How Scientists Figured Out Salmon Use Magnetism To Navigate

salmonToday I wrote about salmon navigation on NPR’s food blog The Salt:

After hatching in a freshwater stream, young salmon make a break for the ocean, where they hang out for years, covering thousands of miles before deciding its time to settle down and lay eggs in the same stream they were born.

So how do these fish find their way back to their home river?

According to one theory, it’s all about magnetism. When salmon are young, the theory goes, they imprint on the pattern of the Earth’s magnetic field at the mouth of their native river. Years later, when the salmon head back home to spawn, they home in on that pattern. In a study published Thursday in Current Biology, the scientists behind that theory now say they have evidence that’s exactly how the fish are navigating.

What got cut from the story is how the scientists made the connection:

The researchers determined that the fish must be using magnetism to navigate by studying sockeye salmon from the Fraser River in Canada. The mouth of that river is blocked from the ocean by Vancouver Island, meaning that the fish can’t directly swim back into the river from the Pacific; they have to choose a route either north or south of the island to get home.

That set up a natural experiment: If the salmon are navigating by the earth’s magnetic field, they should pick the route entrance that has a magnetic field pattern that most closely resembles the one at the mouth of the Fraser.

But it gets a little complicated because the earth’s magnetic field varies by strength and direction of the magnetic lines, and that field wiggles ever so slightly year to year — as evidenced by the changing location of the magnetic north pole.

So over time, the northern route is sometimes a closer match for the salmon, and vice versa. And it turns out that “fish migratory routes are following that wiggling” in the magnetic field, says lead author Nathan Putman of Oregon State University in Corvallis, which means they’re probably using it to find the exit off the Pacific highway and the way to their home river.

Image credit: Current Biology, Putman et al.

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