Salmon have one of the most arduous life strategies in the animal kingdom. Hatched in freshwater streams, young salmon undergo physiological changes that allow them to survive in the salty sea. After several years of relentlessly searching for food in the vast ocean, salmon may swim thousands of miles as they return to their natal streams where they will spawn and die.
The big question for biologists has always been: How do they find their way back?
For years scientists have suspected that the Earth’s magnetic field plays an important role. A recently published study has finally revealed how it might be used by migrating fish.
“What we think happens is that when salmon leave the river system as juveniles and enter the ocean, they imprint the magnetic field—logging it in as a waypoint,” Nathan Putman, a post-doc and lead author on the study, told OSU. “It serves as a proxy for geographic location when they return as adults. It gets them close to their river system and then other, finer cues may take over.”
Looking over 56 years of fisheries data, the study focused on sockeye spawners returning to the Fraser River in British Columbia. Between the salmon and their natal streams lies the natural barrier of Vancouver Island. Returning salmon must therefore decide whether to take a northern or southern route to reach their spawning grounds.
Researchers were able to compare the spawners’ chosen routes with measurements of the Earth’s geomagnetic field. What they found was that salmon were more likely to choose the route with a magnetic field similar to that of their natal stream at the time of their hatching.
“That should get them to within 50 to 100 kilometers of their own river system and then olfactory cues or some other sense kicks in,” said Putman.
In addition to shedding light on the migrations of wild salmon, the study may also have important implications for salmon raised in fish hatcheries.
“If, for instance, hatchery fish are incubated in conditions with lots of electrical wires and iron pipes around that distort the magnetic field, then it is conceivable that they might be worse at navigating than their wild counterparts,” Putman said.
Future studies will focus on the response of captive fish to artificially generated magnetic fields.
by Mike Vernon