WHILE THE schools of bluefin tuna were a little late arriving off the New England coast this summer, the fishy cloak-and-dagger stories surfaced as usual prior to their run. And, arriving on schedule in such Massachusetts fishing ports as Gloucester, Newburyport and New Bedford were the Japanese buyers whose presence--and pocketbooks--makes for all the stories.

Most Americans have little interest in the bluefin's dark, red meat, preferring white-fleshed, filleted tuna. But the Japanese dearly love fresh bluefin tuna, and will pay dearly for it, which makes the fish of considerable interest to those few Americans in a position to cash in on the Japanese appetite.

As those who have contributed to the sushi bar explosion in this country know, the Japanese are fond of raw fish. And the glory of the sushi or sashimi display is raw bluefin tuna topped with a dab of wasabi (green horseradish) and a discreet splash of soy sauce.

The lengths to which the Japanese trading companies will go to obtain the bluefin tuna includes sending buyers to roam the New England coast during the July-to-October run. These tuna technicians, as they are called, go from fish dealer to fish cooperative to select the best of the catch, which they ship back to the prestigious Tsukiji Central Market in Tokyo.

Embellishing the factual are the stories told about clandestine sales of illegal tuna caught to help satisfy the Japanese demand, which is said to exceed the quota set by the International Convention for the Conservation of Atlantic Tuna. The ICCAT was formed about 10 years ago when the bluefin all but disappeard.

It seems that everybody on the streets and docks of Gloucester, a small city north of Boston best known these days for the Moonies' large presence in the fishing industry, has a tale to tell, some not unlike the stories of ship-to-shore marijuana smugglers, none for attribution.

One has a neighbor who knows a stockbroker who goes out on his yacht during vacation after tuna, a good percentage of his catch being illegal. In the parking lot of the supermarket, a lady points to a John Travolta look-alike and says that she'd heard he bought a black Mercedes last year and banked $20,000 on his ill-gotten gains.

Rumors make the rounds about mysterious boats tied up in mysterious coves to receive the contraband, and sometimes they talk about the middlemen who drive down country lanes to meet other vans full of illegal bluefin. One wonders how in the world they get a fish that weighs a ton from one van to the other.

To question federal, regional and state agencies about such goings-on is fruitless. "Black market tuna?" asked one law enforcement officer. "Never heard of it." He refused to be quoted, though.

Another who also wished to remain nameless said that 99 percent of the tuna caught were legal. "How are you going to conceal a fish that size?" he said. "Set it up in the back seat of your car and put a hat on its head?"

This year the government officials may be right. The population of bluefin has increased so dramatically that last year's quota of one fish a week was increased to one fish a day this season. In fact, ICCAT monitors on both sides of the ocean watch the schools of tuna closely enough through the season that at the beginning of August it allowed each person two fish a day. However, last week it was dropped back to one per day.

Another subject of great discussion in Gloucester is, of course, Rev. Sun Myung Moon's Unification Church. His Ocean Challenge program has brought young followers to the city to "teach personal strength and the benefits of cooperation" by sending church members off to sea, three to a boat, to catch tuna. The church owns 35 tuna boats and the International Seafood Company, which also buys tuna from native fishermen.

The locals have resented the church, which bought $2 million worth of property and businesses several years ago, and would prefer to deal with their traditional distributors and cooperatives. The kicker is that the Moonies were offering $3 a pound in July when the other buyers were offering $1.50. For a 1,000-pound fish, that's a big difference, but the current price is closer, $3.50 to $3.

"I don't want to be quoted," one fisherman said, "but you have to give the church some credit. They pay top dollar and they pay it immediately in cash. We don't have to wait a week to get our checks."

Sportsmen are also out there in deep waters catching tuna, which they sell to fish cooperatives or private companies just as readily as the professional does. Sometimes the dealer goes after the sportsman. Labor Day weekend, David Fryberg of the Tri-Coastal Seafood Cooperative in Newburyport bought the entire winning catch of both the Rhode Island and Masters Invitational tournaments held at Point Judith, R.I.

No matter who catches the fish, the regulatory drill is just the same. Each fish is supposed to be tagged the minute it is taken from the boat. A metal disk stamped with a number is clamped to the tail section where it stays until the fish is processed. Daily records are kept by the fish companies and these landing reports are sent on to the government.

All these fish add up to the seasonal quota set by ICCAT, which is almost 85 percent filled. When it is, the buyers will move on--to Newfoundland to board Portugese boats to select redfish, to Point Judith for butterfish, to Washington state for elvers, baby eels and sea urchins.

But when they return next summer, their faces will be familiar. Mr. Yugsuma has been making the trip to Gloucester for 10 years. Mr. Seito, Mr. Yamada and Mr. Doba have become familiar faces in Newburyport, and Mr. Toshi is known in New Bedford.

"They don't want just any fish," said Fryberg. "They select them much as cattlemen select beef and can tell a great deal about the flesh with an educated prod. When in doubt, they cut out a plug to judge the quality."

Atlantic bluefin tuna are immense and, since they are deep-water fish and very swift travelers, much of a mystery to marine biologists. They spawn in southern waters in the spring and then begin their migration north, normally being spotted in May off the Bahamas, but their sleek, spindle-shaped bodies have the leanness of survivors and make indifferent sashimi. Only the sport fishermen are interested at this point.

Then--this year the second week of July--they begin to appear in numbers off the coast of Massachusetts. The early fish being landed average of 750 pounds, but they continue to grow as they gorge their way north through schools of herring and mackerel and by September they average a thousand pounds.

The Tri-Coastal Cooperative is a cheerless, damp place of weathered wood with its face to the water and its back to the parking lot where trucks are ready to take the tuna to Japan Air Lines in Boston or New York. It is eviscerated (or gutted), the head and tail cut off and packed in ice in insulated cartons that are casually referred to as coffins.

Not all of these behemoths go back to Tokyo in mere dressed condition, which is fish market talk for head and tail removed and eviscerated. Lately Seito has been stationed in New Bedford, where his trading company has set up efficient freezing equipment. There giants are processed into two enormous side fillets and two precious belly fillets that are the most prized by sashimi chefs. Their fat content is the important ingredient, and the look is similar to salt pork.

The work schedules for the tuna technicians can be arduous. Sometimes the bluefin is trucked down to the Newburyport Cooperative from Maine ports arriving at midnight and the men must be there to pass judgment. If they are still there at lunchtime, they will disappear into the cooler where the smaller fish from inshore catches are waiting to be packed for market.

"They'll look around in there and decide what they want for lunch--a mackerel or squid or whatever's just come in. They don't need a stove, you know," says David Fryberg's mother, Doris. "They eat it raw." She speaks with the sure knowledge of surprising her audience. "One day they were absolutely delighted to find a few sea urchins in a boat. They eat the roe right out of the shell."

Taped to the wall behind Doris Fryberg are Christmas cards from Japan with season's greetings printed over pictures of the tuna technicians' home office. "We keep track of each other through the year."

If many Americans feel queasy about sashimi, how do the buyers feel about New England food? Can you imagine facing either a dish of Indian pudding or a blueberry muffin? What do they eat in Newburyport and Gloucester?

"Pizza," says David Fryberg without pause. "And most traditional American foods. Sometimes they cook a Japanese meal where they are staying or go into Boston to a restaurant."

Yugsuma began staying at Nina Conti's rooming house during the season when he first came to Gloucester. An energetic, ebullient woman, she is no longer a landlady, but Yugsuma is family now, and when he can't find a hotel room he calls Conti, so sure is he of finding hospitality. She is a ferocious housekeeper and appreciates the fact that Yugsuma takes his shoes off before entering the house.

"What does he eat?" she responds. "Italian food. I always make my spaghetti sauce with fresh tomatoes, and he knows it. Homemade sausage and meatballs--and Italian bread with a good pat of butter spread on it. Then when he finishes, he takes another piece of bread and wipes off the sauce on his plate."

She admits that her Japanese guests also frequent the steakhouses and local restaurants, but always the very best.

"And they don't want all their dinner on one plate, so I put the meat and the pasta and the vegetables on separate plates and serve them at the same time. And breakfast. Eggs, toast, bacon and sausages, lots and lots of coffee. They really like their coffee."

Her fisherman husband, Paul, often brings home some of his more exotic catches to cook on the charcoal grill. Once it was whelk, known as Northern conch in the fish market.

Says Nina Conti, "We get those big shrimp and put them on the grill. 'Don't cook them too much,' Mr. Yugsuma called out and I said, 'Leave them on a few minutes more.' "

She has adapted to Japanese interests somewhat, however, and has a recipe for cooking bluefin tuna and other dense-fleshed fish such as swordfish, shark, and tuna.

"Choose a two-pound piece of tuna for four people," she says. "Make a paste out of chopped fresh mint, some fresh basil, minced garlic, grated provolone, and plenty of black pepper. Then poke holes in the tuna with a knife and fill the holes with the paste. How much? How many? Whatever you want. Then tie it together around the outside like a roast. Heat a little oil in a skillet and brown the tuna on both sides. Cover it with fresh tomato sauce and cook for an hour. Eat the sauce with pasta and serve the fish sliced with a salad as the second course. Or eat them both together. Whatever you want."

If Nina Conti isn't too fond of Japanese food, others have learned from the visitors. Recently Paul Earl, marketing executive for the National Marine Fisheries Service in Gloucester went out to dinner with a group of friends--naturally to a restaurant known for its fresh fish. Bluefin tuna was on the menu. Carefully Earl explained to the waitress what he wanted--a portion of bluefin, but raw for an hors d'oeurve, and to make the request more unbelievable, he also asked for soy sauce and horseradish.

"Everybody at the table got very quiet," he said, "watching me slice the fish and mix the soy sauce with a little horseradish. But when we finished that, we asked for another order. Once people try raw tuna, they really like it."

Shizou Tsuji who wrote the definitive "Japanese Cooking--A Simple Art" (Kodansha International, $17.95) is more detailed, of course, and says that the sauce appropriate to a thick slice of sashimi is not necessarily the best sauce for paper thin slices, even if they come from the same fillet. Basically, to a half cup of soy sauce he adds two tablespoons of toasted crushed sesame seeds or two tablespoons of pickled plum meat put through a sieve (Japanese pickled plum is shocking but delightful) or a tablespoon of grated ginger.

Tsuji says, "Using chopsticks mix some of the spicy condiment with the soy sauce in your dipping bowl. Momentarily rest a slice of raw fish in this sauce en route to your mouth; pop the whole slice in at once and enjoy."