Off South Africa's coast, great white sharks are threatened

On the edge of a boat off this coastal village, Michael Rutzen stubs his cigarette into a soda can and stares pensively out to sea.

He has free-dived with great white sharks for nearly 20 years, and he has never known it to be this difficult to find them.

Extensive research by Rutzen and his marine biologist partner, Sara Andreotti, has found that great whites off the South African coast are rapidly heading for extinction. The area long has been one of the world's best places to see the sharks, which also live off Australia, the United States and Japan.

Streams of fish blood and oil trail in the water behind other cage-diving boats nearby. Rutzen phones one of his...