic ecosystems and triggering widespread economic impacts. With that in mind, the Idaho State Department of Agriculture (the state agency responsible for managing invasive species) has adopted an eradication mindset ever since quagga mussels were first discovered in the Snake River in September 2023. ICL continues to monitor ISDA’s efforts to eradicate quagga mussels from the Snake to ensure that their response to quagga mussels is effective without causing undue environmental harm in the process.
Aquatic pesticide application – round 2
Following the state’s initial quagga eradication efforts last fall, we have all been anxiously waiting to see if quaggas would turn up in the Snake River again this year. Unfortunately, this past September, those fears were partially realized as quagga larvae (known as veligers) were once again discovered in the Mid-Snake near Twin Falls. However, the number of positive samples was significantly lower than in 2023 and did not indicate significant further spread of the mussel distribution. It is not surprising to see some number of veligers present again this year, as quaggas are notoriously difficult to eradicate once they’ve been identified (annually, one female quagga can release 1 million eggs while one male can release 200 million sperm).
This year’s eradication efforts occurred from October 8-19, using approximately 35,000 gallons of Natrix in total to achieve the desired target concentration in the river. ISDA was able to attain the target copper concentration of 1 parts per million in the Mid-Snake River between Twin Falls Reservoir and Centennial Park and maintain that concentration of a total of 200 hours in order to ensure quagga mussel mortality. This is the same section of river that Natrix was applied to last fall, with the exception of one additional river segment upstream of Twin Falls Dam that was included this year due to the presence of quagga veligers during this year’s sampling efforts. ISDA also put additional focus on ensuring that the deeper holes in the river behind Twin Falls and Shoshone Falls dams reached the targeted copper concentrations. ISDA is planning a second phase of this year’s eradication effort in November to target quagga mussels in still water and deep pool sections of the river.
ICL had the opportunity to meet with ISDA staff at Twin Falls Park while the Natrix application was underway to observe those efforts in action. According to ISDA, the pesticide application went according to plan and their work will now be focused on sampling the river for copper concentrations to track if they are dissipating as expected. Thus far, testing shows that the river has already begun to return to background levels of copper. As was the case during last year’s Natrix application, there has been some level of fish mortality documented during the most recent quagga eradication efforts but not as severe as last year (birds and other life were not expected to be impacted).
What’s next?
ISDA will not know the efficacy of these eradication measures until veliger and eDNA sampling is conducted next summer/fall. Quagga mussels only reproduce when water temperatures are above 57 °F, therefore ISDA will have to wait for warmer water next year to conduct this sampling. We won’t be able to consider the Snake River truly free of quagga mussels unless it tests negative for quaggas for at least five consecutive years.
Regardless of whether this second attempt at quagga eradication 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. An ounce of prevention is worth a pound of cure for this issue.
While we are hopeful that this second quagga eradication attempt will be successful, in the event that it is not, we have significant reservations about expanding or repeating this approach in the Mid-Snake or beyond, and emphasize that each occurrence of invasive mussels warrants site-specific consideration. We also have lingering questions as to what the state’s threshold is for whether an eradication effort is still feasible and whether the state will pursue more aggressive prevention measures to prevent the spread of quaggas, such as boat check stations at popular boat ramps across the state and/or boating bans in the impacted stretch of the Mid-Snake.
For more information and ongoing updates regarding quagga mussels in the Snake River, visit ISDA’s official site at https://invasivespecies.idaho.gov/quagga-mussels.
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