How do young, naive salmon with no migratory experience somehow voyage through vast, shifting ocean waters to wind up at specific feeding grounds that are hundreds, even thousands, of kilometers away from the rivers where they were hatched? It turns out these fish may have a magnetic 'map' in their heads that enables them to sense and navigate using the Earth's magnetic fields rather like a GPS.
The findings, published in Current Biology, "likely explain the extraordinary navigational abilities evident in many long-distance underwater migrants," the study authors wrote.
Researchers have long suspected that many animals that travel incredibly great distances to wind up at very particular spots visited by their ancestors must be able to sense magnetic fields. The ability has since been found in a variety of animals, including migratory birds and pooping dogs. Even more remarkable is that many animals seem to use it to navigate with no previous experience, like the Chinook salmon (known formally as Oncorhynchus tshawytscha).
The scientists wondered if the salmon were using Earth's magnetic field the way that sea turtles have been shown to do: like a GPS, to figure out not just which way to point, but also where exactly they are and where they need to go, like a magnetic mental map.
That ability relies on knowing two separate characteristics of the magnetic field at any given point: the inclination and the intensity. Because both of these vary depending on where you are on the globe, by combining the two you can get a pretty good sense of your position.
To figure out whether salmon are also born packing this kind of internal cartography, the research, led out of Oregon State University, tested the abilities of young Chinook salmon that were less than 1 year old and hadn't yet started their seaward journey.
They put the young fish in barrels surrounded with magnetic coils and exposed them to a simulation of the northern magnetic field, both in the strength and the inclination angle. In response, the salmon turned south-southwest. When they exposed them to a simulation of the southern magnetic field, the salmon turned north-northeast.
But how could the researchers be sure that the salmon were using both the magnetic field intensity and inclination in their mental maps? The scientists tested this by exposing them to mixed-up magnetic fields -- a northern intensity with a southern inclination, and a southern intensity with the northern inclination. Sure enough, the fish didn't point anywhere in particular, probably because these made-up "coordinates" made no sense -- they didn't fit anywhere in the salmon's mental magnetic map.
Chinook salmon have a complex life cycle: They hatch in freshwater rivers but migrate out to the ocean and spend several years foraging; their diet ranges broadly, from kelp to jellyfish. Then they buy a one-way ticket home, returning to spawn in freshwater near where they hatched, and die after the breeding season.
So having this kind of complex magnetic sense, hard-coded into the genes, may be vital for their survival, the scientists wrote -- particularly because they and other marine animals face even more challenges than terrestrial travelers. Even if winds blow birds off course, for example, they can adjust by using the ground as a reference point -- and they can land when they're tired. In the vast dark waters of the ocean crisscrossed by overwhelming currents, fish and other long-distance swimmers have no such luxuries.
That's probably why this ability has appeared in animals as different as turtles and fish, they wrote -- and it's likely to appear in other marine members of the animal kingdom as well.
"Given that these salmon make their oceanic migration only once, to locations where they have never been, and without the benefit of following experienced migrants, a navigation system based on inherited instructions is likely to be highly adaptive and possibly necessary," the authors wrote.