Anxious South Bay residents have been watching...

Anxious South Bay residents have been watching their shores with increased vigilance since a tanker poured hundreds of thousands of gallons of oil into the ocean off Huntington Beach earlier this month, say those who monitor the seas.

The Coast Guard said a white, milky substance spotted several hundred yards off Point Vicente in Rancho Palos Verdes last week was a mixture of sea foam and oil that had naturally escaped from the ocean floor.

The filmy substance, reported to the Coast Guard on Wednesday, first by the operator of a private aircraft and later by other residents, has almost completely dissipated and is not the result of any accident, Coast Guard spokesman Craig Jennings said.

Jennings said the 11 million gallons of oil that tainted Prince William Sound in Alaska last March and the almost 300,000 gallons of crude oil that spilled off the Orange County shoreline Feb. 7 have put many people along the beach on edge.

Added to that is anxiety created when a 7.6-million-gallon spill of treated sewage from a Culver City facility closed Los Angeles County beaches recently. “Everything you could possibly report is getting reported,” Jennings said. “We have people concerned. Now we have to teach them about what’s normal and what’s not.”

Residents phoned in another hot tip Thursday of a possible oil spill off Torrance Beach.

The culprit, the Coast Guard later determined, was black sand.