It's a question that has fascinated scientists for decades: When sea turtles and salmon decides to give up the freedom of the open ocean and head back to their birthplaces to breed, how do they find their way back?
Some species of sea turtle migrate thousands of miles across entire oceans back to their birthplaces after leaving more than 10 years earlier. And after hatching in rivers, salmon travel hundreds of miles out to sea before returning home to spawn years later [Press Association].
Now one researcher thinks he has the answer. Marine biologist Kenneth Lohmann believes that these marine animals can detect the distinctive magnetic fields of different spots and use them to navigate.
"What we're proposing is the sea turtles and salmon, when they begin life, basically learn or imprint on the magnetic field that marks their home area," he said. "They retain this information. And years later, when it is time for them to return, they are able to exploit this information in navigating back to their home area" [National Geographic News].
Lohman says this doesn't contradict the existing theory that when salmon reach coastal waters, chemical scents guide them upriver to the particular stream where they were born; those olfactory cues probably have a limited range, he says, and couldn't extend thousands of miles into the ocean to guide the salmon all the way home. For his study, which will be published in Proceedings of the National Academy of Sciences, Lohmann tested the response to magnetic fields by
placing young loggerhead turtles in pools of water surrounded by magnetic coils. When researchers altered the magnetic fields in the water, the turtles changed the direction they were swimming [McClatchy Newspapers].
But testing the hypothesis that turtles and fish use this sense when migrating would be extremely difficult, as it would require tracking and experimenting on the animals in the open ocean. However, the theory has Lohmann thinking about possible applications for conservation programs for salmon in Western rivers; devastated populations are hard to restore, he says, because the fish always return to their birthplaces rather than colonizing other favorable breeding grounds.
If Lohmann is right about salmon navigation, he wants to adjust it, programming newborn fish raised in captivity to colonize now-deserted waters. "We would set up a large magnetic-coil system that lets us dial in the precise magnetic field that we want," he said. "Then we could take fish from a location where they still survive, raise them in the magnetic field of the tanks, and see if they go to the new river" [Wired News].
Image: flickr / JB Mellquist