Transcript for Shark warnings along the California coast

Next tonight, the warning from a helicopter above, to alert swimmers that there were several sharks swimming around them in the water. ABC's senior national correspondent Matt Gutman tonight. Reporter: The warnings, blaring up and down the California coast. Dozens of great white sharks feet from some of the nation's most popular beaches. You are paddleboarding next to approximately 15 great white sharks. Reporter: Orange county sheriffs using a helicopter loudspeaker. Exit the water in a calm manner. Reporter: Sightings from long Beach to San Onofre, where a woman nearly lost her life from a shark bite just two weeks ago. I don't think I'll ever go in again. Reporter: We join shark lab's professor Chris Lowe on a mission to tag as many as he can. I never thought I'd actually see the day where I would see that many in one morning. Reporter: In one 400-yard stretch, we have 20 sightings. Right here! Reporter: Researchers daringly sneaking up on the sharks to get that tag on. Scientists believe the ocean has been so unusually warm the past 18 months that the sharks haven't needed to migrate to Mexico as they normally would in winter. The good news, these sharks would rather eat stingrays than humans. Matt Gutman, ABC news, long Beach.

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