The long-term effect of an oil spill is the subject of much debate among scientists.

Some argue that animals and birds are harmed many years after the event, but others believe wildlife is more resilient.

Oil is not persistent - it's not that toxic

Dr Paul Kingston

According to some scientists, otters and other animals are still suffering nearly 12 years after the Exxon Valdez oil spill in Alaska.

It was not the world's largest oil spill but is widely regarded as the worst in terms of damage to the environment.

The tanker went aground in Prince William Sound in March 1989, spilling an estimated 42 million litres of crude oil - enough to fill 125 Olympic-sized swimming pools.

Huge toll

According to some estimates the spill killed 250,000 sea birds, 2,800 sea otters, 300 harbour seals, 250 bald eagles, up to 22 killer whales, and billions of salmon and herring eggs. And according to some researchers, the population has still not recovered.

Oil makes fur and feathers stick together - it can cause hypothermia

Otter population densities remain at about half their exisitng figure before the spill, according to a report by researchers from the US Geological Survey's Alaska Biological Science Centre, the University of California, Santa Cruz, and the US Fish and Wildlife Service,

The oil, which affected 1,300 miles of rugged coast, penetrated deep into the cobble beaches, especially in the areas sheltered from the winter storms that can help clean the beaches.

Human efforts and natural degradation did remove most of the oil, but some areas still have oil under the surface.

But according to Dr Paul Kingston, from the Centre for Marine Diversity and Biotechnology at Heriot-Watt University, Edinburgh, the problems are minimal.

"It can be argued that you can measure the long-term effects but you really have to search," he said. "It's difficult to distinguish between what's caused by the oil and what is caused by natural changes."

In any spill, the oil itself does not stay around long enough for it to do long-term harm, he argued.

"Oil is not persistent - it's not that toxic," said Dr Kingston. "The animal will ingest it and, if it's not too much for it to deal with, then it will recover."

Survival of the fittest?

He cited the example of the 1974 Metula oil spill site in the Strait of Magellan, Chile. There was no clean-up of the shore and the oil was left to harden.

If you dig below the surface, said Dr Kingston, the oil is still soft. But animals are still living on the land.

A lot depends on the type of oil

Although Dr Kingston recommends that the polluted areas are cleaned quickly, he argues that animals are used to adapting to new conditions.

"On a rocky shore, the animals don't live very long anyway," he said. "Storms kill most of the animals every year so they're adapted to re-colonise very quickly."

Dr Kingston also argued that oil can break down "very quickly" in a food chain, and the amount passed on reduces as it moves up the chain.

According to Greenpeace scientist Paul Horseman, the impact of an oil spill can partly depend on the nature of the coastline and the weather.

Oil is easier to get off rocky coasts than soft marsh of an estuary, for example.

And in colder regions the fuel can stay around longer than it would in hot regions because it cannot evaporate so quickly.

The type of oil will also make a difference. Crude oil has a suffocating, toxic effect and is like a heavy tar. Gasoline is generally more toxic but evaporates fairly quickly.