NEW YORK -- Rick Lofstad sliced an inch-thick steak off a hunk of tuna, red as beets, and offered up the fish’s pedigree: a 40-pounder caught off Montauk, football-sized without the tail and fins.

Lofstad sells his tuna from a stand at the Union Square Greenmarket for $12.95 a pound. Across the street at Trader Joe’s, frozen yellowfin tuna steaks, imported from Singapore, go for $4.69 a pound.

Much as the current foodie mantra exhorts buying local, seasonal and fresh, in this case that comes with a steep premium. Is fresh tuna worth it? Can the average diner really tell the difference?

“In a flash,” said Lofstad, a third-generation fisherman who sells at markets around the city. But then he backtracked, saying that if the fish was frozen at sea at well below zero “it would be very difficult to tell the difference.”

That’s because fish freezing methods have improved so much in recent years that there is little difference between the fresh and frozen products, said Doris Hicks, seafood technology specialist at the University of Delaware Sea Grant.

“In some cases it can be better than fresh,” she said. “As long as it’s been done properly and kept frozen, it can be just as good as fresh or better.”

Fish today is frozen faster and at lower temperatures than before, Hicks said. That’s important because those conditions result in smaller ice crystals. Large crystals disrupt the flesh of the fish, affecting taste and texture.

To test the difference between the fresh and frozen, I seared two tuna steaks — one of Lofstad’s fresh and a Trader Joe’s frozen — in a cast-iron grill pan and served them with freshly made pesto.

The fresh fish was sushi grade, which means it was good enough to be eaten raw. The cooking was brief, just enough to sear the outside and keep the center red. The result? It was so tender it practically melted in the mouth.

The frozen tuna, prepared the same way, had a coarser texture, and both color and flavor were less intense. Of course, the differences could be partly due to variations in the fish themselves.

“When I bit into the fresh it was definitely more tender,” my guest commented. “But the frozen was better than I thought it was going to be. For every day I think it’s fine.”

Corky Clark, a professor at the Culinary Institute of America in Hyde Park, N.Y., disagrees, at least for his own palate. He won’t touch frozen. “I wouldn’t use frozen tuna if my life depended on it,” he said.

But he acknowledged that availability and cost put fresh tuna out of reach for many people. When that’s the case, he offered some advice for when to use fresh and how to make the most out of frozen.

For dishes with mild seasonings, delicate cooking techniques or for use in salads (not the mayo kind), Clark said it’s best to stick with fresh. In those recipes, the tuna is the star, and the flavor and texture of frozen doesn’t work as well.

Instead, for frozen tuna, stick with heavy marinades, or other heavily-seasoned preparations in which the taste of the tuna isn’t as prominent. Sushi, which is typically eaten with flavor-obscuring soy sauce and wasabi, is another good choice.

He also suggests sticking to smaller pieces that are thinly cut and quickly grilled.

Of course, quality is just one issue consumers must sort out when buying tuna. They also must negotiate bedeviling issues such as contaminant levels and how the fish was caught.

The Oceans Alive program of Environmental Defense recommends that adults eat tuna — fresh or frozen — no more than three times a month because of high mercury levels. The group recommends that older children eat tuna no more than twice a month and younger children just once a month.

Groups that track which marine species are being harvested in ecologically sound ways give a green light to yellowfin tuna, but not bluefin, one of Oceans Alive’s “eco worst” choices.

The Monterey Bay Aquarium, which publishes a Seafood Watch pocket guide, lists yellowfin tuna (also called ahi tuna) as a “best choice” or as a second-best “good alternative,” depending on how it’s caught.

Tuna caught with a fishing pole or by trolling, in which a line with a lure or bait is towed behind a boat, are best, according to Karen Jeffries, a spokeswoman for the aquarium.

The other main tuna fishing method is longlining, in which baited fishing lines are strung along a central long line. Longlining can accidentally kill turtles and birds, according to the Seafood Watch guide.

It wasn’t clear how the Trader Joe’s fish was caught, and a spokesman in the company’s corporate offices declined to comment. Lofstad said his tuna is mostly caught by trolling.