Namibia said on Monday that 86 dusky dolphins, including two calves just days old, had been found dead on a remote beach and it had launched an investigation into what happened.
Marine biologist Kolette Grobler estimated that the mammals had been dead for about a week before they were found on Friday by commercial lobster fishermen at Hottentot Bay, some 40 km (25 miles) north of the southwestern coastal town of Luderitz.
"We suspect that one of them was injured or got sick and called on the others for help, and this might have caused them to accidentally beach themselves," Grobler said.
"We, unfortunately, could not take samples due to the decomposition, but we took water samples to determine if the dolphins ate toxic plankton."
The ministry of fisheries and marine resources said it would investigate the cause of the dolphins' death.
The dusky dolphin, known for its aerial acrobatic skills, is found off South America, southern Australia and New Zealand as well as southwestern Africa.
As well as the dusky dolphin, Namibian waters are also home to the bottlenose dolphin and the Heaviside's dolphin.