SOUTHPORT, Maine — A young harbor seal named Copper rescued in March near Christmas Cove after becoming entangled in monofilament fishing line was returned to the Atlantic Ocean on Tuesday after nearly doubling its weight while at the Mystic Aquarium in Connecticut.

Copper was discovered dehydrated March 21 on Southport with severe entanglement wounds around his neck. Staff and volunteers at the Marine Mammals of Maine center in Harpswell removed the fishing line, cleaned his wounds and began treatment before transferring him four days later to the Connecticut aquarium, which specializes in treating recovering marine animals.

Fishing line entangled around his neck interfered with Copper’s ability to catch and eat food, causing him to be underweight, according to Marine Mammals of Maine staff. Copper weighed 48 pounds when he arrived at the Mystic Aquarium, the where he was treated and rehabilitated for four months and gained 36 pounds.

On Tuesday, he was returned to the ocean in Narragansett, Rhode Island, according to a news release from the aquarium.

Several volunteers from Marine Mammals of Maine in Harpswell were there to see him swim away.

“We certainly wish we could have carried out his care here in Maine, but we were just really glad there was a spot for him to go to,” assistant stranding coordinator Dominique Walk said Thursday.

Mystic Aquarium officials hope Copper’s story will remind people not to discard trash into and near the ocean, where seals can become entangled in discarded fishing line and turtles can suffocate after swallowing balloons.