Scientists say an oil spill off the Galapagos Islands last year has caused the death of more than 60% of marine iguanas - a species unique to the Pacific Ocean islands.

The spill happened when an Ecuadorian oil tanker ran aground in January 2001 depositing around three million litres of crude oil into the sea.

We came back after a year and were astonished to find a large number of skeletons on the coast

Scientist Martin Wikelski

Yet compared with other tanker accidents, the concentrations of oil found just offshore were fairly small, and the prevailing view at the time was that Santa Fe, one of the Galapagos Islands, had escaped serious contamination.

But now researchers have concluded that even this small amount of pollution can have severe effects on wild animals.

They are warning against complacency over the impact of oil spills, even when it seems they have caused little environmental damage.

The research, carried out by Martin Wikelski, a biologist of Princeton University, New Jersey, and others, is published in Nature, the British scientific weekly.

The unique wildlife was thought to have escaped the spill

The scientists estimate as many as 15,000 marine iguanas died on Santa Fe in the 11 months after the accident.

The figures are considered highly reliable as the marine iguana population on Santa Fe has been closely monitored for years.

Although it is uncertain why they died, oil washed up on the island's shoreline could have poisoned the reptiles or the algae that they consume, or perhaps they starved to death, the research suggests.

Wildlife on the Galapagos Islands, a World Heritage Site located about 1,000 kilometres (620 miles) west of mainland Ecuador, has evolved separately from the rest of the world.

This separation has turned the islands into a biological time capsule.

Navigational mistake

Martin Wikelski and the Galapagos National Park are suing Petroecuador, Ecuador's state-owned oil company, for damages.

Petroecuador President Rodolfo Barniol would not comment on the study or the lawsuit.

The captain of the tanker admitted his navigational mistake while steering the Jessica into harbour on San Cristobal Island caused the spill.

The accident happened when the tanker ran on to a reef just off the island as it prepared to enter the harbour to deliver fuel.

Captain Tarquino Arevalo, who admitted confusing two landmarks, was jailed for 90 days last year and had his professional licence suspended.