A huge oil slick from the Spanish supertanker Castillo de Bellver, which broke in half Saturday off South Africa's west coast, is threatening to destroy the main concentration of a rare breed of penguin on the "vulnerable" list of the International Union for the Conservation of Nature.

Half the total population of jackass penguins, so called because of the braying sound they make, live on tiny Dassen Island near the spreading oil slick that now is reported to cover nearly 300 square miles of the South Atlantic.

An official in Cape Town said that for the moment, wind and currents were keeping the slick offshore. He said that if the wind holds for two days, the oil should be carried safely past the island and other marine and wildlife habitats in the area.

The slick was only 30 miles from the mouth of Langebaan Lagoon, one of the most important sea bird sanctuaries in Africa, and was also threatening the main breeding waters of South Africa's rock lobsters, many of which are exported as a delicacy to the United States.

The rock lobsters, found only in the cold waters off South Africa's west coast, are a $9 million-a-year export industry, and 35 percent of them are caught in the vicinity of Dassen Island.

William Damerell, port captain of Cape Town, who is in charge of the rescue and counter-pollution operation that began when the supertanker burst into flames and broke in half Saturday, said that a light southeasterly wind and the cold Benguela Current, which sweeps up the coast from Antarctica, are carrying the slick slowly northward past Dassen Island.

If the wind holds for another two days, the penguins and lagoon birds will be safe, he said.

The danger is that the wind might shift to the northwest, blowing the slick inshore. It is winter in the Southern Hemisphere, and the prevailing winter wind off the cape coast is to the northwest.

The southeaster, a blustery wind that usually blows in spring and early summer, is known locally as "the cape doctor" because it is said to blow germ-infested litter out to sea.

"We are very lucky," Damerell said in a telephone interview. "The cape doctor is saving us at the moment. But if the wind changes to a northwesterly, the pollution threat will be appalling."

Meanwhile a salvage tug from Cape Town, the John Ross, is trying to tow the forward section of the supertanker farther out to sea so that the remainder of the 252,000-ton cargo of oil will be discharged as far from the coastline as possible.

The stern section sank Saturday.

Kent Durr, president of the National Council for the Conservation of Coastal Birds, said that his organization had rescue teams standing by but that if the wind changed there would be little they could do to save the birds.

The two-foot-high jackass penguins are particularly vulnerable to oil pollution and particularly difficult to save if damaged, Durr said.

"If they swim into a slick, the oil weighs them down and they drown," he explained. "Even with a light coating their feathers stick together and they can't swim. Then they begin to suffer from exposure."

If they are rescued they can be cleaned by a complicated process that involves several washings with special detergents, Durr said. The trouble is these detergents also remove the birds' natural oils, which means they will die of exposure if released immediately, Durr said.

They have to be kept in captivity for about six weeks until the natural oils return, and during this time they are difficult to sustain because they refuse to eat dead fish, Durr explained.

"They have to be force-fed by hand, one bird at a time," he said. "You have to hold the bird between your legs, open its beak and force slices of pilchard down its throat. It is quite an operation. Obviously we would only be able to cope with a handful of birds. If the oil hits Dassen Island it will be a disaster."

Durr said the jackass penguin is unique to the coast of South Africa and Namibia (Southwest Africa), and only about 200,000 remain.

There were 1.5 million of the little black and white birds on Dassen Island in 1926, Durr said. His council made a count four years ago and found there were only 100,000 left, of which only 20,000 were nesting.

"That shows a species in a cataclysmic descent towards extinction," Durr said.

[Michael Wright, vice president of the World Wildlife Fund, sister organization of the International Union for the Conservation of Nature, said in Washington that the penguins' "vulnerable" classification is one step short of "endangered." Serious damage to their habitat would put them on the endangered list, he said.]