HANGBERG, South Africa — One man vanished diving at midnight. Another was attacked by a great white shark in deep water. Two more drowned, one in borrowed scuba gear he wasn’t qualified to use.

All four men, who died over the past few months, were casualties of an entrenched illicit trade: poaching abalone, a seafood delicacy that sells for enormous prices in Asia.

The shellfish, which once smothered reefs in South Africa, in some places packed as tightly as cobblestones, has become more difficult to find as a result of overfishing, luring untrained divers into deeper and more deadly waters.

“The old reefs have been hit too hard and haven’t had time to recover,” said a diver from Hangberg, an impoverished fishing community in Cape Town, who insisted on anonymity because he did not want to alert the police to his poaching.