SOUTH PORTLAND, Maine - On a drizzling summer afternoon in South Portland, marine biologist Walt Golet is helping attach a quarter-ton Atlantic bluefin tuna to a heavy crane so it can be weighed as part of New England's premier tournament for the giant fish. And this year's derby is different than many in the past - there are far more tuna.

A decade ago, participants in the Sturdivant Island Tuna Tournament went consecutive years in which they didn't catch a single fish in the Gulf of Maine. This year, fishermen set a record with 30, including the 801-pound (363.33-kilogram) winner.

Their record haul is happening amid a turning point for these giant tuna, an iconic species that scientists say appears to be slowly recovering in the Atlantic Ocean. The reemergence of bluefin, which can weigh more than half a ton, has led to debate among fishermen, conservationists and scientists over just how much the big fish have recovered. It remains at a fraction of its population 60 years ago.

"There's probably no fish that's ever been more politicized than Atlantic bluefin tuna," said Golet, a University of Maine professor. "People get a passion for this fish. And people are making a living off of this fish."

The fish have long been at the centre of a battle among commercial fishermen who can make a huge amount of money on a single fish, environmentalists who see them as marvels of marine migration, and consumers who pay a hefty price for them in restaurants.

The tuna's status as a premium sushi and sashimi fish makes it particularly valuable in Japan, as a large bluefin can easily yield hundreds of meals. A single bluefin sold for more than $1.75 million at an auction in Japan in 2013. Fishermen in this month's Sturdivant Island Tuna Tournament, which wrapped up on Aug. 4, said many of the bluefin caught that day would eventually head to Japan.

The fish, capable of crossing the Atlantic in 60 days, have been harvested by man for centuries, and annual worldwide sales total hundreds of millions of dollars. The bluefin is listed as endangered by the International Union for Conservation of Nature, due in large part to years of overfishing.

But international regulators say the species has recently recovered enough that it can withstand more fishing, and U.S. ocean managers implemented an increase of about 17 per cent for this summer, to the delight of fishing groups. The decision prompted environmental groups to renew their calls for holding the current line on quotas.

Bluefin tuna are harvested on both sides of the Atlantic, using primarily harpoon and rod-and-reel in the west and seining nets and longline fishing in the east.

For commercial fishermen like Pete Speeches, who fishes out of Portland, the quota increase reflects a tuna resurgence seen on the water for some time.

"They're definitely more abundant," he said, getting ready to unload a 672.5-pound (305.04-kilogram) tuna from his boat at the Sturdivant Island tournament.

Environmental groups say they fear boosting the quota now and potentially in future years could undo years of conservation work. Shana Miller, The Ocean Foundation's program manager for global tuna conservation, said a quota increase of hundreds of thousands of pounds was excessive.

More fishing is a bad idea because of a possible return to overfishing, and because of concerns that increasingly warming oceans could retard the fish's reproduction, Miller said.

"They pushed it to a level that they know is going to lead to stock decline," Miller said. "Which really makes no sense."

There also remains uncertainty among scientists about the status of the fish's population, said Grantly Galland, a marine biologist and the global tuna conservation officer for The Pew Charitable Trusts.

"Any decrease in the population now is bad for fishermen in the long term," he said.

The quota increase amounts to more than 750,000 pounds (34,0198 kilograms) when allocations for the U.S. and other countries that target Western Atlantic tuna, such as Canada and Japan, are counted.

In the Eastern Atlantic, quotas are going up even more. An international body agreed to increase the quota there by about a fifth for this year, to more than 60 million pounds (27 million kilograms). Millions more will be added in the following two years.

The management of Atlantic bluefin is more complex than for many other fish species because they cross numerous international borders as they migrate to feed and spawn.

The International Commission for the Conservation of Atlantic Tunas, with about 50 member nations, including the U.S., released a report last year that described the species as recovering but still facing challenges.

Although the commission has decided to boost the quota for U.S. fishermen this year, the species will still be tightly monitored going forward, said Brad McHale, a fishery management specialist with the National Oceanic and Atmospheric Administration.

"We can't extract more than can naturally replenish," McHale said. "That's a constant struggle."

Steve Weiner, a 50-year harpooner out of Ogunquit, Maine, said he remains concerned about bluefin health on both sides of the Atlantic.

"I'm glad we're getting an increase in quota on this side, because we deserve it," Weiner said. "But we can't manage this fish properly unless the whole Atlantic is properly managed."

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Associated Press video journalist Federica Narancio in Washington, D.C., contributed to this report.