"Magnetism" helps salmon spawn.

• Position: Reproduction - Migration of sea turtles and salmon

Marine biologists previously proposed a theory that sea turtles and salmon use the Earth's magnetic forces to find their way back to their birthplace after migrating across thousands of miles of open, featureless sea. Now, those same researchers have discovered a way to test that theory in salmon--and found it to be true. "This is the first direct evidence supporting the geomagnetic imprinting hypothesis." says Kenneth Lohmann, professor of biology at the University of North Carolina, Chapel Hill.

In 2008, Lohmann proposed that, when marine animals such as sea turtles begin life, they learn or "imprint" on the magnetic field of their birthplace and later use this information to find their way back home when it is time for them to reproduce. The hypothesis provided the first plausible explanation--a unifying theory--for how marine animals can navigate to their birth areas from distant ocean locations.

To test the hypothesis, Lohmann examined more than 50 years of fisheries data on sockeye salmon born in the Fraser River, which lies off the Canadian coast of Vancouver, British Columbia. Sockeye salmon from the Fraser River typically spend two years at sea, distributed widely throughout the Gulf of Alaska, before heading back to their birthplace to spawn.

Salmon in the open sea cannot reach the Fraser River by swimming directly...