Incredible picture captured off Aussie jetty stuns beachgoers

An Australian beachgoer was stunned to spot a swarm of fish seemingly work together to create a large spinning mass in the ocean, sharing the majestic sight and wondering what was going on.

Despite thinking he'd witnessed something unusual, it is in fact a common behaviour performed by fish for a variety of reasons, experts say.

Last week a man caught sight of a large school of fish, believed to be mullet, swimming side by side to create a rotating ball in the water off a jetty at Fremantle in Western Australia, just near the Traffic Bridge. He snapped a picture and also recorded footage of the swirling mass, with the fish all moving in a clockwise direction.

After sharing the spectacle online another man said he saw a similar thing at Cockburn Sound — an inlet of the Indian Ocean on the coast at Fremantle. Professor Ian Tibbetts from the University of Queensland told Yahoo News he also saw a swarm of fish doing this at the end of last week near North Stradbroke Island on the Queensland coast.

Why do fish behave this way?

There are several reasons why fish behave this way and its likely the mullet were migrating to spawn, Tibbetts told Yahoo. With the fish moving to different waters to mate they move together for "protection" against predators with a "strength in numbers" approach.

• Local man finds 30,000 tropical fish dumped on beach from Great Barrier Reef

• Anglers banned from catching iconic Aussie fish after numbers plummet

• Aussie river catastrophe: Millions of dead fish as far as the eye can see

Professor Culum Brown from Macquarie University told Yahoo News it wasn't "unusual in Western Australia for fish to spawn in near shore areas" and this is what the beachgoer would have seen.

"The spawning season is quite long but end of summer to early winter is about the right time. In this case its called the mullet run," he said.

He explained what the beachgoer saw is called a 'bait ball' and typically they are driven to the surface from threats below.

Do you have a story tip? Email: newsroomau@yahoonews.com.

You can also follow us on Facebook, Instagram, TikTok, Twitter and YouTube.