Inherited Magnetic Ocean Navigation in Juvenile Pacific Salmon (Oncorhynchus species)

We tested the hypothesis that Pacific salmon are guided by the field intensity and inclination angle of the earth’s magnetic field. Our analysis of long-term records of returning adult sockeye salmon (O. nerka) to the Fraser River showed that secular variations in the geomagnetic field account for a significant portion of the variation of their homing pattern. We rear and test the fish under both local, undisturbed geomagnetic conditions and under specified magnetic intensity and inclination conditions representing different geographic locations. Juvenile Chinook salmon and steelhead exposed to magnetic fields characterizing the northern and southern latitudinal extremes of their oceanic range orient in opposite directions, in each case towards their marine feeding grounds. Fish use both the magnetic intensity and inclination angle to assess their location; orientation is random when magnetic coordinates are contradictory (i.e., northern intensity paired with southern inclination and vice versa). Fish tested had no migratory experience, suggesting that their magnetic map sense is inherited. Steelhead reared or tested in a spatially non-uniform and widely varying magnetic field failed to distinguish between the same experimental fields and were not significantly oriented.