Norwich — Sheldon Mathis arrived at his regular fishing spot at Howard T. Brown Memorial Park at Norwich Harbor about 6 a.m. Thursday, with fellow fishermen all abuzz about a dolphin that had been entangled briefly in a fishing line the prior evening.

About 7:30 a.m. Thursday, Mathis grabbed his cellphone as a fellow fisherman apparently hooked the dolphin. The marine mammal leapt in the air and thrashed about until the fisherman cut his line, allowing the dolphin to flee. The last sighting was about 8:30 a.m. Thursday, Mathis said.

Norwich police officers and state Department of Energy and Environmental Protection conservation officers arrived at Brown Park soon after the dolphin encounter and halted all fishing for the dolphin’s safety.

“It’s been out here two days now just gorging on the fish over here,” Mathis said, pointing to the area where the Shetucket River flows into the harbor.

Mathis and Tony Esquilin, 51, of Norwich both said they have been fishing in the harbor for about 25 years, mostly for striped bass and bluefish. They were after striped bass Thursday before the DEEP officers closed the popular fishing spot.

Esquilin said it’s rare to see a dolphin in the harbor, but seal sightings are more common. He described an incident a few years ago when he was fishing for striped bass in his kayak. He caught one fish, but when he reeled it in, he had only half a fish. Same thing happened a second time. The third time, he said the large seal latched onto the fish and did not let go, nearly tipping over the kayak.

Meagan Seacor, vice president of external relations at Mystic Aquarium, said dolphin sightings in the Thames River and Norwich Harbor are rare. Based on the videos posted on social media sites, it appeared to be a common dolphin. The images were too fleeting for officials to speculate on its size, age or weight.

The animal could have made its way up the river for food, she said. The jumping behavior, called “porpoising,” could have been caused by stress and boating activity nearby. She said the aquarium did not receive a report that the dolphin may have been hooked by a fishing line.

Aquarium officials were pleased that the final video showed the dolphin swimming in a calm, more normal behavior. The dolphin apparently left the harbor area by noon or 12:30 p.m., she said.

“We felt good and hopeful that it’s in a stable, healthy condition, that it was making its way back to the ocean,” Seacor said.

Seacor said the aquarium received a call about the dolphin about 7 a.m. Thursday, but aquarium officials quickly learned that videos had been posted on social media as early as the previous evening.

She emphasized that anyone who sees marine mammals in unusual settings should call the aquarium marine hotline at (860) 572-5955, ext. 107.

c.bessette@theday.com