Local and federal researchers are trying to determine the cause of death of a juvenile humpback whale that was found washed ashore Sunday on an island off the coast of Maine.

A resident of Inner Heron Island, a privately owned island off South Bristol, was shuttering their cottage for the winter when they found the whale’s decomposing body on the rocky shoreline and notified Marine Mammals of Maine.

Lynda Doughty, Marine Mammals of Maine’s executive director, examined the carcass of the 31-foot juvenile male on Tuesday, but was unable to determine a cause of death. There were no visible signs of injuries, but the whale, which weighs several tons, was lying on its stomach and couldn’t be fully examined. The National Oceanic Atmospheric Administration is also looking into the death.

“It was too decomposed to make any assumptions about how it died,” Doughty said Wednesday evening. “I can’t flip it over. It’s just too big.”

Doughty took swabs to test for viruses and to check its DNA. Those samples will be sent to labs for further testing, but the results may not be known for weeks or months. The whale was so badly decomposed that Doughty was unable to extract any viable tissue samples. It will be left on the island and eventually will wash out to sea.

She said that humpback whales are one of three large whale species currently experiencing unusual mortality rates along the East Coast. The others are minke and North Atlantic right whales.

Ainsley Smith, NOAA’s regional marine mammal stranding coordinator, said there have been 166 documented humpback whale deaths since 2016 along the Eastern Seaboard. By comparison, there were about 52 documented humpback whale deaths from 2012-16. Since 2016, five humpback whale deaths have been documented along the Maine coast.

“That is a much higher stranding rate than what we normally see,” Smith said Wednesday evening, referring to the number since 2016. “It is definitely concerning.”

Smith said many of the documented deaths have been caused by vessel strikes and rope or fishing gear entanglements. Smith said NOAA has seen no evidence that the humpback deaths are being caused by a virus.

Partial or full necropsies were conducted on approximately half the 166 whales that died and about 50 percent displayed evidence of either being struck by a vessel or entangled, according to NOAA.

Under the Marine Mammal Protection Act, an unusual mortality event is defined as “a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response.”

According to NOAA, humpback whales live in all the world’s oceans. They travel great distances every year and have one of the longest migrations of any mammal on the planet.

Some populations swim 5,000 miles from tropical breeding grounds to colder, more productive feeding grounds. Humpback whales feed on krill – shrimp-like crustaceans – and small fish, straining huge volumes of ocean water through their baleen plates, which act like a sieve.

Marine Mammals of Maine is a nonprofit based in Brunswick that responds to stranded marine mammals and sea turtles from Kittery to Rockland. To report a stranding call 1-800-532-9551.

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