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You're gonna need a bigger marsh: Wayward blue shark spotted in Cape Cod

"That's no otter," says woman who spotted the wrong-way ocean predator in Eastham, Massachusetts.

A wrong-way shark accidentally swam into a Cape Code marsh recently, stunning scientists and witnesses, who captured video of the directionally challenged animal.

Robin Rowe enjoys looking for wildlife from her home near Bee’s River Marsh in Eastham, Massachusetts, and was excited on Nov. 6 to spot what appeared to be an otter flopping in the water.

"And so I thought maybe it was a river otter," Rowe told NBC News on Friday.

"Because that's what I thought it might be, because I just saw this round black head that wasn’t a duck. And so I watched it for like a minute, and then it started swimming around and I was like, OK, well, that's not a river otter. It's got fins."

The blue shark was likely headed south for the winter from the Gulf of Maine when it swam inside the curved mass of Cape Cod, into First Encounter Beach and then Bee’s River Marsh.

"It's a big migration for a number of species," said Greg Skomal, the senior fisheries scientist for the state Division of Marine Fisheries. "The hook of Cape Cod sometimes entraps (southbound) animals."

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Rowe said she was instantly concerned about the creature.

"I knew it was not where it was supposed to be," she said. "I know enough about sharks that they don’t swim in the marsh."

But judging by Rowe's video, Skomal said, the shark appeared to be in good shape and he's confident the predator found its way out of Cape Cod. There had been no reports of any dead sharks washing ashore.

"Yes, it was well inside (the hook of Cape Cod), but they know how to get out if they are healthy and I think this shark was healthy," Skomal said.

Assuming the shark figured out how to motor north and then take right turns around Provincetown to finally head south, Skomal said, the healthy predator could be in ocean waters off the Carolinas by Friday.

Blue sharks can grow to be as long as 12 feet and weigh up to 400 pounds.