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For all the reports about overfishing, it can sometimes be hard to except that we really have a problem. After all, if we’re supposedly fishing out the seas, why is it easy—and cheap—to get salmon, crab, tuna and any other delicacy you want at the local sushi counter? Why have McDonald’s Filet-o-Fish sandwiches skyrocketed in cost? If fish are going, how come they’re still around?

Ocean advocates have an answer for that—as we’ve fished out some parts of the ocean, we’ve simply moved to untapped waters, often with the aid of better and bigger ships. (Callum Roberts’s great book An Unnautral History of the Sea traces our nomadic fishing habits.) That shift, along with the rise of the farmed fishing industry, has kept us swimming in seafood. Now environmentalists have the data to back that claim up. In a study published yesterday in the open-source journal PLoS One, a group of fishery experts and oceanographers showed that global fisheries have expanded geographically over the past 50 years, keeping a fresh supply of fishing—but that the world’s fishing fleets are now running out of ocean. (Download a PDF of the study here.) Charting the movements of fishery fleets since 1950, the researchers showed that boats have been expanding southward at about one degree of latitude a year, moving away from the long-exploited waters of the Northern Hemisphere.

While the fleets migrated, the global fish catch rose from 19 million metric tons in 1950 to a high of 90 million at the end of the 1980s, before declining to 79.5 million tons in 2008. As Daniel Pauly, a researcher at the University of British Columbia’s Fisheries Centre and one of the paper’s authors, told Juliet Elperin of the Washington Post:

Global seafood catch is dropping “because there’s essentially nowhere to go.” The fact that fish catches rose for so many decades “looks like sustainability but it is actually expansion driven. That is frightening, because the accounting is coming now.”

Essentially we’ve been digging into our capital stock of fish, and the bill is coming due. Short of significantly reducing our catch of vulnerable fish, we are looking at a future where wild seafood may become a rarity. (We’re already on that path—half the fish consumed today in the world is farmed, not caught, though that comes with its own set of environmental problems.) But there’s little evidence that governments are willing to restrict fishing—just last month the International Commission for the Conservation of Atlantic Tunas (ICCAT) refused to put severe catch limits on bluefin tuna, despite increasing evidence that the species is headed for extinction. (ICCAT did put restrictions on the capture of vulnerable shark species, though they are considerably less valuable than the bluefin.) The only untapped waters are the high seas, far from coasts—which tend to be unproductive—or the remote waters of the Arctic and Antarctic.

The study’s authors draw an interesting parallel to agriculture. Both fishing and farming underwent enormous expansions in the postwar era, helping to feed an exploding global population. But farmers managed to double global agricultural production between 1961 and 1995, while only increasing the amount of land under cultivation by 10%. (The Green Revolution made existing plots of land far more productive, albeit at an environmental cost.) Over the same period the global fish catch increased by 2.4 times, but it required an 4-fold increase in exploited waters. As the study’s authors conclude:

Our results demonstrate that the growth in the world’s marine fisheries over the past 56 years was driven through a sequential exploitation of new fishing grounds. Fisheries now cover a majority of the world’s ocean, with areas of low productivity and distant waters as the final remaining ‘frontiers’. The decline of newly exploited areas since the late 1990s, which corresponds to a decline in global landings, implies that the era of great expansion has come to an end.

Not every scientist is convinced that the world’s fisheries are headed for an aquacalypse. (Read aquatic scientist Ray Hilborn’s counterpoint on the future of global fisheries, which he views much more positively.) But there’s little doubt that with more people demanding more fish, global fisheries will be strained—and we’ll need smart, tough management more than ever.

Of course, if that’s not enough, we’ll also have to deal with the threat of ocean acidification. In a new study released at the Cancun climate summit yesterday, the United Nations Environment Programme reported that increasing carbon emissions are already beginning to change the pH balance of the oceans—and a more acidic ocean could have major impacts on the food supply. Gathering data on ocean acidification, the report found that corals and shellfish will likely find it more difficult to build skeletons, which could reduce their numbers. The report also found that ocean acidification and ocean warming could interact in a way that will limit the range of species like crabs:

“We are seeing an overall negative impact from ocean acidification directly on organisms and on some key ecosystems that help provide food for billions,” said Carol Turley, a senior scientist at Britain’s Ocean Acidification Research Programme, who headed the report. “We need to start thinking about the risk to food security.”

Of course, ocean acidification—like climate change itself—is incredibly complex, and it’s difficult for scientists to model exactly how marine species will respond to changing pH levels. They may be able to adapt, though there’s no guarantee. But with carbon emissions and temperatures rising—the World Meteorological Organization reported yesterday that 2010 will be among the three warmest years on record—we’ll find out, one way or another.

See also on TIME:

Code Blue—Saving Our Oceans

The Fish Crisis