D: Here's a Moment of Science trivia question, Yaël: Why is a sockeye salmon like a compass?

Y: What? That's quite the riddle, Don. Can you give me a hint?

D: Okay: it has to do with magnets.

Y: Well, a compass points north thanks to the pull of the earth's geomagnetic field. But what does that have to do with fish?

D: Quite a lot, in fact. It turns out that sockeye salmon are also sensitive to the earth's magnetic pull. Scientists have discovered evidence showing that salmon use a nifty trick called "geomagnetic imprinting" to find their way home to their birth rivers.

Y: Geomagnetic imprinting? Wow! So, salmon can detect the precise geomagnetic "feel" of a river's location, and memorize it to help them return later? How did the scientists figure that out?

D: They analyzed migration data for sockeye salmon born in British Columbia's Fraser River. These fish leave the river to spend their adult life in the North Pacific Ocean. Two years later, they return to spawn. Here's the thing, though: they either have to swim around the north or the south end of Vancouver Island to get back. The number of salmon using the northern route has steadily increased over the last century, and the scientists figured out that the earth's shifting geomagnetic fields were involved.

Y: Let me guess: the magnetic field at the mouth of the Fraser River used to line up with the southern route, but these days, it coincides with the northern passage instead?

D: You've got it. And, that, Yaël, is why a sockeye salmon is like a compass. It uses the earth's magnetic field to point itself in the right direction and find its way home!