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LUQUILLO, Puerto Rico • A marine mammal expert in Puerto Rico says rescuers were unable to save a sperm whale about the size of a city bus that was stranded just off the island's north coast.

Marine biologist Antonio Mignucci of the Manatee Conservation Center at Puerto Rico's Interamerican University says a necropsy is planned to determine what caused the death of the adult female. Mignucci says the whale weighs about 15,000 pounds and is 35 feet (10 meters) long. He estimates her age at between 20 and 40 years.

The sperm whale was first spotted early Saturday just off the coast near the town of Luquillo. She died about five hours later.

Sperm whales inhabit the deep ocean near Puerto Rico from January through about August.