Shawn Tibbetts, the vice president of the Casco Bay Bluefin Bonanza, unloads a huge tuna on Wednesday from the Harpswell-based boat Bad Medicine, captained by Blaine Lund. The tuna weighed 643 pounds. Credit: Susan Young / BDN

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During a recent dog walk, I spotted a group of people standing around the biggest fish I had ever seen. It dwarfed the people standing beside the fish as it hung from its tail.

I had stumbled upon the Casco Bay Bluefin Bonanza, an annual fishing tournament, where dozens of massive tuna were caught, and money was raised for scholarships at Maine’s community colleges.

Although I’ve lived in Maine for nearly 30 years, there’s still a lot I don’t know about this state, especially about the goings-on in the sea waters off Maine’s long coastline.

Like the fact that tuna weighing hundreds of pounds are not that unusual. The heaviest tuna tallied on the leaderboard during the five-day bonanza weighed 756 pounds. It was brought to shore on Thursday by My Three Blondes, captained by Joe Pinkham of Five Islands.

The biggest bluefin tuna catch on record was a 1,496-pound monster caught in Nova Scotia in 1979.

Although there are concerns about many marine species in Maine and around the world, bluefin, the largest species of tuna, seem to be doing just fine.

In fact, the fish are so plentiful, that the International Commission for the Conservation of Atlantic Tunas has increased the quota for the total amount of bluefin tuna that can be caught in the U.S.

Walt Golet, a marine scientist with the University of Maine and the Gulf of Maine Research Institute, said that a strong population of menhaden, now a favorite prey of tuna, has helped the tuna population. In addition, tuna can survive — even thrive — in a variety of habitats. They can go from the ocean surface to a depth of 5,000 feet in a matter of minutes, which means they can tolerate a wide range of water temperatures. Golet, who measured the length of each tournament fish and oversaw their weigh-ins, estimated that many of the fish caught during the bonanza were in their teens to twenties.

“The population ebbs and flows,” he told me. But, this summer, there have been a lot of fish around. The number caught during this year’s derby is well above last year’s tournament total.

Walt Goulet, a marine scientist at the University of Maine, measures a 643-pound tuna caught on Wednesday by Bad Medicine of Harpswell during the Casco Bay Bluefin Bonanza. Credit: Susan Young / BDN

The result was a flurry of activity at the Spring Point Marina in South Portland this week. After fishing overnight, boats pulled up to the marina with their catch secured in large zippered bags or covered with tarps. The tuna are typically stuffed with and surrounded by ice.

The fish are hoisted up from the boats by their tails to be weighed. The heads are quickly cut off, which sometimes disgorges live fish that flop on the dock, and tissue samples collected by a team of UMaine students.

The headless tuna are quickly loaded into waiting trucks. Some are taken to the Portland Fish Exchange and may end up on local restaurant plates. Others go to Massachusetts, where they are likely to be flown to Japan.

“It’s a crazy adrenaline rush,” Cameron Thorp, the captain of the Endorfin out of Harpswell, said to describe the experience of catching such big fish. He and a crewmate landed four fish during the bonanza, likely claiming a prize for the most total weight. The largest tuna they caught weighed 737 pounds.

On Wednesday morning Thorp and a crew mate brought in a tuna that weighed 568 pounds and measured 100 and ½ inches. It was their third tuna of the tournament.

It was caught at 2 a.m. and dragged the boat for three miles, Thorp said. It took them an hour and a half to land the fish, which was caught by rod and reel off Seguin Island.

Thorp, who splits his time as a commercial fisherman between Maine and his native New Zealand, said it would take about a week to find out where the fish goes, and how much he’ll be paid for it.

Despite my awe over the big tuna, they are actually harder to sell than smaller tuna, Seth Richards of North Atlantic Traders told me. There isn’t much demand for tuna this big because it is too much for one customer. If the fish is of good enough quality, which is determined mainly by fat content, it is likely to go to Japan where wild fish are prized over farm-raised fish. To the likely outrage of many Mainers, it will be sold as “wild Boston bluefin.” A fish headed to Japan may fetch between $10 to $20 a pound, maybe more, Richards said.

“It’s fortunate for us Mainers that these fish are in our backyard,” Richard said as Thorp’s 568-pound fish was loaded into his truck. “They probably aren’t going anywhere,” he said, noting that fish this big don’t have many predators.

Fortunate indeed, to know that bluefin tuna are thriving off the coast of Maine, to see such big fish up close and to learn more about a part of our state’s marine ecosystem that remains unknown to many people.

Susan Young is the opinion editor at the Bangor Daily News. She has worked for the BDN for over 25 years as a reporter and editor.