Salmon use Earth's magnetic field to navigate

Built-in GPS! Ocean salmon can navigate across thousands of miles of water by sensing the Earth's magnetic field, a new study has confirmed.
Previous research had presented evidence of a correlation between the migration patterns of ocean salmon and the Earth's magnetic field, suggesting it may help explain how the fish can navigate across thousands of miles of water to find their river of origin.
Scientists have now confirmed the connection between salmon and the magnetic field following a series of experiments at the Oregon Hatchery Research Center in the Alsea River basin.
Researchers exposed hundreds of juvenile Chinook salmon to different magnetic fields that exist at the latitudinal extremes of their oceanic range.

Fish responded to these "simulated magnetic displacements" by swimming in the direction that would bring that toward the centre of their marine feeding grounds.
"What is particularly exciting about these experiments is that the fish we tested had never left the hatchery and thus we know that their responses were not learned or based on experience, but rather they were inherited," said Nathan Putman, a researcher in Oregon State University's Department of Fisheries and Wildlife and lead author on the study.

To test the hypothesis, the researchers constructed a large platform with copper wires running horizontally and vertically around the perimeter.

By running electrical current through the wires, the scientists could create a magnetic field and control both the intensity and inclination angle of the field.
They then placed 2-inch juvenile salmon called "parr" in 5-gallon buckets and, after an acclimation period, monitored and photographed the direction in which they were swimming.
Fish presented with a magnetic field characteristic of the northern limits of the oceanic range of Chinook salmon were more likely to swim in a southerly direction, while fish encountering a far southern field tended to swim north.
The fish possess a "map sense" determining where they are and which way to swim based on the magnetic fields they encounter.

"What is really surprising is that these fish were only exposed to the magnetic field we created for about eight minutes. And the field was not even strong enough to deflect a compass needle," Putman said.
Putman said that salmon must be particularly sensitive because the Earth's magnetic field is relatively weak. Because of that, it may not take much to interfere with their navigational abilities.
The magnetic field is likely not the only tool salmon use to navigate, however, Putman noted.
"They likely have a whole suite of navigational aids that help them get where they are going, perhaps including orientation to the sun, sense of smell and others," Putman said.

The study will be published in the journal Current Biology.