The question of how cod stocks fell so low in the waters off New England is almost as perplexing as the question of how to bring about recovery.

The favorite reason - too much fishing pressure - is followed by other explanations, including changes in ocean temperature and degradation of the environment. Perhaps it is a combination of these things.

Pinpointing the cause or causes of plummeting cod stocks is key to their rejuvenation.

But four major parties concerned with the question - scientists at the National Marine Fisheries Service, regional fisheries managers, environmental conservation groups and commercial fishermen - rarely agree. The debate is heated at times.

Adding an edge to the debate is the knowledge that had fish stocks been managed properly in recent decades, there would be more than enough for the commercial fishermen.

"If these stocks were rebuilt there would be such great economic benefit," declared Lee Crockett, executive director of the Marine Fish Conservation Network in Washington, D.C., a coalition of more than 170 national and regional environmental organizations concerned with the future of the ocean's resources.

"How do you get there? The fishermen are concerned that any reduction will drive them out of business," he said.

The reauthorization of the Magnuson-Stevens Fishery Conservation and Management Act, a program of resource management adopted by Congress in the mid-1970s, has set a goal of 217,000 metric tons of living Georges Bank cod, or biomass. Scientists estimate there were 22,564 metric tons of cod on Georges Bank in 2004.

Originally written to address concerns of overfishing by foreign fishing fleets, the Magnuson Act is now concerned with the U.S. management of its fish resources.

Fred Serchuk, a scientist with the National Marine Fisheries Service, believes if the 217,000-metric-ton goal is achieved, commercial fishermen could harvest 35,000 metric tons of cod a year without injury to the fishery - three times what was landed in the year 2001.

For decades, scientists, managers, fishermen and conservation groups have been grappling with how best to preserve and rebuild cod stocks.

Scientists with the National Marine Fisheries Service are charged with assessing the health of ocean fish stocks.

Fisheries managers are responsible for shaping a plan that allows fishermen to harvest stocks without depleting the resource.

Fishermen go to sea and try to earn a living wage from their work.

Conservation groups focus on protection of the resource, and are often willing to challenge plans advanced by government scientists, fisheries managers or commercial fishermen.

So far, fisheries managers have concentrated on reducing fishing pressure, both by closing areas of the ocean to fishing and by limiting the amount of time that commercial fishing vessels can spend at sea. Unsurprisingly, both approaches have proved unpopular with fishermen.

On July 14, fisheries managers and scientists met in Woods Hole to talk about better protecting ocean floor habitat and to develop workable closures.

One approach, advocated by the Conservation Law Foundation, calls for the creation of Habitat Areas of Particular Concern, the ocean equivalent of sanctuaries.

David Pierce, the deputy director of the state Division of Marine Fisheries, said meeting participants agreed on specific areas to protect.

There is growing evidence that the practice of dragging harms areas where plants and seaweed grow on the bottom.

Every time a dragger drops its net into the ocean, the vessel pulls two 1,000-pound doors along the bottom. The doors spread the net outward.

"Bottom trawling and scallop dredging is tearing up important habitat," said Mr. Crockett of the Marine Fish Conservation Network. "If this were happening on the land, you could fly overhead in an airplane and see that habitat is being destroyed. You'd say ‘Oh, my God.'" He continued:

"With the land it is a big issue, people understand. But the ocean is different, you can't see. You have to be Jacques Cousteau to see what is happening down there."

Mr. Pierce said areas south of the Vineyard have been targeted for protection. Other areas have been pinpointed along the northern edge of Georges Bank.

"Ideas have been brought forward by the Conservation Law Foundation and the World Wildlife Fund of Canada. They are proposing Habitat Areas of Particular Concern south of the Vineyard. These areas are historically already known as places where cod have congregated," he said.

"The point of our proposal is that we ought to protect the places where you find juvenile fish," said Priscilla Brooks, director of the marine conservation program with the Conservation Law Foundation, a group that has actively pursued responsible management of fish stocks.

Submitted at the end of April, she said, the proposal is based on data from trawl surveys.

If adopted, the closures would not take place until 2008.

Scientists report two kinds of cod live in the waters on and around Georges Bank. There are cod that spawn in the deeper ocean and those that spawn closer to shore. If the Conservation Law Foundation has its way, more inshore cod will be protected.

"We have evidence cod actually go to nursery areas close to shore," said Paul Diodati, director of the state Division of Marine Fisheries. "One of these aggregations is in the Massachusetts Bay area. We have evidence of large concentrations of cod coming into these same areas over and over again."

Mr. Diodati said similar populations might exist in the waters south of Cape Cod.

Another possible fishery revival technique is a special net that would take haddock rather than cod. Both species share the same Georges Bank habitat.

"We have to work out ways to enable fishermen to fish for haddock, which is abundant, without catching cod," Mr. Pierce said. "What we are using now is a net called a haddock separator."

But he said results so far have not been promising.

"Early indications are that the net is either not working or fishermen are purposely not making it work," he said. "The net is supposed to allow the fishermen to get four haddock to every cod; but if they are getting three haddock to one cod, that has serious implications."

Mr. Crockett said the government must also explore industry management methods similar to those used in agriculture markets.

"Our concern is that the political response has always been to weaken the conservation requirement," he said. "I think it would be more beneficial if the politicians helped these fishermen deal with the economic consequences."

Under one proposal the government could offer fishermen money to sell and scrap their boats, or pay them not to go fishing.

But members of the commercial fishing industry want exactly the opposite. They are calling on regulators to lower the 217,000-metric-ton goal, saying that conservation measures cannot possibly bring about that kind of recovery. A reevaluation has been scheduled for 2008 as part of the Magnuson Act's groundfish management plan.

Among the commercial fishermen faulting the management scheme is Robert Lane of New Bedford, who owns two fishing boats.

"I think it is cyclical," he said of the fisheries. "Years ago, when I went fishing, we never saw haddock. I fished the shoals and never saw much. Now the haddock are in huge abundance. I wonder how reliable their science is."

Paul Howard, executive director of the New England Fishery Management Council, also notes that warmer water found in recent years at Georges Bank has pushed cod away.

Mr. Crockett continues to see overfishing as the main culprit.

"We hear the constant refrain from the fishing community that the scientists are wrong and that there are more fish out there," he said.

But Mr. Crockett said as long as there are fishermen out there trying to catch the last cod, there will be someone willing to pay for it, and the decline is inevitable.