BOSTON — At 5 on a windy winter morning, in a cold, harshly lit warehouse in Boston’s seaport district, Baracat Paiva stared down at the 200-pound bigeye tuna before him and reached for his knives.

Mr. Paiva, as tall and imposing as the dead fish before him, dipped his cloth-gloved hand in warm water, to keep his fingers from numbing. He deftly sharpened an eight-inch knife against a water stone before plunging it into the silver-scaled fish and sawing off the collar, using another, bigger knife to lop off the bone. A few bits of tuna sprayed off his knife and stuck to his hooded sweatshirt and flat-brimmed baseball cap, but he didn’t seem to notice.

Then, in a single motion, he slid the knife down the length of the fish on one side, then made another slice on the other. After a few more strokes, he carefully peeled away the rib cage and splayed open the fish, which split into four neat sections. He picked up one of the fillets, brimming with burgundy-hued flesh, and bounced it around in his arms like a baby.

“Beautiful,” he said.

Mr. Paiva, the highest-volume fish cutter at the popular Boston wholesaler Wulf’s Fish, has become something of a celebrity at a job that normally doesn’t attract much attention.