How salmon find their way home

Nutrition experts say fish is good “brain food” for humans. Well, when it comes to sockeye salmon, their own brains just might be smarter than you thought.

A study published this week in the journal Current Biology, and led by an Oregon State University researcher, provides a clue to a question that has intrigued biologists for decades: How do salmon navigate across thousands of miles of open ocean to find the river where they were born, before journeying back to spawn and die?

“We’ve known for a long time that fish make really long migrations, that they come back to their home river,” Nathan Putman, a post-doctoral researcher at OSU and the lead author of the study, said in a phone interview Friday afternoon. Researchers, in addition to knowing for quite some time that fish use their sense of smell to navigate, have also suspected that a geomagnetic field played a role, Putman said.

“But what was not known was that they use that ability to define their way back home,” he said.

In the study, scientists examined 56 years of fisheries data documenting the return of sockeye salmon to the Fraser River in British Columbia. The route they chose around Vancouver Island showed a correlation with changes in the intensity of the geomagnetic field.

“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,” Putman said. “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.”

The Earth has a predictable, consistent geomagnetic field that weakens as you move from the poles toward the equator. Salmon originating from Oregon that have spent two to four years in the northern Pacific Ocean off Canada or Alaska would, as adults, return toward their home river, the scientists speculate, journeying southward off the coast until they reached a magnetic field intensity similar to that of their youth.

That process should get them to within 50 to 100 kilometers of their own river system, said Putman, who works in OSU’s Department of Fisheries and Wildlife.

Salmon returning to the Fraser River must detour around Vancouver Island, which is about 185 miles long, to reach the mouth of the river, choosing a southern or northern route. Scientists involved in the study found that the “drift” of the magnetic field correlated with which route the salmon chose.

When the normal magnetic intensity level for the Fraser River shifted to the north, the sockeye were more likely to choose a northern route for their return, the scientists found. When the field shifted slightly south, the salmon chose a southern route.

“The idea is not a new one,” Putman said. It was first proposed in the early 1980s by Thomas Quinn, a University of Washington graduate student who is now an aquatic and fishery sciences professor at the UW and one of the researchers on this new study. But the data gathered from the study on the Fraser River “is the first evidence that definitively shows that the fish remember,” Putman said.

The term “magnetic imprint” is “just sort of a technical way of saying ‘remembering.’ ” he said.

Other marine animals, including sea turtles, eels and tuna, have shown similar behavior, Putman said.

“Salmon are a cold-water fish, and all things being equal, they prefer cold water,” he said. “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.”

Putman said that his previous studies of the Columbia River have shown that the magnetic intensity shifts less than 30 kilometers in either direction over a period of three years, which is about the length of time many salmon spend in the ocean.

“Salmon have to get it right because they only have one chance to make it back to their home river,” he said, “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 and OSU fisheries biologist David Noakes plan to follow through with experiments on varying the magnetic field for salmon in a laboratory setting, using the Oregon Hatchery Research Center in Oregon’s Alsea River basin.