On Oct. 17, the tiny freshwater mollusks [golden mussel], which have already laid siege to waterways of southern South America, were found at Rough and Ready Island, near Stockton. Since then, state officials said, it has been in at least one other location, O’Neill Forebay, in Merced County.

"The species poses a significant immediate threat to the ecological health of the Delta and all waters of the state, water conveyance systems, infrastructure and water quality,” staff officials wrote.

The Department of Water Resources is already conducting vessel inspections in the hopes of preventing spread of the mussels. In the San Luis State Recreation Area, officials have been inspecting watercraft exiting O’Neill Forebay, San Luis Reservoir, and Los Banos Creek Reservoir, said Tanya Veldhuizen, the department’s special projects section manager. The inspections are to “ensure all water is drained from livewells and bilges to prevent spread of invasive species to other water bodies.”

The department, she said, is also taking heightened measures to protect the State Water Project — the system of pumps, pipes, and canals that exports water south from the Delta. This enhanced vigilance to mitigate “mussel biofouling,” she said, requires more frequent inspections, as well as cleaning and flushing. The mussels, she said, are likely to build up in screens, strainers, and trashracks.