Environmental organizations have voiced concern that the oil seeping from the sunken tanker Prestige off the coast of Spain could trigger a major ecological disaster. The short-term impact on animals and coastal industries can be devastating. Some experts say the long-term impact from these massive oil spills may not be as dramatic as it seems.

As salvage workers on Spain's northwestern coast struggle to keep millions of liters of fuel oil away from the shore, many are already trying to calculate the environmental and economic damage caused by the spill. The Spanish government has declined to give specific predictions, but the World Wildlife Fund says the spill will devastate Spain's fishing industry, and will destroy the fragile seashore rich in fish, birds, and marine mammals.

But the long-term effect of an oil spill on wildlife is the subject of much debate among experts. Some argue that animals and birds are harmed many years after the event. Thirteen years after the Exxon Valdez disaster in Alaska, for example, scientists say oil still contaminates the coastline.

That spill was not the world's largest, but is considered the worst in terms of damage to the environment. An Alaska citizens group, the Oil Spill Trustee Council, says of all animals affected, only the bald eagle and river otter populations have recovered. Many other species, including endangered killer whales, have made little or no progress at all.

The Exxon Valdez spill involved more than 40 million liters of crude oil, which is more damaging than the fuel oil spilled by the Prestige.

But as in the case of many oil spills that reach shore, images of the Spanish coastline transformed into stagnant pools of sludgy liquid and oil-slicked birds and other marine life are filling the public consciousness. But Paul Kingston from the Center for Marine Diversity and Biotechnology at Heriot-Watt University in Edinburgh, Scotland says the chances of recovery for the wildlife may be better than they appear.

"One sort of envisages death and destruction and if you see a shore as far as you can see covered in black tar with struggling birds on it, it is not a nice sight," Mr. Kingston said. "But as a biologist, if you look at the numbers that are actually killed even in some of the worst oil spills, even though they are high, in terms of the total bird population, they are not that high, and populations do tend to recover."

Mr. Kingston says shore animals have adapted over the centuries to the need to rapidly replenish their populations, which are routinely depleted by harsh sea and weather conditions. In addition, he says fuel oil spills are relatively short lived because the oil has very little toxic residue, unlike other, more noxious types of oil, such as diesel or crude oil. The danger for a spill like the Prestige, he says, lies in its initial stages when the sticky oil comes into contact with marine life. It clings to the animals' skin and feathers immobilizing and sometimes suffocating them.

"Once the bulk oil has been removed, and that is quite important, the shore itself will recover relatively quickly, particularly on a rocky shore where the oil is superficial. It is just attached to the surface of the rocks, and it is cleaned off. And that can happen quite naturally by wave action. The only problem that can arise is if the oil gets buried someway in the sediments, but even then it can sort of sit underneath the sediments and providing it is deep enough, it is not going to effect the surface so again, the animals will settle back again on the surface."

This is what many hope will happen to the oil that has leaked from the Prestige.

But a large amount of the fuel remains in the ship's hold, now deep under the sea. Some scientists believe the fuel will solidify in the cold water and stay there at the bottom of the ocean causing little damage. But environmental groups like Greenpeace call it a time bomb, and say it will slowly pollute the region for years to come as oil continues to seep out.

Making matters worse, 10 years ago the Greek tanker Aegean Sea ran aground in the same area, releasing more than 75 million gallons of crude oil into the sea. Scientists say the area has not yet recovered.

The long-term, global impact on marine life from these and other spills is still not clear.

World Wildlife Fund biologist Susana Riqina says toxins leaked from oil are absorbed and accumulate in the bodies of marine animals. She says as fish and other animals migrate and feed off each other, toxic contaminants spread.

"It is difficult to determine why animals are dying and in different areas in the world. For instance whales and many other animals appear (to be) dying at the coast. Sometimes we do not know why, really," Ms. Riqina said. "But we should be sure that all these problems are connected. And what happened here can produce problems (in) any other part of the world because animals move."

Ms. Riqina says fuel oil can mix with heavy metals, sulfurs and other toxins, and threaten the worldwide ecological balance. But Paul Kingston, the researcher in Edinburgh, argues pollutants from spilled fuel oil break down very quickly when they are eaten by animals, and their strength is further diluted as they move through the food chain.

Still, the immediate damage is considerable, and environmental groups are calling for governments to do more to prevent accidents from happening. The World Wildlife Fund has been lobbying the Spanish government to declare its northwest coastline a protected area, which would keep shipping traffic away from the coast, and minimize damage if other spills happen.