On a warm July morning, in the sapphire-colored waters west of the Spanish Island of Ibiza, six purse-seine boats from three competing companies searched for giant bluefin tuna. The purse seiners—named for their conical, purse-like nets, which are drawn closed from the bottom—were guided by three spotter aircraft that crisscrossed the sky like vultures

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In the center of the action was Txema Galaz Ugalde, a Basque marine biologist, diver, and fisherman who helps run Ecolofish, one of 69 tuna ranching, or fattening, operations that have sprung up through the Mediterranean. A small company, Ecolofish owns five purse-seiners. Its main rival that morning was the tuna baron of the Mediterranean, Francisco Fuentes of Ricardo Fuentes and Sons, whose industrial scale operations have been chewing up giant bluefin stocks.

I was with Galaz on La Viveta Segunda—a 72-foot (22-meter) support vessel that was part of the fleet of dive boats and cage-towing tugs following the purse seiners. Around 11 a.m., the spotter planes spied a school, setting the purse seiners on a 19-knot dash. The stakes were high. Even a small school of 200 bluefin can fetch more than half a million dollars on the Japanese market. Galaz watched through binoculars as an Ecolofish seiner reached the school first and began encircling it with a mile-long net. "He's fishing!" Galaz shouted. "He's shooting the net!"

Source: An excerpt from The Global Fish Crisis: Still Waters, by Fen Montaigne. Republished from the pages of National Geographic magazine