A West Australian sardine fisherman has described how a rare tropical whale shark surfaced near his boat in the Southern Ocean, surprising himself and local scientists.
Whale sharks are the world's biggest fish, growing to more than 12 metres, and are normally found only in tropical waters.
However, the harmless endangered species - which feeds predominantly on plankton - has come under increasing threat from Asian shark-finners, with the Southern Ocean sighting a possible sign of behavioural change, according to scientists.
Commercial fisherman Peter Westerberg described how he was in King George Sound, about 420 kilometres southeast of Perth, in the early hours of Saturday when the blue-and-white-spotted giant bumped up against his sardine net.
"I looked over and there was a giant bulge - it was massive,'' Mr Westerberg told AAP on Tuesday.
"First thing I thought it was a (white) pointer.
"But then I saw four foot of its tail and body out of the water and it just sort of waved at me - I could have reached out and touched it.''
Once Mr Westerberg realised it was a whale shark, he knew he was witnessing a rare event and contacted local marine biologist Brad Norman as soon as he got back to shore.
Mr Norman told him it was the first time a whale shark had been spotted so far south.
"He said its behaviour was typical of whale sharks, which like to rub themselves against fishing nets to see if they can get a free feed,'' Mr Westerberg said.
"It was scratching itself - it gave me a bit of a start.
"When you wake up in the morning to go sardine fishing on the south coast of Australia, you don't expect whale sharks to be on your agenda.
"It was just a beautiful thing to see.''
Mr Norman, who works for non-profit conservation organisation ECOCEAN, said the whale shark had been spotted by a police aircraft on the same day.
He said Mr Westerberg had "clearly recognised'' the rare species.
"He loved the fact it was covered in spots,'' he said.
"Given that whale sharks are normally a warm-water species, it's amazing that this animal was recorded down there.''
Mr Norman said the spotting had followed similar first-time sightings off the coast of Dunsborough, about 280km south of Perth, in January, and off Fremantle two weeks earlier.
He said it was too early to confirm if the sightings were related to behavioural changes brought on by overfishing in tropical waters, but didn't rule it out.
"They're a threatened species and their numbers are low, but this has maybe shown that WA is a bit of a haven for their survival and conservation,'' Mr Norman said.
"However, there is the possibility these whale sharks are far more widely dispersed and distributed than we actually have records for.''
Mr Norman encouraged the public to report other whale shark sightings by going to www.whaleshark.org.