A 655-pound leatherback sea turtle that had been stranded in thick mud in Truro on Wednesday night was released off the coast of Harwich Port Saturday morning, New England Aquarium officials said.
A Massachusetts Audubon Society staff member spotted the 7-foot-long black male turtle in Pamet Harbor Wednesday night as high tide approached, said Connie Merigo, the aquarium's rescue director.
Aquarium staff and volunteers, along with staff members of the Audubon Society and International Fund for Animal Welfare, brought the turtle to the aquarium's Animal Care Center in Quincy near dawn Thursday.
The sea turtle was about 100 pounds underweight and had low blood sugar and an old injury on his front right fin, Merigo said.
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"When he first got here he was fairly lethargic, especially out of the water," head veterinarian Dr. Charles Innis said.
The turtle, which aquarium staff members did not name, was given an injectable sugar solution, vitamin and mineral supplements, steroids, and antibiotics to stave off infection, Innis said.
"We were fairly aggressive with this turtle because we have not been successful the last two leatherbacks we've had," Innis said.
In the last 40 years, the New England Aquarium has encountered only four other leatherback turtles on shore. One, found in Chatham in the mid-2000s, was released. Another, in the late '80s, died on the beach as Aquarium workers tried to care for it.
Two were taken to aquarium facilities for care — one in 2005 and one in 2011. Neither survived.
Atlantic leatherbacks, an endangered species, can be hard to care for in captivity because of their size — they can weigh more than 1,000 pounds — and because the open-sea creatures do not grasp the concept of barriers.
"They're not used to any sort of restraints — they've never seen a wall," Merigo said. "They'll continue to struggle, they'll continue to swim forward."
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To keep the recently rescued sea turtle from repeatedly crashing into the walls of the 24-foot long, 10-foot wide pool, staff and volunteers put him in a harness and turned him away from the walls while he swam.
After being released yesterday, the leatherback swam south from Harwich, approaching a jellyfish colony off the Nantucket coast where he could find food, said Merigo.
She will continue to track the turtle's movements through a satellite feed.
"The difference between the release location and where he is now shows some energy output, and it shows he's still swimming," Merigo said.
Though the turtle grew stronger over his two days in Quincy, Innis said he was not sure why the turtle had washed ashore or whether it will survive in the ocean.
Leatherbacks rarely live more than a few days in captivity because of the stress, so aquarium officials decided to release him.
"The choices were really to euthanize him, keep him in rehabilitation, or release him," Innis said. "He was too strong to euthanize, and too strong to keep. . . . We elected to release him, but with a little discomfort."
Gal Tziperman Lotan can be reached at gal.lotan@globe.com.