Sockeye salmon migrates magnetically

The sockeye salmon uses a magnetic map to return to its spawning ground after navigating across thousands of miles of open ocean, according to 56 years of data analysed at Oregon State University.

The study documented the return of the sockeye salmon to the Fraser River in British Columbia – and the route they chose around Vancouver Island showed a correlation with changes in the geomagnetic field.

“What we think happens is that when salmon leave the river system as juveniles and enter the ocean, they imprint the magnetic field – logging it in as a waypoint,” said Nathan Putman, a post-doctoral researcher at Oregon State University and lead author of the study. “It serves as a proxy for geographic location when they return as adults. It gets them close to their river system and then other, finer cues may take over.”

Our planet has a predictable and consistent geomagnetic field that gets weaker as you move from the poles to the equator. The scientists theorised that salmon born in Oregon that have spent two to four years in the northern Pacific Ocean would return as adults, journeying southward off the coast until they reached a magnetic field intensity similar to that of their youth.

“That should get them to within 50 to 100 kilometres of their own river system and then olfactory cues or some other sense kicks on,” said Putman.

Vancouver Island provides a natural laboratory for salmon studies. Salmon returning to Fraser River have to take a detour around the massive island to reach the river’s mouth and must choose a southern or northern route. In the study, the researchers found that the “drift” of the geomagnetic field correlated with which route the salmon chose.

When the normal intensity level for the Fraser River shifted to the north, the salmon were more likely to choose a northern route for their return. Likewise, when the field shifted to slightly south, they chose a southern route.

Putman was keen to point out that geomagnetic sensing may not be the only tool the fish use to find their way: “Salmon have to get it right because they only have one chance to make it back to their home river, so it makes sense that they may have more than one way to get there. The magnetic field is amazingly consistent, so that is a strategy that can withstand the test of time. But they may also use the sun as a compass, track waves breaking on the beach through infrasound, and use smell.”

Paper: Evidence for Geomagnetic Imprinting as a Homing Mechanism in Pacific Salmon http://www.cell.com/current-biology/retrieve/pii/S0960982213000031