Off the coast of South Africa, fish bunch together to survive. But these "bait balls" attract swarms of predators, from hungry seabirds to sharks and whales.

On Cape Town’s western shore, near a big-wave surf spot called Dungeons, is a low, flat island that seals have made their own. They snooze and bellow and nurse pups, and now and then heave themselves into the Atlantic, where snorkelers can join them in their frolics around reefs and through kelp forests. Sunlight sparkles on air bubbles trapped in their fur, and when they somersault and speed away, they trail a champagne wake.

The island lies within the Karbonkelberg Restricted Zone, a “no take” sanctuary inside a much larger protected area that includes most of Cape Town’s coastline. Karbonkelberg is the kind of place where a person, enchanted by whiskery seal faces staring into his own, can feel that all’s well in the oceanic world.

Unless, as I did, he were to look up and notice a line of men toiling up a hillside path with heavy sacks on their backs. Breaking away from the gymnastic seals, I swam to a tiny cove and stepped ashore onto a carpet of discarded abalone shells. They were the size of soup bowls, and they shimmered with nacreous shades of pink and green, like scenes from an aurora. The air was pungent with the stench of seals and rotting kelp. An ibis stalked among the shells, pecking at scraps of abalone guts. I climbed onto a flat-topped boulder that minutes before had been a shellfish abattoir. Here the men had thumbed the meat out of the shells and filled their sacks.