From this story in the Los Angeles Times:
Officials were trying to determine Tuesday what caused thousands of sardines to turn up dead in Ventura Harbor, another puzzling case of fish that died off after apparently using up all their oxygen. Harbor master Scott Miller said he arrived Monday morning to find patches of dead sardines floating on the surface of the southwest corner of the harbor. Other fish bobbed near the surface, appearing to gasp for air.
After deploying aerators to stir up oxygen below the surface, a dozen volunteers used nets to scoop about 6 tons of fish carcasses from the water before dumping them offshore, he said. The incident comes about six weeks after millions of sardines died in Redondo Beach after swimming en masse into King Harbor and suffocating. In a massive cleanup last month, workers removed about 175 tons of fish carcasses that floated in the marina and began to rot underwater. Although the dead fish in King Harbor tested positive for domoic acid, a poison generated by toxic algae blooms, scientists believe the fish gathered in the enclosed harbor in such huge numbers that they died of critically low oxygen levels, not poisoning