MELBOURNE (AFP) - Fishermen off Australia who accidentally caught a whopping 6.3m basking shark have provided scientists with a rare opportunity to study the second-biggest fish on the planet.

Little is known about the species - smaller only than the whale shark - because it does not need to surface for air to survive and so is not often spotted.

The specimen has been donated to Museum Victoria, in the southern city of Melbourne, whose scientists will use its tissue samples, stomach contents and vertebrae to research its genetics, diet and life history.

The head and fins will also be used to build a full-scale exhibition model of the animal, which is rated vulnerable to overfishing, the museum said in a statement. "These rare encounters can provide many of the missing pieces of knowledge that help broader conservation and biological research," said the museum's senior curator of ichthyology, Martin Gomon.

Basking sharks are slow-moving plankton feeders which can grow up to 12m long. Unlike other sharks, their teeth are tiny - about 2mm long - and they feed by trapping tiny plankton and jellyfish in their huge mouth.

Museum Victoria said the species was better understood in the northern hemisphere, with little known about the southern population. The museum has only ever dealt with three of the animals over the past 160 years.

The shark was accidentally picked up by a fishing trawler in the Bass Strait off the Australian mainland's most southeastern point.

"These animals have been sighted live over the years in Victoria," said the museum's senior collection manager of vertebrate zoology Dianne Bray.

"As they do not need to breathe air like whales and dolphins, they are not so commonly seen at the surface. We have no idea of what their numbers may be."