A teenage boy was mauled to death in front horrified witnesses who tried frantically to save him from a shark that had bitten his leg at a popular South Australian beach.

The 15-year-old talented surfer was understood to be on a day trip with his father at Ethel Beach on the Yorke Peninsula when the tragedy unfolded on Thursday afternoon.

Locals told The Adelaide Advertiser the ocean predator grabbed the boy’s leg in its savage jaws while he was surfing the challenging break.

“The shark took his leg, and so another local guy ran out, jumped on his board and paddled out to help him,” one local said.

“The shark was circling them as the guy pulled the boy out of the water. There was a lot of blood.

“He brought him to shore but I think it was too late by then.”

The Advertiser reports that the boy was on a day trip in the area with his father, who is understood to have witnessed the terrifying incident.

Emergency services were called to the popular surf beach in Innes National Park, about 300 kilometres south-west of Adelaide, with reports of a shark attack at 1.30pm on Thursday.

There, at the beach hemmed in by towering cliffs and named for its Ethel ship wreck, they recovered the teenager’s body.

Just before Christmas, local photographers Adam and Bel Meyer had filmed sharks chasing schools of baitfish off the Yorke Peninsula and informed authorities.

Shark expert Andrew Fox said ocean conditions may have been conducive to a shark attack.

“Today this incident could be related to being overcast, fish running past that surf beach area, and slightly dirty water – are all conditions that help ambush predators become more motivated.”

Adam Meyer filmed sharks chasing baitfish off the Yorke Peninsula. Photo: AdBe Photography

Local surfer Luke Sykora spoke of his shock at the tragedy.

“I have a seen a fair few sharks here over 22 years but that is the first fatal attack that we’ve had down here,” he told ABC News.

“I wouldn’t wish it upon anyone … to even be at the same beach at the same time.

“I think the town would be mourning. I think it’s very sad and very tragic and a bit of an eye-opener.”

Ethel Beach is popular over the Christmas holidays. Photo: southaustralia.com

The attack is the third fatal mauling in South Australia this year and the fifth shark attack.

In October, Tod Gendle, 55, was fatally bitten at Granites Beach on the state’s west coast in front of friends and other surfers, who saw the predator dragging him under water.

School teacher Simon Baccanello, 46, disappeared without a trace in May while surfing at Walkers Rock Beach, about 365 kilometres west of Adelaide.

Police are preparing a report for the coroner.

-with AAP