New theory on how Salmon find their way back to their Scottish birthplace

ONE of nature's greatest mysteries may have been solved after scientists revealed a new theory on how salmon find their way home.

Every year, 20 million of them leave Scottish rivers and travel thousands of miles to Norway and Greenland to feed. Remarkably, they then return to Scotland, often to within 100 metres of where they were hatched, in a process that can take more than two years.

How salmon complete such voyages across sea and ocean without getting lost has baffled scientists for generations. But a new theory proposes that the fish use the earth's magnetic field to locate their origins in Scottish rivers.

Scientists believe that, in a process called "natal honing", salmon imprint the magnetic signature of their home once reaching adulthood.

Kenneth Lohmann, professor of biology at the University of North Carolina in the United States, said: "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."

The Earth's magnetic field varies across the globe – each oceanic region has a different magnetic signature. Researchers believe that by remembering the unique "magnetic address" of their birthplace, fish may be able to distinguish that location from all others.

Salmon and sea turtles often bypass suitable breeding grounds on their vast journeys in favour of the places they were born. Scientists believe the fish do this due to previous breeding success at a particular site.

Prof Lohmann said: "For animals that require highly specific environmental conditions to reproduce, assessing the suitability of an unfamiliar area can be difficult and risky.

"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."

He said it might also be possible to magnetic imprinting to help re- establish salmon in rivers where the original population had been wiped out.

Scientists agree the Earth's magnetic field changes over time and probably helps animals arrive only in the general area of their birthplace. Then, once an animal is close to their target, other senses, such as vision or smell, may be used. Salmon are known to use their sense of smell to locate spawning grounds once they are close.

Andrew Wallace, the managing director of the Association of Salmon Fishery Boards and the Rivers and Fisheries Trusts of Scotland, said: "That salmon have some sort of magnetic map is certainly very plausible and would explain how they can travel thousands of miles and then return to the same tributary.

"There have been theories that birds and mammals use the stars to navigate, but obviously fish can't do that.

"Salmon have an extremely strong sense of smell and if they can recognise their magnetic signature, then it would explain how they can return to the right area. From there they can use their sense of smell to find the correct tributary."

The salmon industry brings in around £95 million a year in Scotland.

James Leeming, from FishPal, formerly FishScotland, said: "This is certainly an interesting idea and it sounds like a step in the right direction to discover the reason salmon can return.

"But the important thing is that the fish do return," said Mr Leeming. "The Tweed is one of the best salmon rivers in the world and brings in large amounts of money for the local economy."