The spread of golden mussels in California has alarmed farmers this year as the invasive species established itself in waterways across much of the state. In the Sacramento-San Joaquin Delta, the freshwater mollusks began obstructing irrigation systems that sustain billions of dollars’ worth of tree nuts, winegrapes, tomatoes and other crops. “It’s a nightmare,” said Trey Steinhart, operations director at Zuckerman Family Farms in San Joaquin County. “There’s nothing you can do to stop them.”