Jonathan Mayhew, a third-generation fisherman from Martha's Vineyard, Mass., knows firsthand that a school of northern Atlantic bluefin tuna is a marvel to behold. A former spotter pilot, Mr. Mayhew used to routinely gaze down on schools of 60, 80, and sometimes 200 of these torpedo-shaped fish, each weighing many hundreds of pounds.

He also knows that schools of giant tuna are a rare sight these days.

"The stocks are not there," Mayhew says. "The biomass of bluefin that comes into the Gulf of Maine is either nonexistent, or it's going elsewhere."

Scientists concur. "Our giant fishery has disappeared," says Molly Lutcavage, director of the Large Pelagics Research Center at the University of New Hampshire, Durham. "The overfishing has gone on too long and it's finally taken down what was once a big fishery."

So far this year, fishermen have pulled in only 12 percent of the 1,533-ton US quota, the worst of several years of steeply declining catches. For US fishermen, the apparent collapse of a fishery once valued $19 million (it was valued at $3.3 million in 2006) is only the latest in a series of setbacks that make it tougher to make a living.

New information on just how far this migratory fish travels has scientists thinking that overfishing in the Mediterranean and eastern Atlantic may be depleting bluefin numbers closer to the US. Others think that, perhaps due to human-driven climate change, shifting conditions in the North Atlantic are further stressing already weakened stocks. Warning of collapse, many are calling for a moratorium on bluefin harvesting, both to allow stocks to rebound and to give scientists a chance to figure out what's happening.

"We had all these complicated regulations to control the fishery [that] seem to have largely failed," says Clay Porch, a research fishery biologist at the National Marine Fisheries Service (NMFS) in Miami. "Drastic measures need to be taken to put us back on track."

At stake is not just the continued existence of a remarkable fish, but a favorable resolution to a greater conundrum: Will humankind figure out how to sustainably manage nature's bounty before there's not much left to manage?

The answer, so far, seems to be no. Despite calls by US lawmakers and scientists for a moratorium on bluefin harvesting, the International Commission of the Conservation of Atlantic Tunas (ICCAT), which decides bluefin quotas, did not meaningfully lower quotas when it convened in November. (Acknowledging overfishing, the European Commission did ban bluefin fishing for the rest of 2007.) As in past cases where regulators didn't act in time, a multitude of competing interests – in this case the 42 countries that participate in ICCAT – forestalled greater, but necessary cuts, experts say.

With the fate of bluefin hanging in the balance, scientists are now calling for a more comprehensive and, presumably, effective "systems" approach, one that takes the entire ecosystem into account when setting fishing quotas.

"Nobody's really done it," says Don Perkins, president of the Gulf of Maine Research Institute (GMRI) in Portland. "And the regulatory system is not set up to consider an individual species from a multispecies perspective." But, he says, by necessity, that's where fishery management is headed.

Fisherman face an Atlantic divide

The warm-blooded bluefin, the largest of the tuna species, travels vast distances during its life. Living up to 30 years, adults fatten up in highly productive northern waters, like, historically, those in the Gulf of Maine, and then spawn in the warmer waters of the Gulf of Mexico and the Mediterranean.

Since the early 1980s, managers have treated North Atlantic bluefin as two separate populations: eastern and western. A north-south line that bisects the Atlantic divides the two stocks. Many scientists assumed a mere 4-to-5 percent exchange between eastern and western tuna. But "no fisherman that I know of ever believed for a second that the two-stock theory was viable," says Mayhew. Enabled by new tagging technology, studies conducted during the past decade substantiate Mayhew's hunch.

"Since the late '90s, we've confirmed that the mixing is much greater than assumed," says Dr. Lutcavage – up to 30 percent. The new evidence has US scientists worried about what's happening in the East, particularly in the Mediterranean. Even by European scientists' account, bluefin catches there are currently at unsustainable levels. The Standing Committee on Research and Statistics (SCRS) recommended a quota of 15,000 tons in 2007. But the ICCAT set the allowable catch at 29,500 tons. Worse, studies put the actual catch somewhere around 50,000 tons. And, says Lutcavage, while 73 inches (about 250 pounds) is the minimum harvest size in US waters, it was only 10 kgs (22 lbs.) in European waters until recently. In late November, ICCAT raised the minimum catch size there to 30 kgs (66 lbs.).

"If you're taking 70,000 metric tons [77,000 tons] and it's much smaller fish, the number of fish you're taking is much greater," she says. For each 250-pounder caught here, up to 10 were potentially removed there. Presumably, these fish never came west.

The Gulf of Mexico and the Mediterranean leave distinct chemical signatures on fish that swim their waters. A laboratory at Texas A&M University in College Station, Texas, is currently analyzing bluefin ear bones to better determine their origin, the results of which should be available early next year, says Dr. Porch.

Calls for a moratorium ignored

In October, meanwhile, Bill Hogarth, director of NMFS, called for a three-to-five year moratorium on bluefin fishing. In November, the US Senate unanimously passed a resolution calling for the same. But despite warnings from both sides of the Atlantic that collapse was imminent, ICCAT implemented only a small reduction. By 2010, the eastern quota will drop to 25,000 tons yearly.

The World Wildlife Foundation called the decision the "final blow for Mediterranean tuna." Rich Ruais, executive director of the Blue Water Fishermen's Association in Salem, N.H., and an adviser to the American delegation to ICCAT, characterizes it as "an utterly preposterous inadequate response to the crisis at hand.... The European Community has just totally refused to address the issue."

The fault, he and others say, lies with tuna-farming interests in the Mediterranean. The recent boom in capturing and fattening tuna in mobile sea pens has created a powerful lobby worried about recouping its investment. "There's nowhere else to point the finger," Mr. Ruais says.

But other ocean anomalies have some thinking that eastern fishing fleets aren't the only stress on western tuna.

The few tuna that still swim the western Atlantic are showing up quite lean. "The quality of the fish that I've seen has definitely declined over the last few years," says Bob Campbell, manager of the Yankee Fisherman's Cooperative in Seabrook, N.H.

After analyzing 14 years' worth of Mr. Campbell's logbooks, Walter Golet, a PhD candidate in Lutcavage's lab, agreed. Bluefin "come here for one reason – they come here to feed," says Mr. Golet, who recently published his findings in the journal Fishery Bulletin. But "they don't seem to be obtaining the forage that they were getting 10 to 15 years ago." (This year, which was not included in the study, has seen a slight rebound in tuna quality, says Campbell.)

This thinning has some wondering about what bluefin eat, particularly herring, a keystone species in the northern Atlantic.

Herring biomass seems high, similar to the way it was before the advent of modern industrial fishing fleets in the 1960s. But both fishermen and conservationists consider this five-inch-long fish important enough that they've formed an alliance devoted to its protection. Known as CHOIR, the Coalition for the Atlantic Herring Fishery's Orderly, Informed, and Responsible Long-Term Development, has repeatedly called for large herring trawlers to leave New England waters. The coalition fears that if herring numbers greatly diminish, everything that feeds on them, from tuna to cod, will suffer.

Indeed, while herring biomass seems healthy, scientists have noted that an individual four-year-old herring is smaller than in the past. "The size at that age has decreased over the last decade or so," says Bill Overholtz, a senior scientist with NMFS in Woods Hole, Mass. The most obvious cause is the "density effect," he says: There are so many herring that each one eats less, making them smaller. And smaller herring could mean that feeding tuna have to work harder for each meal.

Climate change may shrink stocks

But some think that changing oceanic conditions driven, perhaps, by global warming, may be responsible for the tuna's woes. Warmer temperatures explain the abundance of young tuna in the Gulf of Maine. "These are fish normally found off the mid-Atlantic states," says Lutcavage. "Are these fish shifting north because of ocean warming?" (Mayhew says they've always been abundant, but they never return as adults.) Climate change could also explain why Canadian fishermen continue to catch bluefin while their US counterparts don't. "It may be that since herring and bluefin are associated with cooler water … those conditions are better farther north than they used to be," says Dr. Overholtz.

A warming Arctic could also be having a more direct effect on the food chain. In the late 1990s, the western North Atlantic saw an increased inflow of low-salinity water from, scientists think, melting ice and permafrost. (The influx seems to have tapered off more recently.) Andrew Pershing and Jeff Runge, two marine scientists who divide their time between the University of Maine in Orono and GMRI, suspect that the meltwater caused an algal bloom.

Weighing less than the surrounding ocean, the low-salinity water formed a distinct layer at the ocean's surface. Usually limited by churning water, the algae had increased access to sunlight in this relatively still layer. Small shrimp-like creatures called copepods then gorged on the algae, and grew in number. Herring fed on the copious copepods, and multiplied as well. But a decade later, "there are so many adult [herring] now that they're reducing their own food," says Dr. Pershing.

Another possibility: More arctic meltwater knocked the herring-copepod feeding cycle out of synch. Copepods have several stages of development. Not coincidentally, herring spawn when copepods are most fatty – or at least they used to. The freshwater pulses, and perhaps changing wind patterns, may have reduced the number of fat copepods when the herring arrive to feed.

"In the past few years, the abundance of these [fatty] copepods has been low," says Runge. "It may be [that] the numbers ... that are present when the herring need them is changing."

Runge has begun higher frequency plankton sampling from a greater number of sites in the hope of better gauging the climate-plankton link. He's also looking at how copepod and herring abundance relate. Pershing hopes to determine if the changes observed in the Gulf of Maine hold across a greater area. And Golet plans to analyze herring logs the way he did for tuna. Results indicating a decline in herring quality that jibes with other changes in the North Atlantic would greatly solidify these hypotheses.

But the greater story is one of what Perkins calls "a transition to a more systems approach to ecosystem management."

Take everything, from shifting currents and plankton blooms to tuna, sharks, and humans, into account when setting quotas, he says. Otherwise, "When you look at a species in isolation, you miss really important factors," he says. "You could miss the problem or construct a solution that hurts rather than helps."