Mark Healey might be part fish. The pro big-wave surfer can hold his breath underwater for six minutes. He once swam with a great white shark, hanging onto its dorsal fin.

And recently, the freediver got up close and personal with overfished sharks off the coast of Japan so he could tag them for scientists.

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Healey grew up in Hawaii and learned to swim before he could walk. He landed his first surfing sponsorship at age 13 and went pro at 17, according to his online bio. When he’s not surfing unbelievably big waves, Healey does dangerous movie stunts, goes spear-fishing and literally swims with the sharks.

This month he made the cover of Outside Magazine. ”I love having the opportunity to incorporate old knowledge like spearfishing into modern conservation and scientific discovery,” he told correspondent Thayer Walker.

The guy, who has more than earned his “waterman” moniker, learned about the ocean conservation nonprofit Beneath The Waves from fellow diver and marine conservationist  Tre’ Packard. The group needed Healey’s expertise to tag scalloped hammerhead sharks in deep water near the Japanese island Mikomoto.

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Populations of the scalloped hammerhead shark have declined drastically, in part from overfishing. Scientists want to tag them to better understand their migratory patterns and numbers, which will in turn help target their conservation efforts, Walker explained.

This is no simple task. Unlike other scientific shark-tagging operations, these hammerheads can’t be caught, tagged and released. Catching them would kill them. Despite growing up to 200 pounds and eight feet in length, they’re also hard to spot, given their skittish nature.

Healey and the scientific team spent nearly a week aboard the Otomaru before conditions were right for encountering them. The waterman got to work, free diving down to 135 feet, using a special device to launch tracking tags into a specific area behind the dorsal fin, and then resurfaced — without any extra oxygen.

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“I get so much from the ocean,” Healey said in a recent video for GoPro, one of his sponsors. “You have to give back. You have to try to balance the scales.” Watch him successfully tag sharks here:

As a child, Healey used to accompany his father on spearfishing expeditions. “While my father was diving he’d drag me around on the buoy where all the fish were strung up,” he recounted on his site. “Looking back it’s pretty amazing that I never got eaten by a tiger shark.”

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One of his most amazing shark feats was on a dive with great whites near Guadalupe Island in 2011 for a video shoot. He ended up hanging onto one shark’s dorsal fin for a ride — and somehow didn’t get hurt.

With all the shark encounters the man has racked up over the years, they probably think he’s one of them now.