Early European explorers to the Americas encountered an astounding abundance of marine life. White beluga whales, now limited to the arctic, swam as far south as Boston Bay. Cod off Newfoundland were so plentiful that fishermen could catch them with nothing more than a weighted basket lowered into the water. As late as the mid-19th century, river herring ran so thick in the eastern United States that wading across certain waterways meant treading on fish. And everywhere sharks were so numerous that, after hauling in their catches, fishers often found them stripped to the bone.

“It completely bowled me over when I started reading some of these early accounts,” says Cal­lum Roberts, a professor of marine conservation at the University of York, England, and author of “The Unnatural History of the Sea,” which tells much of this tale. “The picture painted is one of an abundance of life which is very hard for us to grasp today.”

Hundreds of years of fishing – and especially the last half century of industrialized fishing – have drastically altered the oceans. Measured by weight, only 1/10th of the large predators that once swam the seas – the big fish and sharks that shape the entire ecosystem – is estimated to remain. And many of these changes have occurred relatively recently. Any middle-aged fisherman will wax nostalgic about the catches of just 20 years ago. Any marine scientist will glumly check off reefs they once studied that are now bleached and overgrown with algae as a result of overfishing and pollution, and the marine life that’s simply disappeared.

“Today’s oceans have got far less in the way of biomass than they used to,” says Professor Roberts. “We’re altering ecosystems in a way that reduces the level of productivity they can support.”

After millenniums of a free-for-all, many foresee the era of open access to the ocean formally coming to a close.

World catches have steadily declined since peaking in the late 1980s. Everyone, from scientists to fishermen, is alarmed. And in the US, all quarters are pushing to develop solutions before the problem becomes unfixable. Fishermen and fishery managers are rethinking management to encourage stewardship. Scientists now say that fish stocks can’t be viewed in isolation; they must be managed in the context of the greater ecosystem. Many, even some fishermen begrudgingly, realize the importance of having some areas completely off-limits to fishing in order to keep ecosystems healthy. And increasingly, a new argument is heard in the debate over fisheries: Marine ecosystems should be preserved not just for their economic value, but also because, like the wilderness preserved in the national forest system, they are part of humankind’s natural heritage.

The debate comes at a time when, driven by both health trends and increasing prosperity in countries like China, demand for fish is rising. In industrialized countries, fish consumption doubled, to 27 million metric tons, between 1961 and 2003, according to the United Nation’s Food and Agriculture Organization (FAO). Per capita, that’s an increase of one-third, to 29.7 kg (65.5 lbs.) per person yearly. (Much of the increased demand is being met by a growing aquaculture industry.) In developing countries, fish continue to provide an important source of protein. The average African gets 17 percent of his protein from fish; for Asians, it’s 26 percent. The typical North American gets only 7 percent of his protein from fish.

Fishery managers have a name for what can be removed without causing stocks to fall: the maximum sustainable yield. In theory, a well-managed fishery should provide free food – save for the cost of catching it – year after year.

And yet, even in the US where stocks are on balance in better condition than in other places, 41 of the 244 stocks for which the National Marine Fisheries Service (NMFS) has information are being fished at unsustainable levels, or overfished. Worldwide, one-quarter of fish stocks are overfished, says the FAO. Another 50 percent are fished to full capacity; they can sustain no more. According to one somewhat controversial analysis, if current fishing trends continue, all the world’s fisheries will have collapsed by mid-century.

What happened? Daniel Pauly, director of the Fisheries Centre at the University of British Columbia, Van­couver, calls the combined cultural, technological, and economic factors “the march of folly.”

It began with a long-held notion of the sea’s endless bounty.

Until relatively recently, fishermen, fishery managers, and scientists alike thought the sea was so vast, so teeming with life, that human activity simply couldn’t diminish it, Mr. Pauly says.

Until the advent of modern fishing technology in the 20th century, it couldn’t.

“The sea was very large compared to the means we had to exploit it,” Pauly says. But beginning with steam-powered trawlers more than 100 years ago, and ending with today’s global-positioning navigational systems, technology has improved fishermen’s reach and efficiency. “We essentially deployed our industrial armada against fish, and obviously we would win: It’s a war against fish,” says Pauly.

Technology made inaccessible fish accessible. Pristine areas used to constantly replenish adjacent areas that were fished, scientists hypothesize. But as technology let fleets fish in areas previously unfished due to remoteness or difficult undersea topography, this replenishment failed. Fish numbers fell, but better fishing technology concealed the trend. World catches remained stable or increased, suggesting healthy stocks.

Then, when local stocks began to collapse, fleets moved ever farther offshore, leading to what Robert Steneck, a professor of marine biology at the University of Maine, Orono, calls “roving banditry”: High-seas fleets fishing stocks to collapse, then moving on. Many countries also subsidized their fleets, increasing capacity far beyond what the seas could absorb. Worldwide, the FAO estimates that by the 1990s, subsidies had pushed fishing capacity some 30 to 50 percent above what the oceans could sustain. (It has since fallen.)

“When the biomass goes down because of fishing, in a sense the stock has a message…. ‘Leave me alone,’ ” says Pauly. “But subsidies, which contribute to the harvesting of fish, enable the fisher to ignore the signal of the stock.”

In the US, which actively developed its domestic fleet throughout the late 1970s and ’80s with low-interest loans and other programs, many thought that fishing overcapacity would self-correct. If fishermen were just another predator, once fish numbers dropped, fishers would, too. Equilibrium would be restored.

But the laws of economics led to a different outcome: “As stocks get rarer and rarer, their prices go up – the so-called ‘ratchet effect,’ ” says Steve Murawski, chief scientist at National Oceanic and Atmospheric Administration Fisheries in Silver Spring, Md. The incentive to catch the few remaining fish increases rather than decreases. “That wasn’t well understood,” he says.

Empty Oceans, a series on the state of the world's fisheries, will be appearing in the Monitor's environment section. For the full series, click here.