Investigators are probing an unusually high number of humpback whale deaths since 2016 off the US Atlantic Coast, where many appear to have been killed by colliding with boats, officials said Thursday.
A total of 41 humpback whales (Megaptera novaeangliae) have died in the waters off Maine to North Carolina, said the National Oceanic and Atmospheric Administration.
Twenty-six died last year, far higher than the average annual number of humpback whales for the area -- just 14.
So far this year, 15 have already washed up dead.
"The increased numbers of mortalities have triggered the declaration of an unusual mortality event, or UME, for humpback whales along the Atlantic Coast," said Mendy Garron, stranding coordinator at the NOAA Fisheries Greater Atlantic Region.
A UME is issued whenever a stranding is "unexpected, involves a significant dieoff of any marine mammal population, and demands immediate response," she told reporters.
Animal autopsies -- known as necropsies -- have been performed on 20 whales.
Ten of the marine mammals showed acute signs of blunt force trauma or large propeller cuts from colliding with ships or boats, suggesting this was the likely cause of death.
The other 10 had no such obvious signs, and researchers are continuing tests to find out what other factors could have contributed to their demise.
- Cause unknown -
"Whales tested to date have had no evidence of infectious disease," said Garron.
Researchers stressed they have yet to uncover the cause of the unusual spike in deaths.
"The answer is really unknown," said Greg Silber, coordinator of recovery activities for large whales with the NOAA Fisheries Office of Protected Resources.
There is no known spike in vessel traffic in the area, but humpback whales move around in search of prey, which could bring them closer to shore, he added.
Humpback whales grow to between 48 and 62 feet (15-19 meters), weigh 40 tons, and are known for their haunting songs that travel great distances underwater.
Most humpback whales are no longer considered an endangered species, after that designation was lifted in 2016 due to a rebounding population.
There are more than 10,000 humpback whales in the North Atlantic Ocean.
But there are 14 distinct populations of the whales, five of which are still endangered, including those in the Arabian Sea, Cape Verde Islands, northwestern Africa, the Western North Pacific and Central America.
An international moratorium on hunting them was established in 1982 and remains in place.
Unusual die-offs of humpback whales in the Atlantic Ocean were also reported in 2003, 2005 and 2006, NOAA said. The cause of those deaths were undetermined.