The deaths of more than 80 gray dolphins in Brazil is proving a mystery to an environmental group which strives to protect them. Environmentalists in Brazil 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.

“From December 16th, we began to collect four to five dolphin deaths every day, a quantity that was monthly. Mortality, which was five a month, was already unsustainable for this population.” According a statement by Gray Dolphin Institute, a environmental organisation, also notes that among live animals, some have been found to be thin and also have skin lesions and the dolphins died over past 17 days in the Bay of Sepetiba, a coastal district about 70 kilometers west of Rio de Janeiro.

The institute says it has retrieved five dead dolphins a day from the bay but what has caused their deaths is still unknown. Lab tests are being conducted and the results are expected by the end of the month.

According to the organization, the main reasons for deaths of dolphins in the region are the advance of port and industrial development around the bay and the predatory fishing that ends with the fish from which they feed. However, there has been a change in the pattern of cases, which may indicate a different cause. Until then, the most frequent victims were male adults, but most recent episodes involve puppies and females.

Rio de Janeiro University’s oceanographer and researcher, David Zee, raises other assumptions. For him, it is necessary to study possible effects of the lunar cycle. “We are in a time when the moon is close to the perigee, the moon is closer to Earth, so the tide fluctuations are more intense,” he explains.

According to him, the phenomenon may be promoting the entry and accumulation of polluted water in Sepetiba Bay, which would explain the large number of puppies among the victims, since younger animals are more susceptible to water contamination. Another hypothesis raised by the researcher is related to the proliferation of harmful algae, which could be occurring due to the influence of intense heat and other factors.