TALCAHUANO, Chile — Eric Pineda, a dock agent in this old port south of Santiago, peered deep into the Achernar’s hold at a measly 10 tons of jack mackerel — the catch after four days in waters once so rich they filled the 17-meter fishing boat in a few hours.

Mr. Pineda, like everyone here, grew up with the bony, bronze-hued fish they call jurel, which roams in schools in the southern Pacific.

“It’s going fast,” he said as he looked at the 57-foot boat. “We’ve got to fish harder before it’s all gone.” Asked what he would leave his son, he shrugged: “He’ll have to find something else.”

Jack mackerel, rich in oily protein, is manna to a hungry planet, a staple in Africa. Elsewhere, people eat it unaware; much of it is reduced to feed for aquaculture and pigs. It can take more than five kilograms, more than 11 pounds, of jack mackerel to raise a single kilogram of farmed salmon.