Another dead North Atlantic right whale has been spotted off Massachusetts, bringing the total number of confirmed deaths in North America this summer to at least 13.

The U.S. Coast Guard documented and reported the latest carcass on Monday, Jennifer Goebel, public affairs officer for the National Oceanic and Atmospheric Administration (NOAA), Greater Atlantic region, confirmed on Wednesday.

The dead whale is currently floating in the southern Gulf of Maine, about 160 miles (about 258 kilometres) east of Cape Cod, she told CBC News.

This is the third dead North Atlantic right whale discovered in U.S. waters, said Goebel. The news comes just one week after another whale was found floating off Martha's Vineyard, the Massachusetts island south of Cape Cod.

Between 10 and 12 dead whales have been found in the Gulf of St. Lawrence since June 7. Preliminary necropsy reports suggest ship strikes and fishing gear entanglements as possible causes for the deaths. A final report is expected by mid-September, and will be made public.

Only about 500 North Atlantic right whales are left in the world, according to fisheries officials.

North Atlantic Right whale wwf

Only about 500 North Atlantic right whales are left in the world, according to fisheries officials. (worldwildlife.org)

"It's a really alarming statistic to have lost, you know, possibly two per cent of the population," Goebel said. "It's very alarming and we're very concerned about the state of this population."

The dead whale was a reproductive female, according to Amy Knowlton, a research scientist at the New England Aquarium.

The aquarium's North Atlantic right whale team was able to identify the carcass as Couplet, (#2123), a whale born in 1991, which gave birth to five calves, the most recent one in 2014, said Knowlton.

'This is not sustainable.' - Brian Sharp, IFAW

The latest confirmed death "shows the breadth and scale of this problem," said Brian Sharp, program manager for the International Fund for Animal Welfare's marine mammal rescue and research team.

"It's incredibly concerning from a species perspective," he said. "This is not sustainable."

More deaths than births

The number of North Atlantic right whale mortalities this year has far exceeded the number of births, according to Charles Mayo, a senior scientist at the Center for Coastal Studies in Provincetown, Mass.

Only about five calves were born this year — a reproductive rate of about one per cent, said Mayo. By comparison, the average calving rate over the past 10 years was about 2.5 per cent, he said.

Mayo also feared the actual number of deaths could be even higher than reported.

He noted some of the carcasses have drifted very far offshore, and suggested there could be others that drifted even farther and may never be found.

"So a population that's been very close to extinction is now a lot closer to extinction — especially if whatever is causing this is allowed to continue," said Mayo.

Traditionally, a "typical year" would see "a couple" of North Atlantic right whale deaths, said Sharp. It's still unclear why the numbers have spiked this year, he said.

"That's the most frustrating part is that we just don't know at this point," he said.

Hurricane Gert complicates efforts

Officials are hoping the latest carcass might offer some clues. They are planning a multi-agency response strategy to try and locate the whale and tow it in for a necropsy, or at least take some samples at sea, said Sharp.

For now, they are waiting for weather disturbances caused by Hurricane Gert to calm down before a search and investigation can begin, he said.

As of Wednesday afternoon, the hurricane was about 250 miles southeast of the area where the carcass was spotted, causing sea swells of between nine and 11 feet, said Sharp.

NOAA and the U.S. Coast Guard are working on some drift analysis based on the winds from the storm and currents to help narrow the search area, he said.

"We're still working on the logistics of that, but because it is a right whale, we're really keen to get some information from it somehow," said Goebel.

They hope to have a plan "sooner rather than later," she said.