'We thought it was a shark': Mysterious giant sunfish spotted off SA's west coast

Sunfish are found all over the world and it is common to see them swimming off the coast, or even washed up on shore.

But despite that, the fascinating marine creatures remain a mystery to scientists.

Whale watcher Rod Keogh saw one recently at Fowlers Bay, on South Australia's west coast.

The bay is a popular whale watching site, about 900 kilometres west of Adelaide.

"We were sitting out in the water the other day and what we thought might have been a shark fin turned out to be a six-foot (182 centimetres) sunfish swimming around," he said.

"This was one of the larger sunfish I've ever seen and it's the third time I've seen a sunfish in Fowlers Bay in 12 years.

"Just after we saw that one we started heading out and we saw another one."

Big, unknown fish

Ralph Foster, fish collection manager at the SA Museum, says sunfish are common in SA waters.

"They don't stick around but they pass by regularly," he said.

"I think they're chasing the jellyfish swarms that build up in our coastal waters."

But despite being a common sight, Mr Foster says the animal remains a mystery to marine biologists.

A new species of the animal was only discovered in 2017.

"We've got so much to learn about these things, and people are only now just starting to get to do work on them," he said.

"Putting satellite tracking on them to see how far they travel and the depths they dive."

Mr Foster said how the sunfish moved throughout the world's oceans was another part of the mystery surrounding them.

"They're a big unknown," he said.

Boat breakers

Mr Foster said that even though sunfish are not dangerous in the same way sharks are they do still pose a threat.

"They've broken boats," he said.

"A few years ago one of the Sydney to Hobart yachts hit one and sunk because of it.

"People see them on their sides on the surface of the sea and what they're doing is warming their bodies.

"Once they've warmed up sufficiently they dive down to 200–300 metres to feed on jellyfish down on the cold depths.

"They'll feed on them while they can and when they've gotten too cold and they'll go back to surface and repeat the process."