Some 15 great white sharks surrounded paddleboarders off Capistrano Beach in Dana Point and San Onofre State Beach in San Clemente, California, on Wednesday.
The Orange County Sheriff's Department spotted the sharks via helicopter and warned the paddleboarders and swimmers about the swarm in the area, according to the Los Angeles Times.
"Attention in the water, this is the Orange County Sheriff's Department. You are paddle-boarding next to approximately 15 great white sharks," warned Deputy Brian Stockbridge in an announcement to the swimmers, CBS News noted. "Exit the water in a calm manner."
Video shot by the sheriff's department and posted on YouTube had received more than 105,000 views as of Friday.
Most of the sharks reportedly looked like juvenile great whites – somewhere from six to eight feet long and about a year old.
"Shark populations are increasing," marine biologist Chris Lowe, who runs the shark lab at California State University, Long Beach, told CBS News.
"The reason why I think we're seeing more sharks is because we've protected them. They've been protected in U.S. waters since 2005," Lowe said. "The other thing is, there's lots of food. So along our beaches, the sting ray populations have been kind of exploding over the last 50 years."
Lowe said the younger sharks are looking for food in safe spaces and are not necessarily interested in snacking on humans.
"We haven't seen signs of those young sharks actually being a potential threat to humans," Lowe told CBS News. "When they're babies, they're afraid of pretty much everything, so they may not really be that different from us."
The San Diego Union-Tribune reported that on April 29, Leeanne Ericson was attacked by a shark while swimming off San Onofre beach. The shark bit her on the right buttock and upper leg to the bone.
The Union-Tribune reported last week that Ericson remained in critical condition after two surgeries at Scripps Memorial Hospital in La Jolla, California as doctors tried to save her right leg.