Marine animal magnetism is homing salmons' secret

Roaming marine animals like salmon and sea turtles use more than their noses to find their way back to their birthplace, scientists have suggested.

Researchers from the University of North Carolina have suggested they use variations in the Earth's magnetic field to find their way home to reproduce.

The average life of a salmon or sea turtle is impressive – after being born they often cross entire oceans and are absent for over a decade before returning home to reproduce.

Now a paper published in the Proceedings of the National Academy of Sciences journal suggests how this is possible.

'Natal homing' involves young salmon and sea turtles detecting the Earth's magnetic field and using it to sense direction during their first migration away. This helps them return to the general area of their upbringing, after which they use more specific detection methods – like smelling – to find their way home.

"What we are proposing is that natal homing can be explained in terms of animals learning the unique magnetic signature of their home area early in life and then retaining that information," study author Kenneth Lohmann said.

For now scientists are forced to fall back on hypothesising about how the marine animals evolved their remarkable homing instincts.

Why they do it in the first place is also unknown, but Dr Lohmann believes the instinct to return home is a practical one based on the highly specific environmental conditions required to reproduce.

"Assessing the suitability of an unfamiliar area can be difficult and risky," he said.

"In effect, these animals seem to have hit on a strategy that if a natal site was good enough for them, then it will be good enough for their offspring."

Sea turtles may be overstretching themselves, however. Only one out of about 4,000 baby sea turtles survives to adulthood and returns to its natal site to breed.