From this story in the Seattle Times:
Three weeks after a huge fish die-off in Southern California, officials said Thursday they have a body count but still can't say what drove 175 tons of sardines into the marina where they died. As many as 2.5 million sardines created a silvery blanket on the surface and floor of King Harbor Marina on March 8, said Dave Caron, professor of biological sciences at the University of Southern California. Several theories have been offered about the unusual behavior. Some said the sardines were lost. Others suggested the fish had been chased by marine predators or ingested toxins that confused them. Redondo Beach City Manager Bill Workman said he even heard from people who believed the sardines may have sensed the coming earthquake in Japan and fled.
_Once in the marina, the sardines used up all the oxygen and died. Residents who live on boats reported hearing what sounded like hail but was really fish coming to the surface gasping for oxygen. Boats were temporarily trapped by the fish carcasses in the south Santa Monica Bay harbor that shelters about 1,400 boats. Hundreds of volunteers and city workers scrambled to remove the remains to ease the pungent smell and the potential threat to other sea life. They may have gotten some unexpected help from the tsunami in Japan, which caused tidal surges and helped to flush fish out of rocks and crevices, Workman said. Oxygen levels were slowly returning to normal in the harbor in large part because the city was able to clean up the mess in less than a week. Officials estimated the effort cost between $300,000 and $500,000.