SAN DIEGO (CBS 8) - Lifeguards are on alert after new shark sightings along the Southern California coast.

A shark sighting prompted a safety warning Wednesday at a North County beach where a swimmer was gravely injured less than two weeks ago in an attack by a great white or similar large sea predator.

"Any shark is potentially dangerous. Most of the white sharks we have in Southern California are juveniles - less than 10-12 feet. They are mostly eating fish," said Andy Nosal, marine biologist. 

Marine Corps Community Services Camp Pendleton issued the alert pertaining to San Onofre Beach, a stretch of coastline open to the public on the grounds of the USMC base north of Oceanside.

"The fact that we have so many sharks and so many surfers in the water with so few incidents, it's a testament that they don't normally make mistakes," said Nosal. 

The shark spotted Wednesday morning at a surf break known as Church was exhibiting "aggressive behavior," according to the announcement. The resulting warning will remain in effect for 48 hours, officials advised.

On April 29, a shark attacked 35-year-old Leeanne Ericson of Vista as she swam at the beach, tearing off much of her right buttock and thigh.

Ericson, who nearly drowned following the strike, was admitted to a San Diego trauma center in critical condition and underwent extensive emergency surgery. Six days later, her physicians reported that her recovery was progressing well, particularly considering the severity of her wounds and resulting blood loss.

Also on Wednesday afternoon, about five sharks have been spotted in the water about 50 feet off the Long Beach shoreline.