New England Aquarium officials coordinating the rescue effort said 34 Atlantic white-sided dolphins died Friday, 16 of which were euthanized. An additional 20 became stranded Saturday afternoon, officials said.
Three dolphins who survived Friday's beaching had been released at Provincetown on the tip of Cape Cod after being equipped with identification tags.
The dolphins, which weigh up to 400 pounds, come to land stressed and disoriented, said Andy Stamper, the aquarium's chief veterinarian. Removed from water, their organs malfunction and fill with blood.
A similar beaching occurred last year, when 97 dolphins stranded themselves on the Cape between January and February. Whales have also become stranded there.
The cause of such beachings remains unknown. Some biologists believe that a new moon coupled with unusually high tides could contribute to the phenomenon.
Aquarium spokeswoman Sue Knapp said it's possible dolphins become confused by the Cape's unusual geography and cannot find their way back to the open ocean.
"Emotionally, it's very tough to have to euthanize these wonderful animals," Stamper said. "Hopefully someday we'll be able to prevent this."