Veteran fisherman James Nickerson was mackerel fishing at the Cape Sable Island causeway on Sunday morning when he witnessed a primordial ocean spectacle: a large shark exploding out of the water, attacking a seal.

"The whole shark was out of the water — the whole body. It was huge. It was a big shark," Nickerson said. "It was as big as the boat I was in, put it that way."

Nickerson grabbed a camera and captured the aftermath as the shark circled in foggy weather. He posted the video on Facebook, where it had been seen more than 45,000 times by Monday.

Experts weigh in

Warren Joyce, a shark technician for Canada's Department of Fisheries and Oceans, watched Nickerson's video.

"From the video, it's hard to make a clear identification. My two suspects are a large mako or white shark. They are both common in these waters this time of the year and they both take seals," Joyce said.

Joyce said there are about three or four white shark sightings each year off Nova Scotia.

If the attack witnessed by James Nickerson was a white shark, he said it's the first sighting in 2017.

"They are here almost all year round. They are still rare. We don't see or hear of them that often," Joyce said.

Manuel Dureuil and Chris Harvey-Clark, researchers at Dalhousie University, also have opinions about Nickerson's video.

They agree a definitive identification is not possible, but both said in emails to CBC News the shark is most likely a white shark.

Harvey-Clark has also posted a video of a juvenile grey seal at Sambro Island that had survived an apparent encounter with a shark.

The video shows large bite marks on the seal's side.

A tagged shark?

In August, two white sharks tagged as part of a global shark tracking network were tracked off Nova Scotia.

A signal from Hilton, a male, was recorded in Mahone Bay on Aug 19. Savannah, a female shark, was recorded Aug. 14 on the Eastern Shore.

George great white shark

George, a three-metre great white shark, was detected earlier this month in the Bay of Fundy off Saint John, N.B. (Ocearch.org)

Joyce said the Cape Sable Island encounter could have been George, a tagged white shark last recorded off Saint John, N.B., in the Bay of Fundy on Aug 9.

"It's possible that [George] could have come down, but I suspect it's one that hasn't been tagged," said Joyce. "That is the majority."

White sharks are a protected species in Canada. It is illegal to harm or harass them.

Joyce said people should avoid seal colonies.