The California Department of Water Resources announced Monday that for the first time it spotted quagga mussels, a non-native species, in Castaic Lake.

Staff from DWR and the California Department of Fish and Wildlife found two quagga mussels shells in Castaic Lake on Aug. 17. DWR has monitored the lake since 2008, according to Maggie Macias, a representative for the agency.

“The pathway of introduction and the time of introduction is currently under investigation,” she said in a statement to The Signal.

Quagga mussels pose a threat to California’s native species and can clog water systems, colonize hard surfaces, alter food webs in ecosystems and damage boat engines, according to DWR.

In response to the discovery, the Los Angeles County Department of Parks and Recreation, which oversees the Castaic Lake Recreation Area, will inspect and drain boats leaving the freshwater manmade lake.

“Boat inspectors will check all boats to ensure drain plugs are pulled, ballast tanks are pumped, live wells are drained, and bait buckets are properly emptied,” said Macias.

Boats departing Castaic Lake will receive a tag to indicate that they were last used in a waterbody containing mussels. Inspections of boats entering Castaic Lake have been required since 2011.

The new boat exit inspection requirement is meant to prevent the spread of the invasive mussels, which were found in Pyramid Lake in 2016 and Lake Piru in 2013. Pyramid Lake is upstream from Castaic Lake and both bodies of water are part of the state’s water delivery system.

In 2016, officials worried that the mussel would travel to Castaic Lake from Pyramid Lake, where a few quagga mussels have recently been found.

Macias said Castaic Lake is monitored twice a month during the mussels’ peak breeding season from April through October. During the cooler months of November through March, she said, the lake is monitored once a month.

“Currently, one live mussel has been found after conducting extensive surveys of the shoreline and collecting water samples to examine for the microscopic larval stage,” said Macias. “Over the coming months, DWR will evaluate the feasibility of eradicating the mussel.”

Last month, DWR also announced that construction had started on the Castaic Dam tower access bridge, which is being retrofitted “to reduce seismic risks during a major earthquake,” according to the agency.

A temporary water level drawdown of more than 100 feet was completed in May.

“There will need to be sufficient rainfall and runoff in 2022 to refill Castaic Lake to normal operating levels,” Macias told The Signal last month.