Another rare North Atlantic right whale has been found dead, adding to an already unusually high mortality rate this year, according to an animal welfare organization.

The International Fund for Animal Welfare reported the decomposing carcass on an island off the coast of Massachusetts on social media Monday, along with photographs from the Coast Guard.

"With this 16th confirmed mortality the North Atlantic right whale population has experienced an alarming number of deaths in 2017," Brian Sharp, IFAW's marine mammal rescue and research manager, said in a statement.

There are fewer than 500 North Atlantic right left, according to scientists, and the population has been declining steadily since 2010. In most years, just a handful of deaths are reported.

Twelve of this year's deaths have been reported in Canada's Gulf of St. Lawrence, with the other four in U.S. waters. Many of the deaths were attributed to vessel strikes and entanglement in fishing gear.

The whale death reported Monday was spotted on the rocky shore of Nashawena Island, part of the Elizabeth Islands chain off the south coast of Massachusetts.

The Yarmouth, Massachusetts-based IFAW and the National Oceanic and Atmospheric Administration are working together to determine the cause of death.

"Time is of the essence and we must work together to determine how to best protect this critically endangered species," Sharp said.