The InstitutoBotoCinza, a non-governmental organization, reported the dolphin deaths on social media, Efe news agency reported.

The dead tucuxis (Sotaliaguianansis), a species known in Brazil as boto-cinza, started appearing in mid-December, having beached themselves around Sepetiba Bay, located some 80 km west of Rio de Janeiro.

The dead dolphins represent about 10 per cent of the roughly 800 tucuxis estimated to be left in Sepetiba Bay.

The tucuxi is a freshwater dolphin found in the rivers of the Amazon basin and estuaries along the Atlantic coasts of Central and South America.

Up until now, the worst years for the tucuxis had been 2010, when 32 of the dolphins were found dead, and 2016, when 69 of the marine mammals died.

InstitutoBotoCinza researchers have not yet determined what killed the dolphins, but they have ruled out poaching.

Necropsies will be performed on the dead dolphins in an effort to identify the cause of death, the environmental group said.