Startled beachgoers vacated the ocean on Saturday after a four-foot marine animal was spotted swimming along the South Shore — but a sharp-eyed Hamilton resident proved correct when he recognised the giant as a harmless sturgeon.

Peter Mason was at a popular South Shore beach celebrating his son’s 9th birthday when he saw a dark shadow in the water. At first, he thought it was a rock and went to warn the children.

Mr Mason said: “The kids were playing in the water and I went over to make sure it wasn’t sharp — and then it moved.

“It disappeared and came back a little later, swimming leisurely up and down the bay.”

Mr Mason, who has lived on the island for a couple of years, said he had never encountered anything like it before in Bermuda’s waters.

A government official said the footage appeared to show an Atlantic sturgeon, Acipenser oxyrinchus, and that, despite its formidable appearance, the fish is not believed to pose any risk to humans.

He added: “However, members of the public should leave it alone if they encounter it for the protection of the animal.

“Based on historical records from the 1800s, Atlantic sturgeon were known to occasionally turn up in Bermuda waters.

“They are not normally open-ocean fish, so past sightings most likely involved individuals being swept off course during their migrations in North American coastal waters, possibly by extreme weather or unusual ocean conditions.”

He said Atlantic sturgeon were fished to the brink of extinction for their meat and caviar, and are now listed as threatened or endangered across much of their native range.

He added: “Conservation efforts have been ongoing, and some populations appear to be showing signs of recovery. A sighting in Bermuda waters may be an encouraging indication that numbers are improving.”

Mr Mason said: “I was unsure what it was until I caught up with it. I think it was a sturgeon. I couldn’t find anything else that vaguely resembled it. The first half an hour it was pretty stationary, thought it was a rock.

“Once I had seen it from above, I knew it wasn’t a shark. I get in the water a lot over the summer and I’ve never seen anything that big, and I’ve never seen a sturgeon here.

“It didn’t seem bothered by people; it was swimming underneath the kids.

“A few people came out of the water, thinking it could have been a tiger shark. It took a while for anyone to get close enough to it to identify it.

“I spent a long time thinking it should be something else. I couldn’t find anything else that looked like it. They swim around the Atlantic but it’s unusual to see them this far off the coast.

“It was my son’s birthday — he was happy to see it and keen to go out in the water again to see if we can spot more fish.”

According to National Oceanic and Atmospheric Administration Fisheries, Atlantic sturgeon live in rivers and coastal waters from Canada to Florida.

It says: “Hatched in the freshwater of rivers, Atlantic sturgeon transition to the marine environment, and return to their birthplace to spawn, or lay eggs, when they reach adulthood.”