Whoever said you can’t go home again has never met a sockeye salmon, which navigates more than 2,485 miles (4,000 kilometers) to spawn in the same stream in which it hatched. (See “5 Amazing Animal Navigators.”)
Now, scientists have finally solved how the species accomplishes its navigational feat—the fish uses Earth’s magnetic field to steer itself home.
“To find their way back home across thousands of kilometers of ocean, salmon imprint on [i.e. learn and remember] the magnetic field that exists where they first enter the sea as juveniles,” study leader Nathan Putman, of Oregon State University, said in a statement.
“Upon reaching maturity, they seek the coastal location with the same magnetic field.”
Like several other species of salmon, sockeye hatch in many of the streams and tributaries of the U.S. Pacific Northwest.
After hatching, they live and mature in the gravel beds of these freshwater streams for one to three years. Then, the salmon make their way from their freshwater nurseries to the open waters of the Pacific Ocean, where they spend another several years feeding. Eventually the fish make their way back to the streams in which they were born to spawn and begin the cycle anew.
Navigating Salmon a Mystery
What scientists didn’t know was how the salmon managed to do this. Navigating in the open ocean is a difficult task even with a GPS, yet even with such tiny brains, salmon can identify one stream out of several thousand options.
So Putman and colleagues hypothesized that salmon were using variations in the Earth’s magnetic field to figure out where “home” was. If this was true, then the researchers could see if a salmon’s ability to navigate changed over time with small, naturally occurring variations in the global magnetic field.
Putman and colleagues used 56 years of fisheries data to study a group of sockeye salmon that spawned in the Fraser River in British Columbia and spent much of their adult lives in and around Alaska‘s Aleutian Islands. The researchers studied the likely routes the salmon took in transit between these two locations and compared it to data on the strength of the Earth’s magnetic field at the time. (Also see “Alaska’s Clash Over Salmon and Gold Goes National.”)
The key to this study was a major navigational obstacle the fish had to traverse. Vancouver Island blocks the entrance to the Fraser River, forcing the salmon to swim around either the northern or southern end of the island to get to the spawning grounds (see map below). If the fish really did use the Earth’s magnetic field to navigate, then their choice of routes around Vancouver Island would vary depending on the current strength of the magnetic field at the time.
No Place Like Home
Putman’s hunch was correct: The navigational choice depended largely on which route most closely matched the magnetic signature of the Fraser River when the salmon first left the area for the saltier waters of the Pacific.
“These results are consistent with the idea that juvenile salmon imprint on the magnetic signature of their home river, and then seek that same magnetic signature during their spawning migration,” Putman said in a statement from the National Science Foundation, which helped fund the research.
“As the salmon travel that route, ocean currents and other forces might blow them off course. So they would probably need to check their magnetic position several times during this migration to stay on track. Once they get close to the coastline, they would need to hone in on their target, and so would presumably check in more continuously during this stage of their migration.”
This study, published February 7 in Current Biology, is the first to document an animal’s ability to learn to navigate via the magnetic field. The other animals scientists have studied, like lobsters and birds, either have this knowledge imprinted in them from birth or remember the magnetic signature of home, rather than actually learning to navigate. (Also see “Bats Use Magnetic ‘Compasses’ to Navigate, Study Says.”)
These results help explain why salmon raised in hatcheries so frequently become lost in the ocean. Since many fisheries are crisscrossed with electric wires, magnets, and metallic objects—all of which alter perception of magnetism—the fish never learn the magnetic “feeling” of home.
Putman said the results could also be used to forecast where salmon will be in future seasons by studying how the magnetic field changes over time.