Salmon are known for their homing skills, returning annually to the streams they hatched in to spawn.
What has been something of a mystery, at least until recently, is how they did so. Their journey of thousands of miles across open ocean is undertaken years after they’ve left their rivers of origin.
Scientists at Oregon State University (OSU) studied 56 years worth of fisheries data on the sockeye salmon to British Columbia’s Fraser River. They found that the fish chose routes around Vancouver Island that varied based on fluctuations in the geomagnetic field.
In other words, Mother Earth is their GPS.
“To find their way back home across thousands of kilometers of ocean, salmon imprint on [i.e. learn and remember] the magnetic field that exists where they first enter the sea as juveniles,” said the study’s lead author, Nathan Putman, a post-doctoral researcher at Oregon State University, in a statement. “Upon reaching maturity, they seek the coastal location with the same magnetic field.”
Basically, the scientists said, the Earth’s “predictable, consistent geomagnetic field” gets almost imperceptibly weaker the closer one gets to the Equator, subject to geomagnetic field drift, as Smithsonian Magazine said. What set the Fraser River apart in terms of approach was the fact that there are two approaches, both around Vancouver Island, which blocks the river’s entrance. The fish would have had to pick one or the other, and they consistently chose the route that most closely matched the magnetic strength and signature of the Fraser River at the time they left, the researchers said.
The theory is that salmon imprint the magnetic field as a waypoint when they leave their home river system, Putman said. It gets them into the general vicinity of the same river system when they return, “and then other, finer cues may take over.”
Their results were published in the journal Current Biology on February 7.