Since early last year, a fishing flotilla from the island of Vieques periodically sets to sea for battle, but it never fires a shot. Fishing gear is no match for the powerful weapons of the Atlantic Fleet.

The fishermen have declared war, and if they win, the Navy could lose its second greatest battle in the Caribbean this decade. It lost the first in 1975 when it pulled out of this tiny island municipality off the eastern tip of Puerto Rico

A third battlefield is the U.S. District Court in San Juan, where the fishermen and the Pureto Rican government have filed suit to stop naval training exercises on Vieques.

There, the Navy conducts submarine warfare and air operations, shells beaches and storms "enemy" bases with amphibious landing forces.

The suit claims that the transfer of military operations from here to Vieques 11 miles away has deteriorated the quality of life there, retarded economic growth and endangered marine and wildlife.

The Navy denies this, while emphasizing that the maneuvers are vital to the national defense.

"It's the only place like it in the world where such diversified training can be conducted in one single complex," a Navy spokesman said.

In action reminiscent of the six-year battle to get the Navy off Culebra, Vieques fisherman and their sympathizers sporadically block naval training exercises to dramatize their protest.

Last week, a trial for 21 protesters, including three ministers and four attorneys, was postponed. The protesters, charged with trespassing on a federally restricted beach on Vieques during military maneuvers, are expected to go on trial in U.S. District Court later this year.

The Puerto Rican government suit, requesting the court to issue a temporary restraining order against the Navy, is expected to be resolved in a few months, but most Culebrans, recalling similar judicial battles against the Defense Department, predict that this time the Navy will win.

The anti-Navy feelings of many of the islanders are deeply rooted

While the Navy purchased property from landowners on Vieques in the 1940s for military training, it took the land on Culebra.

"We were never consulted, we were booty of the Spanished-American War," said former Culebra mayor Claro A. Feliciano.

After more than 70 years under the naval boot, Culebrans are determined to keep out industrial invasions; they are resisiting industrialization to remain a tropical paradise.

Half the size of Vieques, this 28-square-mile, snake-shaped island (Culebra means "snake" in Spanish) virtually has been overlooked by 20th century innovation.

"We've rejected proposals to build big hotels, low-cost housing and middle-class urbanization," said Mayor Ramon Feliciano, a soft-spoken man who led the fight to get the Navy out. "Progress will be made without destroying or changing the ecology."

Four years after the Navey's withdrawal from one-third of the island, there is still no bank, no movie, no department store, no law office and no pharmacy. potable water is available only four hours a day.

Yet, the lack of modern conveniences doesn't seem to upset the residents. As one teen-ager put it: "I love Culebra. Here lie my roots; here I have the peace of mind I won't find any place else."

In the quaint atmosphere of an eight-block town suggestive of those at the turn of the century, there is little room for crime - only five robberies this year, police said.

A decade or so ago, a lagging economy and high prices were forcing many Culebrans off the island, but now, though jobs remain scarce, they are returning at a place that has doubled the population (to 1,642) in the past eight years.

Despite continued high prices - roughly 25 percent more than on Puerto Rico - Ramon Feliciano notes that per capita income increased from $300 a year in 1968 to a record $893 in 1977.

After 1975, he said, tourism and commercial fishing operations increased and a factory expanded operations to employ 315 islanders. He said the Navy had discouraged industry, limited fishing to nonrestricted waters and hurt tourism.

"When the Navy was here," Feliciano said, "we feared for our lives. They stepped up military maneuvers with more sophisticated surface-to-air missiles they couldn't fully control." He claimed one missile accidentally fell near his home.

Remnants of the maneuvers remain. Flamingo Beach, the entrance to which is fenced off, contains live ordnance, according to the mayor.

This is a future most Culebrans do not wish for their land neighbors.

Although the Culebrans vividly remember being turned down when they invited the Viequens to join in the fight to get the Navy off the two islands, they are supportive of the struggle now going on 11 miles away.

The Viequen's inital refusal was based on several factors - their beaches were used only for amphibious landing operations, soldiers from Camp Garcia spent money in town and several dozen civilians worked on the Navy base.

Now, with the Navy occupying 76 percent of the 54-square-mile island and the 8,500 civilans sandwiched between two military reservations, one islander complained, "We're cramped like sardines. There's just no place to go even if there are 1,500 cars."

Culebra's mayor, Radames Tirado, predicts that only political pressure on Congress and President Carter will get the Navy off Vieques and he says the "lack of consensus" among the islanders will "prolong the issue."

Indeed, three days after the 21 protesters were arrested last month, 1,474 petitions from Viequens supporting the Navy's presence were sent to Navy Secretary W. Graham Claytor.

Tomorrow, the anti-Navy Viequens will carry their fight to Capitol Hill for a briefing with some members of Congress and a press conference. The battle continues. CAPTION: Picture, The island of Culebra (above), which won its fight to get rid of the Navy in 1975, is supporting a similar move by its neighbor Vieques, 11 miles away. By Roso Juan Sabalones for The Washington Post; Map, no caption, By Richard Furno - The Washington Post