Salmon Use Magnetic Maps To Navigate Home
February 8, 2013

Salmon Use Magnetic Maps To Guide Their Homeward Journey

April Flowers for - Your Universe Online

A new analysis of over 56 years of data has revealed that sockeye salmon use a magnetic map to return to their spawning grounds after years at sea. The findings of this study were published online in the journal Current Biology.

"To find their way back home across thousands of kilometers of ocean, salmon imprint on the magnetic field that exists where they first enter the sea as juveniles," said Nathan Putman of Oregon State University. "Upon reaching maturity, they seek the coastal location with the same magnetic field."

For years scientists have suspected that some animals — including fish, sea turtles and seals — might imprint on magnetic fields, giving them a sort of "sixth sense" or an internal GPS system. However, researchers have found it very difficult to explain exactly how this mechanism works.

Sockeye salmon from British Columbia's Frasier River were the focus of this new study. Like all species of Pacific salmon, sockeye leave the river for the sea shortly after they mature. However, there is a hitch to their return.

"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," said Putman. "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."

In the case of BC´s Frasier River salmon, there is a particularly large and prominent impediment to their homeward journey.

"When they attempt to return, they are confronted with a giant obstacle: Vancouver Island is blocking direct access to their river!" Putman explains. "So the fish must make a choice: Do they use the northern inlet or the southern inlet in their detour?"

The team hypothesized that if fish really do follow magnetic fields, their choice should show a predictable pattern over the years because of geomagnetic field drift. The Earth's magnetic field is not constant, and it gradually changes over time based on the chaotic movement of the Earth's liquid outer core.

Scientists don't know how often salmon check the Earth's magnetic field in order to identify their geographic locations during their trip back home. "But for the salmon to be able to go from some location out in the middle of the Pacific 4,000 miles away, they need to make a correct migratory choice early — and they need to know which direction to start going in. For that, they would presumably use the magnetic field."

Putman continues, "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."

If the fish are using magnetics to return, they should show a greater preference in a given year for the inlet that most closely matches the magnetic value of the Fraser River when they departed about two years previously. The team performed a careful comparison of data collected by fisheries since the 1950s with a model that predicts the shifting magnetic field.

"Salmon are a cold-water fish, and all things being equal, they prefer cold water," said Putman, who earned his PhD in biology from the University of North Carolina at Chapel Hill. "But the fact that they also demonstrate geomagnetic fidelity in choosing a route shows that this could be a major instrument in their biological toolbox to guide their way home."

"Salmon have to get it right because they only have one chance to make it back to their home river," Putman explains, "so it makes sense that they may have more than one way to get there. The magnetic field is amazingly consistent, so that is a strategy that can withstand the test of time. But they may also use the sun as a compass, track waves breaking on the beach through infrasound, and use smell."

Putman also told BBC News that in order for salmon to find their way home, "they remember the magnetic field that exists where they first enter the sea as juveniles, and once they reach maturity, they seek that same coastal location, with the same magnetic field. In other words, salmon remember the magnetic field where they enter the ocean and come back to that same spot once they reach maturity."

Putman believes that the results of his findings will of interest to more than just biologist, since there may be practical applications for both the wild and farmed salmon industries.

"If you want to predict the distribution of salmon, then having information on their mechanisms of navigation is really important," Putman said, and any distortion to the magnetic field may be a problem.

"The Earth's magnetic field is quite weak compared to the magnetic fields that humans can produce," he explained. "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."

The high stray rates of farm-raised fish might be explained by the shifting magnet fields, or the tiny magnetic wires used to tag hatchery salmon.