Natal Homing and the Geomagnetic Imprinting Hypothesis for Salmon and Sea Turtles

Several marine animals, including salmon and sea turtles, disperse across vast expanses of ocean before returning as adults to their natal areas to reproduce.  How animals accomplish such feats of natal homing has remained an enduring mystery.  Salmon are known to use chemical cues to identify their home rivers at the end of spawning migrations.  Such cues, however, do not extend far enough into the ocean to guide migratory movements that begin in open-sea locations hundreds or thousands of kilometers away.  How salmon navigate from the open ocean to coastal areas near the correct river mouth has never been determined.  Similarly, how sea turtles reach their nesting areas from distant sites is unknown.

     Both salmon and sea turtles detect the magnetic field of the Earth and use it as a directional cue.  In addition, sea turtles can derive positional information from two magnetic elements (inclination and intensity) that vary predictably across the globe, enabling turtles to exploit the Earth’s field as a kind of navigational map.  An interesting aspect of the Earth’s field is that it varies predominantly in a north-south direction, as do most continental coastlines.  As a consequence, different locations along a continental coast typically have unique ‘magnetic signatures’ associated with them. 

     We propose that young salmon and sea turtles imprint on the magnetic field of their natal areas and use this information to return years later.  For salmon, this putative mechanism of open-sea homing is hypothesized to bring fish into the vicinity of the home river, close enough for chemical cues to guide the fish to the final spawning area.  Similarly, in turtles, geomagnetic imprinting might be supplemented by additional sensory cues that assist turtles in reaching specific nesting areas once they have arrived in the vicinity. 

     The geomagnetic imprinting hypothesis provides the first plausible explanation for how marine animals can navigate to natal areas from distant oceanic sites.  The hypothesis appears to be compatible with present and recent rates of magnetic field change (secular variation).  One intriguing implication, however, is that unusually rapid changes in the Earth’s field, as occasionally occur during geomagnetic polarity reversals, may affect ecological processes by disrupting natal homing, resulting in widespread colonization events and changes in population structure.