The 10th dead humpback whale in the region so far this year washed ashore in East Sandwich between Sunday night and Monday morning, the latest in a disturbing trend in recent years, advocates said.

“We have an unusual mortality event that’s going on with humpback whales in the region, up and down from Maine to Florida,” said Tony LaCasse, a spokesman for the New England Aquarium. “Unfortunately, this is the 10th dead humpback in the region so far in 2019, which projects out to almost exactly the average over the last three years, when that number has been really elevated.”

Normal mortality for the region’s humpback whales is about 11 per year, LaCasse said.

The body of the 45-foot-long, 40-ton female humpback named Vector was first spotted on Saturday and made landfall between Sunday night and Monday morning on Stellwagen Bank, said Melanie Mahoney, a spokeswoman for the International Fund for Animal Welfare, which will perform a necropsy to determine the cause and manner of the whale’s death.

Jamie McWilliams, education director of Cape Ann Whale Watch in Gloucester, said she spotted Vector floating dead but didn’t notice any sign of entanglement in fishing gear, one common cause of whale deaths; another is collisions with ships.

“It’s devastating,” McWilliams said. “We see these animals we know and love. This is our backyard and their home.”

Vector was first documented by researchers from the Center for Coastal Studies in 1984 and was seen regularly in the Gulf of Maine over the years, during which she had five confirmed calves, said Cathrine Macort, a CCS spokeswoman.

Humpback whales are tracked using photo identification of their flukes, or tails, each of which has a pattern as unique as fingerprints are to humans, Macort said.

In 1970, the federal government listed all humpback whales as endangered after commercial whaling drastically reduced their numbers. But North Atlantic humpback whales were removed from the endangered-species list in 2016.