Yet tuna still aren't fished sustainably, something that conservationists and big U.S. tuna companies are trying to fix. This illustrates one part of the pressure on the world's oceans to feed a growing global population, now 7 billion. It also underscores the difficulties people have in balancing what they take against what must be left in order to have enough supplies of healthy wild fish.

"It's serious. On a global basis, we've pretty much found all the fish we're going to find," said Mike Hirshfield, chief scientist at the advocacy group Oceana. "There's not a lot of hidden fish out there. And we're still heading in the wrong direction, taken as a whole."

Some 32 percent of the world's fish are overfished, up from 10 percent in the 1970s and 25 percent in the early 1990s, according to the United Nations Food and Agriculture Organization.

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