How salmon manage to find their way back to the river of their birth is one of the great mysteries of the natural world. Now scientists believe they may have solved this mystery.

From this story in the (UK) Daily Mail:
Ever wondered how salmon navigate across thousands of miles of ocean without getting lost? After years feeding at sea, the fish swim through vast expanses of featureless water back to the rivers where they hatched. Now scientists may have finally answered a mystery that has baffled them for decades, after finding evidence suggesting salmon use the Earth’s magnetic field to guide them back to their spawning grounds. Researchers believe that when the fish first enter the sea, they memorise the location’s magnetic field and use it as a home address. The magnetic field varies across the globe, allowing animals to use it as a ‘map’ and determine their location.

According to the story, salmon imprint on and basically remember the magnetic fields that exist when they first enter the sea. When they return to spawn, they seek out that same magnetic signature. Scientists tested the theory by looking at historical data for the Fraser River in British Columbia. Vancouver Island sits at the mouth of the Fraser river, so returning salmon must either swim to the north or the south of the island to enter the river.

Since the earth’s magnetic field is constantly undergoing minor shifts and variations, scientists correlated the preferred route of the salmon with the magnetic field data from the same year the fish went to sea.

What they found was that salmon tended to choose their route around Vancouver Island based on the distinct magnetic signature of the river when they first went to sea two years earlier.

Prety cool stuff. But I always just assumed the salmon used GPS like the rest of us. Doesn’t everyone?