An extremely rare fish, commonly known as a doomsday fish, has been spotted washed up on a beach in western Tasmania.

The two-metre long oarfish was spotted on Ocean Beach in Strahan by local woman Sybil Robertson on Monday.

Robertson told the ABC she spotted the elusive deep-sea fish while walking her dog.

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“When I first saw it, it just looked like a long silver streak on the beach,” she told the publication.

Oarfish are the longest bony fish species in the world and have a silvery, iridescent colouring.

“It’s extremely rare to see them washing up anywhere on the coast, anywhere around the world,” Tasmania University biologist, Associate Professor Neville Barrett, told www.20304050.best.

Barrett said oarfish are a mesopelagic species, meaning they live out in the open ocean at a depth of between 200m and 500m deep — although some could even live as far as 1000m deep.

“There’s not much happening out there in that part of the world,” he said.

“We don’t fish it, we don’t go diving out there ... we rarely catch them, and we rarely see them.

“Typically, you’ve got to go 30 or 40 kilometres, or more, offshore to get to that kind of depth that they live in.”

Oarfish have ridged bodies, and swim using a fin on the top of their head and, unlike most other fish, do not have a swim bladder.

This means when they die, they usually sink to the ocean floor.

“They’re so far offshore where they live, they very rarely make it to shore,” Barrett said.

“(When they die) they’ve sunk to the bottom well and truly before then, usually.”

The last recorded sighting of an oarfish in Australia was over 10 years ago near Hobart.

One of the many reasons oarfish perplex scientists is they are almost impossible to age.

“This is a big conundrum with this particular type of species,” Barrett said.

“No one’s yet worked out a way to actually age them.”

Oarfish can grow to over 10m in length, and scientists think they could live to almost 100 years.

“Deeper species tend to grow slower. So just on basic principles, you’d imagine the really big ones can probably be 80 or more years old and the smaller ones at a couple of meters, well, they could be, you know, 20 or 30-years-old,” he said.

“We just don’t know.”

Why are they called the Doomsday fish?

Oarfish are also labelled doomsday fish due in part to its presence signalling natural disasters.

“Historically, they looked a bit like serpents because they’re so long, so they look a bit like a mythical creature if you like,” Barrett said.

“They’re often been associated with things like tsunamis and earthquakes in the Pacific, particularly around Japan.”

It was reported that 20 oarfish washed up in Japan in the year leading up to the 2011 Tohoku earthquake and tsunami that killed over 20,000 people.

“In those places often the deep water is really close to shore, so species like this can be literally living one kilometre offshore, unlike here, where you’ve got to go out about 40 or 100 kilometres,” Barrett said.

“So, they wash up there more often.

“When you get a tsunami or an earthquake, that can be quite an intense shock in the water, and it can cause some of these deeper water species to be pushed to the surface.

“However, when people have actually gone and looked into it, there doesn’t seem to be any relationship whatsoever.”