Two days earlier it had been swimming off Spain. Now it was in a Japanese supermarket in Edgewater, N.J. A man with a microphone and rubber gloves patted its smooth belly and flexed its fins. He showed how the pectoral fins tucked into grooves in the torso, for speed.

It was like an auto show, with seafood: the Giant Bluefin Tuna Cut Performance, which comes to Mitsuwa Marketplace one weekend a year. Shoppers huddled close to ogle enormous fish, hear the spiel and buy bluefin sashimi at $54.99 a pound.

This fish, the man said, was 8 to 10 years old, young for a bluefin, which reaches spawning at 12. How lucky we were to see and soon eat such a precious commodity. He was right: unless you are an ocean fisherman, a seafood wholesaler or another bluefin, you won’t often encounter something so rare and beautiful before it is disassembled.

A man in a blue T-shirt pulled the pectoral fin out and hacked it off. Another mounted the table with a knife as long as a sword. He thrust it in and pulled along the lateral line, cutting through bones: click, click, click. Another long pass along the belly, and one quadrant was loose. It took two men to lift the dark red log away, to gasps and applause.

With the ribs exposed, other men scraped the bones with spoons, piling soft flesh onto plastic platters. Massive hunks were swiftly downsized: from ham to steak slab to sushi sliver. The fish, in many hundreds of pieces, was wrapped, weighed, stickered and passed to thrusting hands.

The bluefin tuna is at the pinnacle of the sushi and sashimi kingdom, and suffers greatly for it. Its numbers have plunged 90 percent since the 1970s. The bluefin is nearing collapse, and its abundant misfortune is passed on to countless other creatures — the unwanted fish, turtles and seabirds killed as bycatch, the immense tonnage of smaller fish vacuumed up to fatten captured bluefin in Mediterranean “farms.”

Governments and conservationists have long sounded the alarm, but no one really controls the open seas or has found the limits of the human appetite for luxury seafood. An international conference in Turkey last month could have slowed the carnage, but didn’t.

In a book about another ferocious and majestic fish, “Blues,” John Hersey defended fishing for sustenance while mourning the unhinged global slaughter that shreds fragile webs of ocean life. “We’d better marvel while we can,” he wrote.

LAWRENCE DOWNES