New Yorkers were shocked earlier this week at the sighting of a large aquatic mammal swimming back and forth in the East River. Onlookers close enough to the water captured the animal flipping its tail up and posted photos of the creature online, alerting the U.S. Coast Guard.

Social media users decided they must've been seeing a humpback whale, but a Coast Guard spokesperson, Logan Kaczmarek, said the agency wasn’t sure whether it was a humpback or a North Atlantic right whale, an endangered whale species.

After the pictures circulated on the internet, the Coast Guard put out a marine public service broadcast to ask boaters to be careful to avoid collision with the whale. Whales are protected by federal law, which prohibits harassment from humans in US waters, specifically commercial areas.

“We do get a lot of marine mammals in the New York Harbor. It’s not rare by any means, but the sightings are less frequent," Kaczmarek explained.

The spokesperson added that there can be up to 80 reported whale sightings annually in the New York area, especially during the fall and spring migration seasons. He added that whale spotting in the East River is special because boat traffic and pollution make it difficult to see underwater wildlife. Since it is the off-season, Kaczmarek told the Gothamist: "I have no clue where that whale specifically was going."

The New York Coast Guard reported that the whale entered the Buttermilk channel Monday and traveled North to the East River. "This one is on a mission to do something," Kaczmarek concluded.

Experts believe the whale population off the coast of Brooklyn is growing rapidly. Mitchell Steinhardt, a naturalist with American Princess Cruises and Gotham Whale has said the number of sightings on trips is rising.

“First year that we were out, we spotted five whales,” he said. “The entire season. Now we have whales 97% of the time on our trips.”