A dead leatherback turtle washed ashore on Red Beach in Aquinnah on Saturday, the first sea turtle stranding across the Cape and Islands in 2025.

Leatherbacks are one of four species of sea turtles found around the Island. All are federally protected as endangered or threatened.

Bret Stearns, the indirect services administrator for the Wampanoag Tribe of Gay Head (Aquinnah) natural resources department, said he estimates the turtle is more than 500 pounds.

“It’s a big, beautiful animal,” Mr. Stearns said. “It’s a terrible shame.”

Karen Dourdeville, the sea turtle stranding coordinator for Mass Audubon Wellfleet Bay Wildlife Sanctuary, said her team, including two colleagues from the New England Aquarium, is performing a necropsy on Monday.

They have only seen photos of the turtle and won’t know more until they see it in person, but Ms. Dourdeville said there doesn’t appear to be any evidence of a vessel strike or entanglement wounds.

Leatherback turtles are listed as endangered and are the largest turtles on Earth, according to the National Oceanic and Atmospheric Administration. They can grow to be as heavy as 2,000 pounds, are highly migratory and tend to visit the Vineyard in the summer.

According to Mass Audubon, the largest nature-based conservation organization in New England, it’s important not to carry sea turtles back into the water.

If a turtle is found on the shore and appears to be alive, Mr. Stearns said to call the Center for Coastal Studies entanglement hotline at 800-900-3622. From there, a team will collect the turtle, analyze its condition, give it treatment or send it to a larger facility, such as the New England Aquarium, for help.

If a turtle is found dead or alive in the ocean, Mr. Stearns also urged people to call the Mass Audubon sea turtle sighting hotline at 1-888-732-8878. The organization collects data that helps inform conservation efforts.