A scheming seal was spotted again Sunday swimming in the Charles River, not far from the dam locks where researchers believe the animal may be sneaking in from Boston Harbor to hunt for freshwater fish.

Mike Squires, a captain with Boston Duck Tours, said he saw the seal lounging on a dock as his duck boat exited the river.

“We have been seeing him fairly frequently,’’ he said. “He’s been there all day, just laying in the sun, having a good time.’’

A Globe photographer also observed the seal by the Leonard P. Zakim Bunker Hill Memorial Bridge Sunday afternoon, and again, later, closer to the locks.

Harbor seal sightings have been frequent on the Charles River recently. Since March, seals have been detected near the Boston University Bridge, the Massachusetts Institute of Technology, Community Boating Inc.’s boathouse, and close to the Museum of Science.

New England Aquarium officials have said it’s unclear if it’s one or more seals visiting, but the animals are coming to the Charles to feast on large river fish that are easy to catch.

Experts from the aquarium believe that the seal — or seals — have probably figured out how to enter the river via the dam’s locks.

Crews from the Massachusetts Department of Conservation and Recreation open and close the locks as part of their duties, including to let spawning fish pass through. When the gates open, the seals are close behind, as if they’ve been waiting for a chance to enter the river. Workers have let at least two seals travel through the locks this month from the Charles River side to get to the harbor.

Harbor seals can survive in fresh water for more than two weeks before they need to head back to the ocean to take in needed salts from sea water.

Steve Annear can be reached at steve.annear@globe.com.