Even young hatchery salmon with no prior experience of the world outside will orient themselves according to the Earth's magnetic field in the direction of the marine feeding grounds frequented by their ancestors. These findings, reported in Current Biology, a Cell Press publication, on February 6th, suggest that Chinook salmon inherit a kind of built-in GPS that always points them home.
"In essence, the fish act as though they have a map based on the magnetic field," says Nathan Putman of Oregon State University. "When the fish experience a magnetic field that is north or south of their typical ocean range, they change their swimming direction to go back."
Putman and his colleagues previously showed that migrating sea turtles rely on magnetic fields to detect both their north-south and east-west positions (see http://www.eurekalert.org/pub_releases/2011-02/cp-mst021611.php). Prior to that discovery, there had been considerable doubt that magnetic fields could explain how migratory animals guide themselves in the east-west direction. The researchers later reported evidence that sockeye salmon have a magnetic sense of direction, too (see http://www.eurekalert.org/pub_releases/2013-02/cp-mmg013013.php).
The new work shows that in salmon, as in sea turtles, the key is reliance not on a single feature of the magnetic field, but on a combination of two: the magnetic intensity and inclination angle. By picking up on subtle differences in both characteristics, salmon can discern their position and guide themselves accordingly. Remarkably, those navigational skills seem to require no prior life experience.
"Our findings are certainly suggestive that before the fish even hit the ocean, they have information about how they should orient to reach, or remain in, favorable locations," Putman says.
It's not entirely clear whether the fish rely on this mechanism to stay within a particular range or to find their way back once they've traveled far, he adds. Either way, the work implies that salmon are particularly sensitive to magnetic fields, something to keep in mind when rearing hatchery fish in unnatural environments built of concrete and iron rebar.
"The changes we made in our experiments were not even strong enough to deflect a compass needle," Putman says. "The fields that humans potentially expose fish to are much stronger and could easily overwhelm their ability to perceive Earth's magnetic field at a given location."
Current Biology, Putman et al.: "An inherited magnetic map guides ocean navigation in juvenile Pacific salmon."