The fishing industry has long been hard to monitor. Its global footprint is difficult even to visualize. Much fishing takes place unobserved, far from land, and once the boats move on, they leave behind few visible traces of their activity.

But this week, the journal Science published some remarkable maps that help fill that gap. John Amos, president of an organization called SkyTruth, which helped produce them, issued a statement calling the maps "a stunning illustration of the vast scope of exploitation of the ocean."

SkyTruth and its collaborators tracked most of the world's fishing vessels through an entire year by monitoring radio transmissions that most vessels now emit automatically in order to avoid collisions with each other. The researchers were able to distinguish between different kinds of vessels — trawlers that drag nets behind them, for instance, versus vessels that deploy drifting "longlines" that are often used to catch tuna.

The maps show the most intense fishing activity along the coasts of heavily populated areas like Europe and southern China. But fishing also covered much of the high seas. According to the researchers, commercial fishing operations covered at last 55 percent of the world's oceans. That area, it calculates, is four times larger than the area devoted to agriculture on land.

The researchers also were able to distinguish between fishing vessels from different countries. According to the study, five countries — China, Spain, Taiwan, Japan, and South Korea — accounted for 85 percent of all high-seas fishing.

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