LONDON: Oil-covered seabirds expiring on the shore are the first image of a tanker disaster and often spark frantic rescue efforts.

But apart from the immediate impact on wildlife there are claims that spills cause little lasting damage. The theory goes that nature will break down the oil with or without human intervention.

In some circumstances this is true but the type of oil, the climate and the time of year of the spill have a big impact.

The point is illustrated by perhaps the two most famous oil spills of modern years: the Exxon Valdez, wrecked off Alaska in 1989 in a disaster which killed thousands of seabirds, sea otters and other rare wildlife, and the Persian Gulf oil spills ordered by Saddam Hussein in 1991 .

The Iraqi spills, in which untold thousands of tons of oil were allowed to pour into the Gulf in an attempt to defend Kuwait, appeared to be an appalling act of environmental crime. Yet six months later, when the war was over, scientists found to their astonishment that the oil had all but disappeared.

The reason is that the Gulf is full of highly active oil-eating bacteria. Regular spills mean a permanent population of oil-hungry microbes is waiting to pounce. The massive spills of the Gulf war spurred the already numerous bugs to multiply rapidly.

The Exxon Valdez was the other extreme. Although in comparison the spill was tiny — 38,800 tons — it happened in freezing conditions in an enclosed area.

A second important factor is the type of oil. Some crude is light and volatile and most of it evaporates harmlessly into the atmosphere. Heavy oils are harder to shift.

In the Braer disaster, off Sumburgh Head in Shetland in 1993, 130,000 tons of oil were spilt but most of it evaporated before it made landfall, helped by violent storms which broke up the slicks.

Although this disaster was not as bad as predicted, many salmon farms suffered great losses and were forbidden from selling stock because of fears of hydro-carbons in the fish. Ten years after the event, langoustine, or Norway lobster, an expensive shellfish which burrows in the contaminated sand, is still banned from sale.

The damage from the Spanish spill, the extent of which is still being calculated, will also last for years.

Those shellfish that survive will be stunted and may not be able to reproduce. Even mussels that appear healthy cannot be eaten because of toxins. Birds like gannets, which are wintering off the Spanish coast after breeding in Britain, are vulnerable as they dive through the slicks hunting for fish.

Sharon Thompson of the UK’s Royal Society for the Protection of Birds said the tanker was carrying heavy crude, which did not easily evaporate.

“It will be at least six months to a year before the shellfish colonies begin to recover and even longer to be sure they are safe for human consumption,” Dr Thompson said, adding that the best way to deal with the oil was to shovel it off sandy beaches. On rocky shores, it was best to leave it alone as cleaning rocks would kill everything. —Dawn/The Guardian News Service.