John Emberly, his wife and their two young sons were enjoying an afternoon at Martinique Beach east of Dartmouth, N.S., when he suddenly noticed people scrambling.

"We saw a family who was beside us freaking out at their kids, trying to get them out of the water," said Emberly, a local resident, junior high teacher and avid surfer.

He assumed a seal or jellyfish was causing all the commotion Thursday. When he looked out, he saw something swimming only nine metres from shore, and it rattled him.

- John Emberly

"It was huge. It was almost surreal, it was like I was watching a movie. It was big, it was really big and I really wasn't far away from it," he said.

"This was definitely a shark. It was a very triangular fin, it was a really big fin, it was insane how big it was. I couldn't believe how big it was."

But a wildlife biologist at Acadia University is throwing cold water on the idea.

Soren Bondrup-Nielsen has viewed the video and says he is "99 per cent" sure the marine animal was actually an ocean sunfish.

The species is the heaviest known bony fish in the world, and Bondrup-Nielsen says he has spotted one before in waters off Nova Scotia. He also has seen hundreds in the Mediterranean Sea.

"What they do, they lie on their side on the surface of the water and their one pectoral fin flops in the air, and it looks like a shark fin," Bondrup-Nielsen says. "And that's why people often mistake that fin for a shark, as opposed to a sunfish."

Is that a shark or a sunfish?

This 'shark fin,' left, was spotted recently in Mahone Bay, N.S. Even the biologists who saw it initially thought it was a shark. Turns out, however, it was an ocean sunfish, right. (Tom Herman)

The fin in the video appears to remain in the same area, Bondrup-Nielsen says. Sharks, however, swim around and their fin is stiff and cuts through the water.

Emberly says he estimates the dorsal fin was about the size of a large pizza box, and believes it belonged to possibly a three-metre-long shark.

"I've surfed in South America, Central America, Hawaii, California, so I've been in water with sharks before. For me to see this, it was definite."

With his family not in harm's way, he did what seemed instinctive. He grabbed his iPhone, ran to the water and started to shoot a video of a big fish that was moving slowly and methodically down the length of the beach.

"It was crazy. I've been going to that beach in the same spot, and just down there a lot over the years. And this is the first that I've ever seen anything like that. This was wild, just out of a movie."

He says it also seemed surreal on the beach with people "going into a frenzy" getting out of the water, but he says it doesn't appear the lifeguards took notice. Everything was happening about a half kilometre from the lifeguard station, Emberly adds.

If it is a big shark, Emberly hopes it's long gone from the area, but says the experience won't deter him from getting back into the water.