Although magnetism has been known to play a role in the remarkable homing ability of salmon, a new study clarifies just how the fish use magnetic fields to travel thousands of miles to their natal rivers to spawn. Researchers at Oregon State University solved this mystery by studying 56 years of fishery data involving the millions of sockeye salmon that annually pour into British Columbia’s Fraser River. Vancouver Island sits in front of the Fraser, and the routes the salmon took around the island in different years offered clues to how the fish decipher shifting magnetic fields. When the magnetic field of the northern passageway around Vancouver Island was similar to that experienced by the fish when they left the river two years earlier, the returning salmon tended to chose the northern route; the reverse was true when there was a more southerly magnectic field. Lead researcher Nathan Putnam said this showed that juvenile salmon imprint on the magnetic signature of their home rivers and then seek their way back using that signature. The research was published in Current Biology.