Captive sharks can sense magnetic fields, a finding that adds evidence that the animals use an internal compass to navigate across oceans.

Scientists have long suspected that sharks use the Earth's magnetic field, along with their keen sense of smell, to swim in straight lines for long distances.

Until now, evidence for a magnetic compass in sharks has been circumstantial. For example, scalloped hammerhead sharks have been known to gather at seamounts in the ocean with anomalous magnetic fields.

Scalloped hammerhead shark (AP photo)

Carl Meyer of the Hawaii Institute of Marine Biology and his colleagues trained captive sharks to associate food with an artificial magnetic field.

A copper coil surrounded the sharks' seven-metre-diameter tank to create the magnetic field. Scientists activated the field when food was placed on the tank floor.

When the field was activated, the conditioned sharks stopped swimming around the perimeter of the tank, sped up and gathered at the food target, the scientists reported in the Journal of the Royal Society Interface.

Once trained, the six sandbar sharks and one scalloped hammerhead shark participated in 11 videotaped trials over six weeks.

Sometimes the field was switched on at random times, without any food. The sharks still headed for the feeding site, which shows they could detect the field.

The findings offers scientists a way to determine how sharks detect magnetic fields and to measure how sensitive they are at doing so, the researchers conclude.

In November, scientists at the University of North Carolina used a similar study to show pigeons use tiny magnets in their upper beaks to navigate long distances.