Salmon form a mental map of the Earth's magnetic field which they use to relocate the river in which they were born.
Armed with Canadian fisheries data from the last 56 years, as well as geomagnetic information, Oregon State University scientist Nathan Putman and his colleagues compared the routes taken by sockeye salmon returning to spawn in the Fraser River in British Columbia.
En-route, the fish face an usual geographical hurdle in the form of Vancouver Island, a 300 by 50 mile long landmass that sits across the mouth of the river.
Returning salmon therefore need to swim around the island, either to the north or south, to enter the river mouth. And because the island is so large, natural annual changes in the Earth's magnetic field (known as secular variations) lead to differences between the magnetic patterns in the north and south approaches.
The scientists reasoned that, if the salmon are imprinting in their memories the magnetic signature of the river mouth through which they first entered the sea, then they should preferentially take whichever of the two approaches - north or south - bears the closest magnetic resemblance (i.e. has varied the least) to the signature in their memory.
Plotting graphs of the difference in magnetic intensity between the river mouth and the different routes and the proportion of fish swimming that way showed a very strong correlation that was highly statistically significant.
According to the researchers, their analyses "suggest that Earth's magnetic field plays an important role in oceanic movements of sockeye salmon and that variability in their routes is influenced by geomagnetic secular variation."
The team conclude their paper in Current Biology by suggesting that the same mechanisms may apply to other natally-homing species like turtles, marine mammals and migratory birds.