Salmon use magnetic map to travel thousands of miles home

Salmon use the Earth's magnetic field to navigate across the ocean as they return to their home rivers to breed, a new study has found. When migrating, salmon typically swim up to 6437 km into the ocean and then, years later, navigate back to the upstream reaches of the rivers in which they were born to spawn their young. Scientists, the fishing community and lay people have long wondered how salmon find their way to their home rivers over such epic distances. A new study, published in the journal Current Biology suggests that salmon find their home rivers by sensing the rivers' unique magnetic signature. Researchers used data from more than 56 years of catches in salmon fisheries to identify the routes that salmon had taken from their most northerly destinations, which were probably near Alaska or the Aleutian Islands in the Pacific Ocean, to the mouth of their home river--the Fraser River in British Columbia, Canada. This data was compared to the intensity of Earth's magnetic field at pivotal locations in the salmon's migratory route. Earth has a magnetic field that weakens with proximity to the equator and distance from the poles and gradually changes on a yearly basis. Therefore, the intensity of the magnetosphere in any particular location is unique and differs slightly from year to year. Because Vancouver Island is located directly in front of the Fraser River's mouth, it blocks direct access to the river's mouth from the Pacific Ocean. However, salmon may slip behind the island and reach the river's mouth from the north via the Queen Charlotte Strait or from the south via the Juan De Fuca Strait. Results showed that the intensity of the magnetic field largely predicted which route the salmon used to detour around Vancouver Island; in any given year, the salmon were more likely to take whichever route had a magnetic signature that most closely matched that of the Fraser River years before, when the salmon initially swam from the river into the Pacific Ocean. "These results are consistent with the idea that juvenile salmon imprint on (i.E. Learn and remember) the magnetic signature of their home river, and then seek that same magnetic signature during their spawning migration," said Nathan Putman, researcher at Oregon State University. It has long been known that some animals use Earth's magnetic field to generally orient themselves and to follow a straight course. However, scientists have never before documented an animal's ability to "learn" the magnetic field rather than to simply inherit information about it or to use the magnetic field to find a specific location. This study provides the first empirical evidence of magnetic imprinting in animals.