A four tonne sperm whale and eight dolphins have been washed up on Senegal beaches over the past month, the Worldwide Fund for Nature (WWF) said Friday.

The mammals were washed up a year after the mystery over 100 whales that were beached north of the Senegal capital Dakar. Forty-eight were rescued and put back to sea.

The dead sperm whale washed up on the beach at Ngasobil, in the Mbour region on Wednesday, WWF official Mamadou Diallo told AFP. It was probably killed in a collision with a boat, he added.

The eight dolphins washed up on a beach at Tivaouane Peulh, north of Dakar. The WWF believes they were killed by a red seaweed which has spread "at an extraordinary rate" on the coast, Diallo said.

"The seaweed contains a toxin that is deadly for fish and dolphins," he said.