The baby dolphin rolled over on its side, floating on the surface. Its mother repeatedly nudged its head above the oily waves in a lagoon in Mauritius, where environmentalists are demanding an investigation into the deaths of dozens of dolphins following an oil spill.

The footage - obtained by Reuters from a fisherman who tried to help rescue the dolphins - showed the last desperate moments of the mother and her calf before they both died.

At least 40 dolphins have been found dead in Mauritius - 38 who washed ashore, as well as the mother and her baby - since Monday, when a Japanese ship was scuttled after running aground in July and leaking oil. 

The death toll may rise: Yasfeen Heenaye, the 31-year-old fisherman who filmed the mother and baby, said he saw nearly 200 dolphins inside the reef Friday morning, 25-30 of them dead.

"Some were injured and some were just floating," Heenaye said. Fishermen were trying to herd the dolphins out of the lagoon into the open seas.

"Inside the reef there is oil spill on the water - if they stay inside maybe all of them will die - but if they go outside maybe they will survive. We were trying to push the dolphins outside the reef, making noise in the boat to make the dolphins go outside the reef," he said.

"There was a mother and her baby...He was very tired, he didn’t swim well. But the mum stayed alongside him, she didn't leave her baby to go with the group. All the way she stayed with him. She was trying to protect him...to push the baby to get back with the group."

But the baby wallowed on its side and died in front of them, floating on the waves, he said.

"When I was seeing this, there was tears in my eyes. I am a parent of a little daughter, it is very difficult for me to see the mother struggle and try her best to save her baby," Heenaye said.

A short while later, the mother had convulsions and died too, said another witness, Reuben Pillay.

(With input from agencies)