Mauritius is a biodiversity hotspot with a high concentration of plants and animals unique to the region.

"The wind and the water currents are not helping, they are taking the oil towards the areas that have vital marine ecosystems," Sunil Mokshananda, a former Greenpeace strategist, who is on an island near the oil-spill site, told the BBC.

The Mauritian marine environment is home to 1,700 species including around 800 types of fish, 17 kinds of marine mammals and two species of turtles, according to the UN Convention on Biological Diversity.

Coral reefs, seagrasses and mangroves make Mauritian waters extraordinarily rich in biodiversity.

"There are very few such marine areas with such rich biodiversity left on the planet. An oil spill like this will impact almost everything there," said Dr Corina Ciocan, a senior lecturer in marine biology at the UK's University of Brighton.

"It is not just about the light oil slick you see on the surface of the water caused by the spill.

"There will also be soluble compounds from the oil that will dissolve in the water, a mousse-like layer underneath the surface of the water, and then very heavy residues on the bed - so the entire marine ecosystem will be affected."