Every year, millions of chinook salmon clog the rivers and creeks of British Columbia in a migration that has puzzled scientists for generations.
The fish travel hundreds of thousands of kilometres through the ocean and then fight the current swimming upstream to spawn, and die, in the fresh water where they hatched.
The juvenile salmon then make the same journey in reverse to find the ocean foraging grounds of their forebears.
"Given that the animals have never been there before, how do they find their way?" asks Nathan Putman, a biologist at the Department of Fisheries and Wildlife at Oregon State University and lead author of a new study that may have the answer.
It has been a widely held belief that ocean current delivers the salmon to these spots, but Mr. Putman and his colleagues found that does not appear to be the case.
"While this can certainly help in some situations, more recent studies suggest that it isn't entirely reliable," he said in an e-mail.
A study by the same researchers last year on B.C. salmon in the Fraser River suggested adults used the magnetic field to find home.
To test that theory further, Mr. Putman and his colleagues built a wooden frame that they wrapped with copper wires running horizontally and vertically, through which they ran electrical current to simulate Earth's magnetic field.
They placed juvenile fish from the Willamette River Basin in Oregon in large buckets within the frame. Photographs from a camera overhead showed the chinook detected subtle changes in the magnetic field that they used like an internal GPS to change direction.
The combination of the magnetic intensity, which grows stronger toward the poles, and the inclination angle of the magnetic field create a sort of grid that the salmon instinctively use as an inherited magnetic map, Mr. Putman said.
"The idea that animals can know where they are on the globe by taking readings of the subtle features of Earth's magnetic field does, admittedly, seem fantastic – and there have been plenty of skeptics," he said.
But the evidence is convincing.
Salmon need to know where they are, where they are going and how to get there, he said. They also need to be able to correct for errors when they arise.
The magnetic grid gives them that.
"Without this ability, fish would be forced to wander at random, which could be very costly – wasting energy, or worse, going to unfavourable places that lead to starvation, high predation risk, inappropriate temperatures, etc.," Mr. Putman said.
"Really, the ability for salmon to navigate is what holds their whole life cycle together and makes it possible."
A previous study by the researchers found that migrating sea turtles do the same.
The study suggests salmon are extremely sensitive to changes in the magnetic field.
"What this means is that some of the strong magnetic fields humans expose fish to – in hatcheries from iron pipes, hydroelectric dams, wave-energy devices and coded-wire tags, to name a few – are certainly perceptible to these fish," Mr. Putman said.
Whether such distortions cause navigational problems is an area for further study.
The article is published in the latest edition of the scientific journal Current Biology.