Marine officials banned swimming and surfing in the waters off Huntington Beach on Friday morning after a shark exhibited "aggressive behavior" and bumped a surfer.
It marked the first time the Huntington Fire Department's Marine Patrol has closed water access, said Lt. Claude Panis.
But the shark's behavior was so aggressive that marine officials had no other option, he said.
Waters from Beach Boulevard to the Huntington Beach Pier and Huntington State Beach will remain closed until 8 a.m. Saturday. The beach, however, remains open.
At about 8:45 a.m., a surfer off lifeguard Tower 17 was bumped by a 7-foot great white shark, Panis said. The surfer's report was deemed credible.
The closure left many beachgoers surprised and disappointed as some planned to surf and receive lifeguard training.
The shark encounter capped off a week of shark sightings at Sunset Beach, where they have been seen in the past. The sighting prompted marine safety officials to post signs along the beach, warning visitors of the sharks, he said.
"The ocean is like a big wilderness," Panis said. "There are many sharks out there."
In May, more than a dozen 5- to 7-foot juvenile sharks were spotted swimming and feeding in the water about 50 feet from the shore. In the past, the sharks have not been aggressive and have not attacked humans.
After the group was spotted, marine researchers returned to the water and tagged seven great white sharks with transmitters. The transmitters track the sharks' movements and provide researchers with other information.
The latest sighting comes days after a Stanford University report indicated that the risk of shark attacks in California has dropped by more than 91% since 1950.
Researchers said the decrease was likely due to human behavior. More beachgoers know where and when to enter the water to avoid shark encounters, they said.