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A seal may have made itself at home in the not-so-dirty water

A seal was spotted lounging on a dock along the Charles River on Tuesday.
A seal was spotted lounging on a dock along the Charles River on Tuesday.(Ed Bajwa)

At first glance, Katie Slivensky and her coworkers at the Museum of Science thought the shiny, football-shaped object floating in the Charles River behind their office was debris.

But when it gracefully dipped down into the darkish water, leaving behind a trail of ripples, they realized it was the head of a seal.

“It was very clearly a large mammal,” said Slivensky, an educator at the museum. “We thought we were all maybe hallucinating or something.”

It wouldn’t be the last time that workers from the museum, and people running on the water’s edge, would spot the creature.

Since Slivensky saw the animal in March, a seal has been feasting on the abundance of freshwater fish that the Charles River has to offer.

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The prevailing theory among experts at the New England Aquarium is that the animal has likely learned that it can get from Boston Harbor to the Charles through the locks of the Charles River Dam.

“It appears that we have maybe one or more harbor seals that have figured out that the locks present an opportunity to be able to get to a really easy and rich food source on the freshwater side of the Charles,” said Tony LaCasse, a spokesman for the aquarium.

The dam has three parallel locks separating the harbor from the river. Staff members from the Massachusetts Department of Conservation and Recreation are responsible for opening and closing the locks for various reasons, such as allowing spawning fish to pass through. When the gates come open, the animal, identified as an adult harbor seal, is likely right there, eager for a chance to dine on carp and perch.

“Seals are exceptionally intelligent,” LaCasse said. “They’re acting like people queuing on the Zakim Bridge at rush hour — they’re waiting to get their turn to make a living in the city.”

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On Monday, dam workers from DCR found a seal swimming in one of the three locks. After notifying aquarium officials, workers opened the gates and let the animal swim out into the ocean.

“That adult harbor seal was really comfortable, and he had no stressed-out behaviors in the lock,” said LaCasse, “which meant he was familiar with the lock.”

A seal was also ushered through the locks, and into the harbor, on Saturday.

There have been sightings up and down the river over the last 11 days. Whether it’s the same seal remains unclear, but sightings have been reported as far away as the Massachusetts Institute of Technology and the Boston University Bridge, miles from the locks.

Harbor seals can stay healthy in fresh water for weeks at a time. Eventually, LaCasse said, the animal would need to return to saltier waters to keep its blood chemistry in balance. Seals passively ingest needed salts from sea water.

One of the latest sightings came Tuesday morning. A seal was caught lounging on a dock near Community Boating Inc.’s boathouse.

A DCR spokesman said that workers had reopened the locks since the seal’s release Monday afternoon, so there was a chance the animal slipped back through unnoticed.

Ed Bajwa, director of intensive care at Massachusetts General Hospital, was jogging by the dock when he saw the torpedo-shaped pinniped sprawled out.

Bajwa did a double-take, then kept running. On his return past the dock, he stopped to a take a photo.

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“I was shocked,” he said. “I’ve never seen anything like that on the Charles.”

Employees from Community Boating Inc. took photos of the seal and shared an image on Twitter. In the picture, the seal’s head is perked up, as if to say, “There’s nothing to see here, please move along.”

By 9:45 a.m., as a brisk, mild wind spattered raindrops at the boating center, the seal had vanished.

But if the creature is as smart as they say, it won’t be his last adventure upriver.

“If people are running along the Charles, and they think they see a seal somewhere, they don’t have to adjust their eyesight or their sanity,” LaCasse said. “There are probably some seals in the water.”


Steve Annear can be reached at steve.annear@globe.com. Follow him on Twitter @steveannear.