Yet tuna still aren't fished sustainably, something that conservationists and big U.S. tuna companies are trying to fix. This shows one aspect of the pressure on the world's oceans to feed a growing global population, now 7 billion. It also demonstrates 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."

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 (FAO).

In the U.S., limits on fishing have allowed some fish populations to grow. In international waters, however, covering more than half of the world's oceans, no single country oversees ocean conservation. Instead, regional multinational organizations make the decisions. These groups began operating after World War II, when their job was seen as dividing up what was then thought to be the unlimited wealth of the seas, said Amanda Nickson, who oversees Pacific tuna conservation efforts at the Pew Environment Group. Today, Nickson said, these groups aren't doing a very good job of restoring tuna populations and making sure they can be fished sustainably.

One of these groups is the Western and Central Pacific Fisheries Commission, which oversees more than 60 percent of the world's tuna catch. Its members include Pacific island nations and the homes of the world's large industrial fishing fleets — the U.S., Europe, Japan, China and Taiwan.

Nickson said it's an unfair matchup of island nations pushing for sustainable fishing versus the large fishing nations, which block the restrictions needed to achieve it. The Pew Environment Group is pressing it to set limits on the amount of fish that can be caught for each species; to take action to protect sharks, which are unintentionally caught along with tuna; and to reduce the number of juvenile bigeye tuna, an overfished species, caught by ships fishing for skipjack tuna.

Skipjack, the most common tropical tuna, is very heavily fished in some places, but isn't yet overfished, said William Fox, a biologist and the World Wildlife Fund's U.S. vice president for fisheries. Skipjack is the only tuna species that hasn't been overfished, according to the United Nations FAO.

"We're in a race with time, because we're trying to get the regional fisheries organizations to make sure that doesn't happen," Fox said. For U.S. fishing fleets, former President George W. Bush signed a law in 2007 that required annual catch limits based on scientific analyses in order to end overfishing by 2011. The limits were in place by the end of 2010.

"We presume those catch limits have ended overfishing, but we have to verify it, and we won't declare it the end of overfishing until the scientific assessment is complete," said Eric Schwaab, director of the U.S. National Marine Fisheries Service.

Limiting the number of fish that can be caught is only part of the picture, Schwaab said. Damage to habitats along the coasts and in the ocean "will probably continue to challenge our ability to rebuild stocks to their past levels," he said.