Idaho State Department of Agriculture invasive-species investigators have found more quagga mussels in the Snake River near Twin Falls, a year after an initial discovery there prompted large-scale treatment of the state’s largest waterway.

The new detection included a “small number” of quagga mussels, according to ISDA. Current sample results show a smaller quagga mussel presence in the affected stretch of the river since a pre-treatment survey was conducted in 2023.

The department did not state the number of new mussels found.

“We hesitate to put hard numbers on it,” ISDA public information officer Sydney Kennedy said. “Staff are currently conducting delimit sampling.” The sampling and data analysis involve where the new mussels were discovered as well as the river stretch treated last year.

Quagga and zebra mussels can clog irrigation and drinking water systems and hydropower equipment.

The tiny mussels can attach to boats and other watercraft and remain there during transport.

In 2023 sampling, ISDA staff found an adult quagga mussel and larval “veligers” below Shoshone Falls. Treatment with a chelated copper product, which killed mussels and other organisms, occurred over a six-mile stretch from Twin Falls Dam to Centennial Park. The department established a 16-mile dissipation area downstream from the park.

Post-treatment sampling has continued.

New positive detection locations include the Shoshone Falls pool, Pillar Falls and Twin Falls Reservoir behind the Twin Falls hydroelectric facility. Treatment is planned for 15 days starting in mid-October.

It is unlikely that a fouled watercraft entering the system in 2024 is the source of the new mussels, which were detected in already closed areas except for Pillar Falls, Kennedy said. The department conducts mandatory pre-launch inspections at the Pillar put-in site.

“What we do know is that quagga mussel reproduction is most active this time of year,” Kennedy said. In sampling,”you could go all summer getting negatives, and then get a positive in the same spot even if nobody is bringing a boat in.”

Quagga mussels reproduce quickly. A female adult quagga mussel can release up to 1 million eggs a year, she said.

The legislature this year spent more money on the longstanding invasive-species program and enacted more stringent requirements for boaters. The money went toward increased statewide river and lake monitoring and sampling, watercraft inspection and public awareness.

“While we never want to see confirmation of quagga mussels, we know that pest eradication is rarely ‘one and done,’” ISDA director Chanel Tewalt said in a release. “We have made significant progress and now must focus on the next phase of eradication.”

“With an effective, robust invasive species program in place, we are far better off today than we were even a year ago,” Gov. Brad Little said in a statement in which he also thanked the legislature. The 2023 quagga response in the Snake River was successful, “but we knew we would be on watch for a period of five years.”