Sockeye salmon migrate 4,000 miles in the Pacific Ocean each year to return to their home river to spawn. It’s known that they rely on chemical cues to guide them short distances in their freshwater spawning rivers, but researchers, fisherman, and interested citizens alike have long pondered how salmon know how to make this journey. People have been so curious, you could say sockeye salmon have a certain captivating animal magnetism about them.
New research from the University of Oregon reveals you’d be right. According to the study results, which appeared in Current Biology this past Thursday, these fish—dubbed “diverse main migrants” by the head researcher—navigate with an internal magnetic map based on earth’s magnetic field.
The study examined 56 years of fisheries data about a subpopulation of sockeye salmon from British Columbia’s Fraser River—a group with options for its migration route. Vancouver Island blocks entry into the Fraser River, forcing the salmon to select either a northward entrance via the Queen Charlotte Strait or a southward entrance via the Strait of Juan de Fuca.
The earth’s magnetic field is strongest at the poles and weakens toward the equator. Each hemisphere has its own field with its own angle, and these inclinations gradually shift over time. This is called geomagnetic field drift or secular variation. The data revealed that the swimmers used the river opening that most closely resembled the angle of the opening the fish had originally experienced, proving that these salmon rely on the earth’s magnetic field to navigate open water.
There were some outliers, however: Even if one river experienced minimal magnetic drift, some salmon used the other. For example, in 2008—a particularly cold year—more salmon used the southern route even though the northern route had the lowest magnetic drift out of all of the years studied. Why? Cold temperatures encouraged the fish to begin their migration in warmer waters, prompting them to stick to the southern entryway. Therefore, the researchers discovered, the combo of magnetic inclination and sea surface temperature proved to have the biggest effect on salmon migration.
Other species such as sea turtles, seals, and other anadromous fish exhibit this natal homing behavior, but it is not well understood for any species. It is now known that they do it, but how they developed the connection with magnetism remains unknown.
Regardless of how they do it, magnetic mapping has its advantages. “Doing so likely enhances the benefits of their anadromous life history,” notes the study. “Efficiently navigating from oceanic foraging grounds to the correct coastal location maximizes time available for feeding, minimizes loss of energy stores in transit, and ensures that the fish reach spawning sites at the appropriate time.”
This hypothesis is now answered, meaning that future studies of natal homing and magnetism in migration for salmon, marine mammals, and migratory birds can potentially help guide their conservation. With an internal compass powerful enough to tap into the earth’s magnetic field, it’s no wonder humans are so drawn to sockeyes.