At least 40 dolphins have died mysteriously in an area of Mauritius affected by an oil spill from a Japanese boat, officials and witnesses have said, as a witness described the moment one mother dolphin died in front of him. Residents who had ventured out in a boat alerted Reuben Pillay to a dolphin swimming around her dying baby. He went to try to find them but the baby had died by the time he arrived, he said on Friday, but the mother initially looked normal. “But in a few minutes she went on her side, one fin in the water and one out of the water, and then she started flapping her tail really really rapidly,” said Pillay, a drone operator and environmentalist who is providing video to Reuters. “She swam in circles in front of the boat, she moved her tail very violently and after about five minutes she just stopped moving, and she sunk … We heard cries, I thought it was a woman on the boat – but they told me, no, it was the dolphin.” As they watched, the mother stopped moving and slowly sank, tail first, beneath the waves. The body of the baby floated on the surface. “We didn’t know what to do. It was heartwrenching,” Pillay said. Earlier in the day, Jasvin Sok Appadu from the Mauritius fisheries ministry said 38 carcasses had washed up on the beaches so far. Autopsy results on 25 dolphins that washed ashore on Wednesday and Thursday were expected in the coming days, he said. So far, veterinarians have examined only two of the dolphins, which bore signs of injury but no trace of hydrocarbons in their bodies, according to preliminary autopsy results. The two autopsies were conducted by the government-run Albion Fisheries Research Centre. The dolphins have been dying in an area affected by an oil spill caused when a Japanese ship, the MV Wakashio, struck a coral reef last month. The ship was scuttled on Monday. On Thursday, Greenpeace called on the Mauritius government to launch an “urgent investigation to determine the cause of the deaths and any ties to the Wakashio oil spill”.