WATCH: Rare Shark Sighting In Shallow Cape Cod Marsh

EASTHAM, MA — Robin Rowe likes to look outside her window overlooking the Bee's River Marsh in Eastham. It's the perfect for spotting birds, turtles and small fish. But earlier this month Rowe spotted something much more unusual in the marsh — a dark mass in the water that turned out to be a shark.

Rowe told Boston.com she thought she was about to catch a glimpse of a sea otter and got her camera ready. When she saw the fin, she realized she was witnessing something even more rare.

"I got excited because I’ve been wanting to see a river otter," Rowe told Boston.com. "So I grabbed my binoculars and my camera — my phone — and went out onto the porch to see if it popped up again. And then I saw it swim and I said, 'This is not an otter. Definitely not an otter.'"

Rowe, a member of the Eastham Chamber of Commerce, took a video of the blue shark and shared it on the chamber's Instagram page.

"You never know what you will see in Eastham," Rowe wrote in the social media post. "Our nature and wildlife is truly amazing. "

According to the Massachusetts Division of Marine Fisheries, blue sharks are common off the coast of Cape Cod, but it's unusual for them to swim in shallow marshes.

Greg Skomal, a senior scientist with the division, told Boston.com it's possible the shark may have made a wrong turn in trying to migrate South, getting caught in Cape Cod Bay.

The International Union for the Conservation of Nature estimates that 20 million blue sharks are caught each year as bycatch and has classified the species as "Near-Threatened."

The male blue shark can be anywhere between 6 and 9 feet long at maturity, whereas the larger females can grow to be almost 11 feet long.

Blue sharks rarely bite humans. Of the 828 recorded world-wide shark attacks to date only 13 of them have been attributed to Blue Sharks, according to the Florida Museum of Natural History. Compare that with the White Shark, which has been blamed for 326 attacks, according to the museum's data.