An ailing dolphin was euthanized early Thursday morning after stranding on land in Southold, the board president of the New York Marine Rescue Center said.
The dolphin, one of two dolphins being observed Wednesday by marine life rescuers for signs of distress after they swam into Jockey Creek, was in poor health, said the president, Charles Bowman.
"It was euthanized on the spot," with an injection, he said.
The euthanized dolphin was an adult and larger than the other, smaller one, he said, adding: "We don't know where the other one is. Hopefully it left the area."
About 300 dolphins strand in the United States each year, according to a 2014 report.
Bowman said Wednesday his organization was called by the Southold bay constables after the dolphins, which appeared to be a mature animal and a younger animal, were spotted in the creek.
"We can't really do anything until one or both of them strands and that point it indicates there's something physically wrong," he said.