Officials from the New England Aquarium were called to help guide a lone dolphin from a shallow inlet in Winthrop to a deeper part of Boston Harbor.
Two white-sided dolphins stranded themselves shortly before midnight Sunday, but only one survived the night.
“It is extremely rare to see these larger dolphins come into these little, shallower, cove-type areas,†Biologist Adam Kennedy said.
The Winthrop harbormaster and residents who live nearby paddled out in boats until the dolphin was eventually led into deeper waters.
The surviving dolphin was guided several miles off Coughlin Park and was last seen going toward Deer Island. It is likely the dolphins were swimming after fish.
A necropsy will be performed on the deceased dolphin.