With the stench of petroleum already searing their nostrils, residents along the deep-cut bays of Brittany are nervously waiting to see how far today's a highest tides of spring will carry the black death from the wreck of the supertanker Amoco Cadiz.

As winds shifted eastward toward France's Normandy coast yesterday, the British Royal Navy relaxed its efforts to protect the Anglo-Normandy channel islands which may be spared.

But Normans along bays that have so far been spared by the pollution braced for the arrival of the black tide that has already ruined more than half of the Maine-like northern coast of Brittany, one of the world's most striking resort areas and one of its richest fisheries.

The remains of 1.6 million barrels of oil is still pouring into the sea from the broken hulk of the Amoco Cadiz that is sitting like a giant beached whale on the rocks just off this little Breton fishing port.

For years, bird and marine life along this coast may be drastically reduced. Some rarer species of birds and fish are doomed to extinction, according to the marine biologists here.

The livelihoods of many Bretons will be destroyed by the black tide, and their anger at the French government for not taking adequate poreventive measures and for having done too little, too late seems bound to give a major boost to the separatist movement in Brittany.

Brittany is already turning leftist. The graffiti left over from demonstrations at the elegant 18th Century maritime prefecture in Brest read "Black Tide - Capitalist Waste."

The worst previous oil disaster in these waters was the wreck of the Torrey Canyon at Easter time, 1967. It only spilled 210,000 barrels into the sea, however. Not only was that one-seventh of the Amoco Cadiz spill, but the Torrey Canyon oil was a heavy type that formed knots of tar and was in the end far less damaging than the cargo of the Amoco Cadiz promises to be. It was carrying top grade Saudi Arabian crude that is light, liquid and penetrating.

The wind anc currents carry the relatively thin film farther and faster. On the beach, it penetrates more deeply and, biologists say, seems more toxic than any petroleum product that has been spilled in these waters before.

"Even the shellfish that live up on the beach are coming up to the surface to die," said Gilbert Deroux, laboratory director of the Oceanographic and Marine Biology Studies Center at Roscoff , 50 miles east of the wreck.

The great bird sanctuary of the Seven Isles off Perros Guerrec is by all accounts doomed. It is in the middle of the currents bearing the oil film eastward. The colony of 25,000 auks, puffins, gannet sand cormorants was nearly exterminated by the oil from the Torrey Canyon. The birds had just about reestablished themselves.

"The plunging and fishing birds, the ones that dive into the sea for fish, have had it," said Deroux. "They get the oil on their feathers, then they preen themselves to get rid of it, and they are condemned by the poison they have swallowed. Those species will disappear from our coast. As for the marsh birds, that depends on the spring equinox tides Sunday, Monday, Tuesday."

The tide charts show that it will be 30 feet high, enough to wash up into all the heavily indented bays. Heavy winds could bring the water further inland. Good weather means high air pressure, and that keeps the swells down.

All along the coast, there are clinics where schoolchildren and fishermen have been bringing the oil-soaked birds for treatment. At Roscoff, more than 150 birds were brought in but "they were more dead than alive," said the director, a Breton woman in her late 20s. Pointing to plastic garbage bags full of dead birds she repeated the litany heard at all the rescue centers: "We save one in a 1,000."

"We opened that center," said Deroux, "because you can't discourage people. The birds are what they see in a disaster like this. We are getting calls here fkrom people in paris offering to send us their children to help. But we don't save any. We just make a census of the species that are hit."

At Portsall, the site of the wreck, the workers at the bird center are far less philosophical.

"We are grave diggers," cried Loic le Carvantec, the angry young Breton in charge. "Stop the inanities. We are not here to save birds."

Then, he thrust an oily but alert razorbill under my nose and gave it a violent handchop on its nape.

"We have to finish them off ourselves," he said angrily handing the bird to a colleague. Le Carvantec went to the window facing the harbor to sob by himself. His blow, however, had not killed the bird. His colleague placed it still fluttering in a wooden crate and strangled it.

"It was better that way," said a young woman barely audibly. "The bird swallowed oil. It was sentenced to death and it would only have suffered."

"This is the fourth time I've seen the black tide in Brittany," said Le Carvantec. "If this isn't the last time it's not birds we'll be killing. It's genocide. And it's not just the birds - the whole food chain - the alga, the plankton, the sandworms, the shellfish. This is the coup de grace. We will leave a chamberpot for our children. They'll manufacture plastic birds with motors so that kids can see what a bird was like.

"Ever since the Torrey Canyon, we have been yelling. But nobody listens to us because we have long hair. Ten years, what was done?"

"There is a real danger," said Deroux, a Parisian who has lived in Brittany since 1954, "that the Breton coast could become a desert. People must understand that these disasters have a cumulative effect. It is not just the four bigh oil tanker spills since the Torrey Canyon. There are little ones here all the time that don't make much news.

"It won't be absolute biological death. Even in the sahara things bloom when it rains. But some species won't come back. You can't kill the gulls. They eat so much garbage anyway, and they can go inland and survive. But some of the other bird species may be reduced below their critical mass. They won't be able to withstand the attacks of the gulls at the nesting sites.

"The real question in my mind is the plankton that the fish feed off of. Will enough of it be uncontaminated?"

The disaster touched off a political debate in Paris as well as Brittany over the government's failure to have a preventive program or the means to fight spills after they happen. The conservative financial newspaper Les Echos criticized the use by tankers of flags of convenience, noting that nearly 40 percent of the world's tankers are registered in countries like Liberia and Panama that tax the ships lightly and demand little in the way of safety standards.

The Amoco Cadiz was an American-owned, Liberian-registered ship with an Italian crew and captain. Spokesmen for Amoco, the trademark of Standard Oil of Indiana, sit in Brest, the port around the southern from the oil, and pass out reports about the disaster 25 miles to the north.

Arrested and charged with polluting, Capt. Pasquale Bardari, 37, was running with a defective deep-sea anchor that would not hold when he needed it most after his rudder gave out. He waited for more than 12 hours after the rudder went out of action to give a distress signal that came after the ship had already run aground.

In between, he was dickering over the payment terms with the captain of the heavy tug Pacific, a boat from Hamburg that was the only heavy sea-rescue vessel of its kind in or near Brest.

By the time the deal was struck, the Pacific proved to be too weak to handle the supertanker in the storm that had come up. Another tug arrived after the Amoco Cadiz was already hopelessly grounded.

Adm. Jacques Coulondres, the maritime prefect in Brest, told the press that the French Navy had no power, no means and no reason to intervene.

Even it the Amoco Cadiz had asked for help, the French Navy was in no position to give any. For years, the navy has been requesting aerial reconnaissance equipment and two supertugs for the Channel coast, but it has always been put off on budget grounds.

The law of the sea does not give the French Navy the right to move unless the skipper requests it.

Now there is a constant stream of tour buses along the searoad, full of hundreds of curious schoolchildren, a large portion of them clean-scrubbed English prep schoolers with Instamatic cameras over in France for the Easter holidays.

There is an outcry in Brittany over government plans to use detergents and chalk to disperse and absorb the oil. The fishermen and biologists say that only makes things worse for the marine life, although it may make the sea and beach look cleaner. The French government has at least momentarily given up detergent cleaning.

The French also deployed large orange rubber buoys in an effort to keep the oil off some beaches. At Roscoff, a line of them, looking like linked sausages, were stained black and the water inside the links had the same characteristic thin layer of oil as the water outside the ineffective barriers.

The oil has not only hit the sea, affecting Brittany's rich oyster beds and fishing. Farmers inland report a thin film of oil on their land. There are claims that cattle are getting sick. Truckloads of Breton cauliflowers, the year's first major crop, have been refused at markets because of their petroleum smell.

Along the sea coast, the oil seems to burn the lungs. "I've stopped jogging along the beach," said a man in his early 30s. "When I run I feel it in my lungs."

Older men fishing garb stood in a knot near him staring helplessly out to sea. "We were awakened at night by the smell that first night," said Marianne Lodoec, a short 80-year-old woman wearing a black scarf and dressed in the traditional black of Breton peasant women as if in perpetual mourning. Her husband was a sailor lost at sea years ago. She stood by the house she was born in overlooking the Portsall reefs on which the Amoco Cadiz foundered.

"We thought it was a car accident. Finally, we saw them sending up lights from the ship. My son went to town to give alert," she said.

Mrs. Lodoec said that she and a neighbor gave the sailors coffee after the rescue helicopters brought them ashore.

"Those sailors were lucky it didn't happen next day when the storm was really up," she said. "None of them would have gotten off alive. The sea is terrible here. I've seen many things happen on this coast through all the years. But the coast is ruined almost everywhere now."