According to a newly published study, performed by researchers from Oregon State University, young hatchery salmon can use the earth’s magnetic field to navigate towards marine feeding grounds. Published in the Feb. 6 issue of the journal Current Biology, the findings suggest that Chinook salmon use something akin to a “built-in GPS” that provides them with a sense of direction. The conclusions from the study shed new light over the longstanding mystery of how juvenile marine creatures successfully navigate to feeding grounds that are hundreds or thousands of kilometers from their place of birth, despite having no previous migratory experience.
Migration of the Chinook Salmon
Chinook salmon (Oncorhynchus tshawytscha), also known as king salmon, are anadromous fish; they migrate from the sea to fresh water to spawn. Native to rivers across western North America and the north Pacific Ocean, Chinook salmon are the largest of the Pacific salmon, measuring up to 58 inches in length and weighing an average of 10 to 15 pounds. The blue-green fish have long bodies and conical heads with a silver belly and sides.
The Chinook salmon typically spend an average of around three years in the ocean, before migrating back to their home rivers for the purposes of spawning. Their breeding season is usually between September and December, with the female salmon occupying spawning nests in deeper waters than most other species of salmon. After hatching, the young salmon stay in fresh water for around 12 to 18 months and then move out to sea, ultimately, spending several years foraging in the ocean. Afterwards, however, the young salmon make a single migratory trip back to the general whereabouts of their original, freshwater natal sites.
Nathan Putman, based at Oregon State University, discussed his team’s intriguing findings during a recent press release. Putman explains that the salmon, essentially, behave as though they possess a “map,” constructed from the magnetic field lines of the earth:
“When the fish experience a magnetic field that is north or south of their typical ocean range, they change their swimming direction to go back.”
Migration Dependent Upon Magnetic Field Strength and Inclination Angle
The group experimentally tested Chinook salmon from the Willamette River Basin, situated in Oregon. They searched for evidence of orientation preferences – in hundreds of juvenile Chinook – that could suggest the utility of the afore-mentioned magnetic map. The studied fish, defined as “navigationally naive,” were less than a year old and had not yet embarked upon their migration to the sea.
Putman and colleagues demonstrated that the Chinook salmon used two features of the magnetic field to obtain a sense of direction, including the strength of the magnetic field and the inclination angle; the inclination angle is the angle at which field lines intersect the earth’s surface. The group suggest that long-distance migration is, perhaps, guided by “orientation responses,” where regional magnetic fields improve the navigation potential of the Chinook salmon.
The research team introduced the fish to a tank and let them acclimatize for approximately 10 minutes. Using a magnetic field system, the team subjected the test fish to different magnetic field types by adjusting the electric currents that ran through them. They were surprised to find that a significant majority of the fish orientated themselves towards the magnetic fields that are present within their oceanic feeding grounds. The fish consistently responded to alterations in the magnetic field around them. If the experimenters adjusted the field to mimic those found in the northern extreme of the salmon’s range, the fish would orientate southwards. In contrast, introducing a magnetic field similar to that observed in the southern end of the salmon’s range resulted in the fish pointing north.
Previously, Putman had already demonstrated that loggerhead sea turtles relied upon magnetic fields during transoceanic migration, before returning to the shores of North America. As with the Chinook salmon, the sea turtles were found to exploit the strength of the earth’s magnetic field, as well as the inclination angle, to navigate.
In concluding, Putman explains that the fish possess knowledge of how to orient themselves to reach specific, favorable locations, prior to even entering the ocean. Putman indicates that their tests used magnetic fields that were “… not even strong enough to deflect a compass needle.” In light of this, he cautions individuals when rearing hatchery fish and suggests that artificial environments constructed from concrete and iron rebar could adversely affect the ability of young fish to perceive the earth’s magnetic field.
By James Fenner