In this Friday, Dec. 29, 2017 photo provided by the Instituto Boto Cinza, a gray dolphin
© Leonardo Flach/Instituto Boto Cinza
In this Friday, Dec. 29, 2017 photo provided by the Instituto Boto Cinza, a gray dolphin floats dead in the Bay of Sepetiba, on the coast of Rio de Janeiro State, Brazil. Environmentalists say they are trying to figure out why more than 80 gray dolphins have died in less than a month on the coast of Rio de Janeiro state.
Investigators are trying to uncover why dozens of endangered dolphins have died in less than a month off Brazil.

The bodies of more than 80 tucuxi dolphins, also known as boto cinza or gray dolphins, have been found in the baffling case.

According to the local Al Dia News, it's the worst mass death of the dolphin this decade. The second-worst was in 2016, when 69 died.

The Instituto Boto Cinza (Gray Dolphin Institute), a non-government organisation which monitors the dolphins, said 88 dolphins have died in the past 18 days - 10 percent of the population in the area.

Workers at the organisation have been collecting the bodies and taking them for autopsies to try and unravel what's going on.

"Without this job, the death of these animals would be silent, they would die and no one would know that this is happening," the organisation wrote on its Facebook page.

While the cause of death is not yet known, the institute is calling for part of the bay to be protected to help preserve the species.

"We will not let this matter go unnoticed and not be forgotten," the institute wrote.

"We will unite our forces so that the species will have a chance to continue!"

Lab tests investigating the cause of death are expected to be completed by the end of the month.