BENTON — A harbor seal that’s feasting on fish in the Sebasticook River this week appears “fat and happy,” said a fish biologist with the state’s Department of Marine Resources.

Jason Valliere said seals seem to make their way up the Kennebec River to Augusta and Waterville, but this is the first time he has seen a seal as far north as Benton, about 60 miles from the ocean.

Lynda Doughty, the Department of Marine Resources’ marine mammal stranding coordinator, said this is the first time in her 12 years on the job that she has heard of a harbor seal as far upstream as the Benton Falls fishway and Benton Falls Hydro Project.

To reach Benton, the seal likely swam from the Gulf of Maine into the Kennebec River through Bath to Winslow, where it entered the Sebasticook River and swam to Benton.

With no dams to impede travel and fresh fish to eat, she said, there’s no reason a seal shouldn’t or couldn’t be in Benton. “They can live in fresh water,” she said. “And with the alewives running through that area, the food is plentiful.”

Valliere said the seal, which he estimated to be 2½ feet long, has been in the vicinity since around Friday. Doughty estimated it is 6 weeks old.

Sometimes, said Doughty, seal pups do not learn to feed by the time they separate from their mothers, at 3 to 4 weeks of age. That’s apparently what happened with a harbor seal that was captured last weekend in Benton.

Doughty said that 3- to 4-week-old pup was captured Saturday and taken to a rehabilitation center, where he is reportedly progressing well.

“He was on his own but hadn’t successfully figured how to eat,” she said.