By AL BURT and DON BOHNING

A fleet of armed, flagless mystery ships prowl the Caribbean and Gulf of Mexico ramming and firing upon Cuban fishing vessels suspected of doing more than fishing.

First reports a week ago said there was only one such ship, but new information Saturday revealed at least four such "ghost" ships, of varying size, carrying out a sea vendetta.

The repeated attacks on Cuban vessels began March 12 and have occurred mostly in the area off Mexico's Yucatan Peninsula.

From Cuba, reports say Premier Fidel Castro became so infuriated that he personally drafted a strong note of complaint to the Mexican government.

Mexican sources say the attacks have panicked the Cuban fishing industry and that Cuban crews bitterly complain because the Cuban Navy and air force do not protect them.

However, because Cuban fishermen cover such a wide area and because the phantom ships appear and disappear so mysteriously, it has been impossible for the military to offer protection.

The military already is preoccupied with protecting Cuba's long coast from exile forays and from "pirate" expeditions which Cuba regularly attributes to the U.S. Central Intelligence Agency.

There are reports that some Cuban fishermen may balk at going out unless they receive military escort.

In response to the Cuban protest, Mexico beefed up its coastal patrols in the vicinity of the Yucatan. There has been no success reported in catching or identifying any of the phantom fleet. Shaken crewmen of the Cuban craft, who put into Yucatan after some of the incidents, spoke of being attacked by a two-stacked vessel more than 100 feet long.

The attacker used ramming tactics and in one incident nailed a Lambda, a type of fishing boat, on its first swift pass and then fled. The crewmen also said they were fired upon.

Continuing conflicting estimates of the attackers by the Cubans led to the knowledge that at least four ships are operating from an unknown port.

The bolstered Mexican patrols boomeranged on Cuba in their one success. They seized a 60-foot Cuban vessel for poaching in Mexican territorial waters. Its crew was turned over to Mexican authorities at the Isle of Mujeres (Women), a favorite harbor of Cuban fishermen.

Much of the Cuban fishing operation focuses in the Mexican area. A curtailment there would be a serious blow to food-rationed Cuba and to suspected subversive activities of the fleet in Central America.

Prime target of the mystery ships has been the well-equipped Lambda class Cuban vessels. More than 110 of these 75-footers constitute the backbone of Cuba's gulf fleet. For most, the home port is Havana, where the Russians are developing a multi-million-dollar fishing complex that also is suspected of being interested in more than fishing.

In addition to the Lambdas, a few of the 60-foot Rho class and the 50-foot Omicron also have been spotted in areas far from Cuba. The 47-foot Cardenas and the 33-foot Sigma normally are used closer to home, although two of the Cardenas type were picked up by the U.S. Coast Guard in the Gulf near Dry Tortugas in February, 1964.

Since Castro came to power, the Cuban operation has expanded significantly into international waters. Cuban tuna vessels and trawlers sail the Atlantic from the Newfoundland Banks to the Brazilian coasts.

The Cuban trawler fleet consists of five 500-ton Okean class Russian-built vessels manned by a mixed Soviet-Cuban crew. The trawlers are often used in complementary fashion to Soviet flag vessels of the same type.

The five Cuban tuna clippers are of Japanese design. The Cubans have contracted some 150 Japanese technicians to teach them tuna fishing.

Twenty-six more large fishing vessels are on order from Spain. They include four 700-ton cod fishing boats, two 700-ton trawlers, and 20 280-ton tuna vessels, according to a recent report in the Havana newspaper El Mundo.

The cod fishing boat Manjuari, first of the 26 vessels to be delivered, arrived in Havana March 20 under a Soviet master and crew.

Several aspects of the Cuban fishing operation are suspect.

Cuban schools which train the fishermen are organized along military lines. Students at the Victoria de Giron school in Cardenas, Cuba, are trouped in squads, platoons and company formations like the formal military. They are instructed in weaponry.

Communications facilities of the Cuban fleet give them excellent potential for transmitting information. Most of the equipment is supplied by the Soviet Union or Eastern Europe with a few items still coming from Canada.

Cuban fishing vessels regularly have been spotted in areas where fish in commercial quantities are not known to exit, notably near Venezuela, Colombia, Haiti, Jamaica and Mexico. One area mapped out for exploration as early as 1964 was the Dominican Republic.

Venezuelan security officials say privately that they possess substantial proof that Cuban fishing vessels have been used to carry arms and explosives to the pro-Castro terrorist organization, the Armed Forces for National Liberation (FALN), which is trying to overthrow the government.