For generations scientists have puzzled at the mystery of how salmon can locate their breeding grounds from thousands of kilometres away.

For generations scientists have puzzled at the mystery of how salmon can locate their breeding grounds from thousands of kilometres away.

But the answer may lie in the species' hidden ability to tune in to the Earth's magnetic field, according to new research.

If the theory is true, it could enable the reintroduction of salmon to rivers where they have been fished out. Previous efforts have met with difficulty, because salmon are notoriously picky about where they have offspring and will usually only reproduce in the place where they were born.

Professor Kenneth Lohmann, a marine biologist at the University of North Carolina, suggested that salmon may be able to use magnetic fields to record a unique imprint of their home breeding ground.

When they reach adulthood they could then access this imprinted memory and find their way home from across the oceans through a process known as "natal homing".

Professor Lohmann said the idea, which may also apply to some species of sea turtle, could have great implications for conservationists and fisherman.

"If the theory turns out to be correct, then it might be possible in future to steer salmon and sea turtles to areas where we'd like them reproduce," he said.

"There are many rivers where they used to spawn, but have since been fished out, and it can be challenging to reintroduce them. In theory, this might provide a way to make them return."

The theory could also provide a boost to conservation efforts by enabling environmentalists to identify dangerous areas in migration routes, with lengthy journeys often undertaken as salmon eschew suitable breeding grounds in the search for the specific areas where they were born.

Salmon famously fight their way upriver to spawning grounds, apparently on the principle that if a natal site is good enough for them then it will also be appropriate for their offspring.

Professor Lohmann's theory is revealed in the journal Proceedings of the National Academy of Sciences.