Sharks navigate using magnetic field

Sharks use the earth's magnetic field as a sort of natural GPS to navigate journeys that take them great distances across the world's oceans, scientists have found.

Researchers said marine laboratory experiments confirm long-held speculation that sharks use magnetic fields as aids to navigation - behaviour observed in other marine animals such as sea turtles.

Their study, published this month in the journal Current Biology, also sheds light on why sharks are able to traverse seas and find their way back to feed, breed and give birth, said marine policy specialist Bryan Keller, one of the study authors.

"We know that sharks can respond to magnetic fields," Keller said.

"We didn't know that they detected it to use as an aid in navigation ... You have sharks that can travel 20,000 kilometres and end up in the same spot."

The question of how sharks perform long-distance migrations has intrigued researchers for years.

The sharks undertake their journeys in the open ocean, where they encounter few physical features such as corals that could serve as landmarks.

Looking for answers, scientists based at Florida State University decided to study bonnethead sharks - a kind of hammerhead that lives on both American coasts and returns to the same estuaries every year.

Researchers exposed 20 bonnetheads to magnetic conditions that simulated locations hundreds of kilometres away from where they were caught off Florida.

The scientists found the sharks began to swim north when the magnetic cues made them think they were south of where they should be.

Keller said the study could help inform management of shark species, which are in decline.

A study this year found worldwide abundance of oceanic sharks and rays dropped more than 70 per cent between 1970 and 2018.

Researchers say the bonnethead's reliance on earth's magnetic field is probably shared by other species of sharks, such as great whites, that make cross-ocean journeys.