BOSTON (AP) — The New England Aquarium says six dolphins have turned up dead on Massachusetts beaches.

The most recent was as 4-foot long, dark grey harbor porpoise that washed up with the overnight high tide on Revere Beach Saturday The aquarium says its rescue biologists handled seven harbor porpoises in a week on the Massachusetts coast.

One was found alive and is doing well at the University of New England’s Marine Animal Rescue Center in Biddeford, Maine.

Despite the number of dead porpoises found in the short time, Aquarium officials say it appears to be seasonal. The harbor porpoises that were found were yearlings that were underweight and probably were only recently weaned from their mothers in the early winter.