At least 18 dead dolphins have washed up on the coast of the island of Mauritius, in the Indian Ocean, where a Japanese ship, the bulk carrier MV Wakashio, has spilled more than 1,000 tons of fuel after running aground on a coral reef last month.
Other dolphins have stranded on shore and appear seriously ill, environmental groups have said.
The ship has leaked fuel into the waters of the Mahebourg Lagoon, which includes a protected wetlands area, mangroves and a small island that was a bird and wildlife sanctuary.
The spilled oil has also reached the soft soil of mangrove forests along the coastline.
Experts said they found no major damage, nor any sign of oil on the ocean bottom or on the coral reef, but that the remaining wreck appeared to be grinding against the reef as waves push it back and forth.
"If this situation continues, it could cause stress to the corals and could kill them," said Noriaki Sakaguchi, an ecosystems expert with Japan International Cooperation Agency.
Greenpeace has called for an investigation.