Normally, the only dorsal fins Courtney Hemerick and Joey Trucksess see when they're surfing belong to dolphins.

But then they heard a rumour: great white sharks had been spotted in the waters they normally surf an hour south of Los Angeles. So Hemerick and Trucksess paddled out to see for themselves.

The minute they got outside the wave breaks of Seal Beach, they saw it.

Trucksess says it was like seeing a dinosaur.

"My knees locked, you start getting sweaty palms. You're tripping out, because there's this giant creature under you. You're definitely not top of the food chain anymore," Trucksess says.

Using GoPros and two selfie sticks, the surfers got footage of their encounter. In the water beneath their boards: not one but two great white sharks.

"They were six feet, about the same size as I am," Hemerick says.

Juvenile great white sharks are increasingly being spotted in the waters off Seal Beach, Calif. (Cal State Long Beach Shark Lab)

Great whites can grow more than six metres long, but luckily for the two surfers, these sharks were just juveniles, which normally hunt bottom-feeders such as stingrays and aren't yet interested in eating mammals.

"What else surprised me is how gentle they were," Hemerick says. "They were just swimming alongside us. They didn't care we were there."

What the sharks were doing there nonetheless puzzled marine biologists. Southern California waters are a nursery for great whites in the eastern Pacific, but two-metre juveniles don't normally come so close to shore.

Now, they're being spotted in growing numbers at some of the most popular beaches in the Los Angeles area.

"Typically in the past, when our waters get cold in the winter, those little sharks have migrated all the way to Baja [Mexico]," says marine biologist Chris Lowe. But the head of the Cal State Long Beach Shark Lab says this summer, they're seeing something very different.

Using state-of-the-art technology -- including underwater robots and airborne drones -- to shadow the sharks, scientists and safety experts are trying to figure out why the creatures are coming so close.

Tracking sharks

The metre-and-a-half-long robot looks like a white missile with orange fins and a black nose cone. Lowe and his team drop it into the water and launch it on its mission: to follow the sharks and get a 3D picture of their world.

Chris Lowe, head of the Shark Lab at Cal State Long Beach, says the emergence of juvenile sharks near California beaches is 'a sign that we've done things better in the ocean to bring these populations back.' (Kim Brunhuber)

"The robot's programmed to move up and down through the water to measure all the parameters of the ocean while it knows exactly where the shark is in 3D space and time," Lowe says. "By combining all that information, we can ask questions as to why did it go down, why did it go up, was it the water temperature, did it see a school of fish?"

Lowe says warmer ocean water, likely due to El Niño, may be responsible for the increase in young sharks, who are congregating in shallow water close to beaches to feed on stingrays.

"Our water temperatures have never gone below 15 degrees Celsius. And therefore the sharks stuck around," Lowe says. "Along our beaches particularly in the summer, stingray populations are quite healthy. So this is a great place for them to come to find an abundant food source. The water's warm and it's generally safe from predators."

But is it safe for humans? On any given day in southern California, there can be as many as 10,000 people in the ocean – people that might look appealing to maturing sharks that have grown tired of eating stingrays.

Joe Bailey, marine safety chief for the city of Seal Beach, gets ready to fire up his drone. (Kim Brunhuber)

Great whites had never been an issue on Seal Beach before. So when they started showing up this summer, Seal Beach Marine Safety Chief Joe Bailey had a novel idea. Instead of sending lifeguards on jet skis to keep an eye out for these potentially dangerous new visitors, why not use a drone?

"The reason this is better is it gives us a really high vantage point," Bailey says. "The lifeguard on the jet ski is only about five feet off the water, so it was difficult to see sharks. So this is really making it easy for us."

That's why, on a sunny morning on the beach, Bailey is wearing a towel over his head.

"I know it's goofy," Bailey says, "but it cuts all the glare so I can see the screen."

Surveilling from the sky

This aerial shot from a drone shows how close this shark is to the surf, not far from Seal Beach in California. (Joe Bailey) Bailey manipulates the joystick and the drone – the same type of quadcopter you can buy on Amazon for about $1,200 US – flies off on patrol. About 20 seconds later, Bailey calls out. He's already spotted one.

"Right underneath it is a shark," Bailey says. In four minutes we see four great whites. And it's not a particularly busy day.

"Two weeks ago, we saw 15 individual white sharks in this area," Bailey says.

Chris Lowe may be the only person who thinks that's great news.

"Seeing all these babies around our beaches is exciting," Lowe says. "To me, it's a sign that we've done things better in the ocean to bring these populations back."

Lowe expects his robot to find out exactly what happens when the young sharks and humans mix.

"The robots are moving by people and they have video cameras and image sonars, and all that information allows us to understand how the sharks interact around people. We've had two generations not used to sharing the ocean with sharks. Now that we're bringing them back, we have to learn how to get used to it."

Bailey will use his drone to keep an eye on the sharks. He's watching for aggressive behavior, like the sharks bumping surfboards or swimmers' limbs. One or two over-curious young sharks have forced lifeguards to close other beaches. But not Seal Beach. Yet.

"It's when they get bigger, when they might confuse a surfer or a swimmer for a seal that we become real concerned," Bailey says.

'It's changed my perspective'

The scary thing is how close they are to the swimmers; several of the animals are gliding near the wave breaks only about 20 metres from shore. Just on the other side of the breakwater, a line of more than 100 young surfers streams towards the water like aquatic ants.

For surfers Courtney Hemerick and Joey Trucksess, their encounter has left them with greater respect for the life underneath their boards.

"It's changed my perspective," Trucksess says. "That I'm a guest – it's not my break, it's not my wave, it's not my ocean, I'm definitely in someone else's territory."

Hemerick says that with a baby on the way, there will be no sequel to his video for him.

"I'm kind of scared, to be honest," Hemerick says. "I've been surfing all my life. Thinking of sharks has always been in the back of my mind. Now that we've seen them… you think it about more."