HALF MOON BAY (KPIX) — Sightings of great white sharks off the coast of Half Moon Bay have prompted an alert from the sheriff’s office.

The San Mateo County Alert System notified of great white sightings from Pillar Point Harbor to the Ritz Carlton in Half Moon Bay.

The sign posted at Francis Beach carried an ominous warning: “Advisory, shark sighted. Enter at your own risk.”

The sheriff’s office tweeted, “Multiple pilots flying over the San Mateo County coast have spotted great white sharks in the water today. If visiting the area please use caution.”

Surfers continued to ride the waves into the evening but Sam Huo said she was reconsidering the activity.

“Uh, yeah … that’s a game changer for me,” she said.

Marine biologist Giancarlo Thomae told KPIX that more sharks are being spotted along the coast. A group of more than a dozen great white sharks was captured on video Tuesday in Aptos in Santa Cruz County. Thomae said the young adult great whites are temperature sensitive so, if they are spotted near the shoreline, they are usually just basking in the sun.

“I would not worry about seeing these sharks,” Thomae said. “In fact, they are not in feeding mode. What they are doing is they are finding the warmest water that they can just to warm up.”

Fisherman John Thompson said he sees them all the time. And he’s not worried.

“Like any animal, they’re really afraid of you,” Thompson said. “You’re something different. You go up and see a deer, it’s going to run off.”