This time last year the fishermen of the United States tuna fleet centered here were complaining bitterly about tough new government regulations protecting porpoises which, they said, were creating "a doomaday situation" for their industry.

They spoke openly of leaving this sunny harbor, selling their boats and giving up the life of the sea - for many, a family tradition spanning several generations.

But today, almost miraculously it seems to some, the American tuna men are thriving and talk of a possible record tuns-catching year is common around the docks. Men who last season were accusing federal provisions protecting porpoises caught in their nets of bankrupting them, now speak hopefully about learning to live, and prosper, under the government's rules.

"We didn't even think we'd be in the tuna business this year." said tuna boat captain-owner Julius Zolezzi, head of an Italian-American family which for three generations has been a leader in the tuna industry here. "Now I think there's such a good future that I'm out fishing for another ship."

The problems here started after the passage in 1972 of the Marine Mammal Protection Act, which sought to restrict the numbers of porpoises commonly caught swimming along with the yellowfin tuna common in the Pacific.

Kills of the mammals, although inadvertent, reached over 100,000 animals in some years, according to the National Marine Fisheries Service which administers the new fishing regulations.

Last year, when the limit was 62,000 porpoises, fishermen afraid of exceeding their kill quotas and suffering stiff federal fines, stayed home during four whole months in the important spring fishing season. This year, although the quotas are a little tighter, only 7,000 porpoises have so far been killed, leaving the fleet with an almost unlimited opportunity to fish all season long.

In fact, the U.S. tuna industry has caught twice as many fish - some 150,000 tons - as it did in the same period last year. Unemployment, a major fear among the 4,300 workers directly involved in the San Diego tuna industry this time last year, is almost forgotten today as the docks are crowded with men working to repair nets and unload fish by the calm, blue Pacific.

Where in 1977 the American tuna men suffered some $70 million in losses, according to industry spokesmen, this year a possible profit of some $100 million is being predicted. For the average crew member signing up for the nine-month fishing season, the increased profits could boost his pay from $12,000 last year to almost $25,000 this year, said one ship's captain.

Despite all the good news, there is widespread concern among fisherman that this year's low porpoise kill is simply the work of nature or luck, something which can change over the next few years. This year fishermen have had the good fortune to catch large numbers of skipjack tuna rather than the more common yellowfin, the species the porpoises most often swim with. The more skipjack caught, usually, the fewer porpoises are likely to be killed.

"This year it's like somebody was watching over us," said August Fellando, general manager of the American Tunaboat Association, which represents over 100 owners here. "This year we've had all the skipjack that has no porpoise on it. Next year could be a tremendous yellowfin year so who knows? Go ask the tuna - I sure don't know."

Officials at the National Marine Fisheries Service, however, insist this year's success proves that their quota system is workable. Dr. William Fox, chief of the oceanic fisheries division of the federal government's southwest fisheries center in La Jolla, claims fishermen have consistently overexaggerated the problems engendered by propoise quotas.

"We've always thought the industry could work within the regulations and nothing we've seen this year discourages us," Fox said. "You hear every year that it's an unusual year each has its own character but even if they got a lot of yellowfin their mortality rate would still be way below quota."

Both Fox and tuna industry spokesmen agree recent technological changes in fishing methods may be contributing to the sharp drop in porpoise fatalities. Such innovations as new nets, better hydraulic systems and faster boats have all contributed to making tuna fishing more efficient and less deadly to propoises.

Whether these changes will, in the long run, keep afloat the current boom in the U.S. tuna boat industry is an open question. But there is no doubt that the mood on the docks has changed dramatically from last year's gloom to an attitude of optimism and hope.

"We used to feel everything was against us, that the government was trying to put us out of business," Julius Zolezzi said as he looked out from one of his ships at the gleaming highrise skyline of San Digeo. "Now I think, the government has decided they want a strong U.S. fishing fleet.And as long as the porpoises cooperate I'll stay right here because I love this business."