WSJournal. For Salmon, Magnetic Fields Point the Way.

Sockeye salmon return to the Fraser River to spawn, taking one of two routes around Vancouver Island.

Salmon find their way home guided by a natural magnetic map, confirming that fish are attuned to the planet's force field and use this sixth sense to navigate hundreds of miles in open water, scientists reported Thursday.

To find out how fish get their bearings, researchers compared records of local magnetic fields and 56 years' worth of commercial fishing data documenting routes taken by homing sockeye salmon during the annual Fraser River salmon run—which historically yields about $1 billion or more in fish in the Pacific Northwest and adjacent Canada.

By a fluke of geography, these spawning salmon must detour north or south around Vancouver Island to reach the waters of their birth along the Fraser River in British Columbia. The researchers, led by Nathan Putman at Oregon State University, determined that a salmon's route was shaped by the magnetic setting of its internal guidance system, which appeared to shift to keep the fish aligned with changes in the intensity of the local magnetic field.

"The fish are basically forced to make a decision" about their route, said biologist Kenneth Lohmann at the University of North Carolina, who was part of the research team. "That decision about the direction of their approach can be related to very slight changes in the earth's magnetic field."

No one is certain how the salmon sense magnetic fields, but previous laboratory tests with sea turtles have shown those creatures readily detect changes in the magnetism around them. Earlier this year, researchers discovered that the flesh of rainbow trout contains microscopic crystals of magnetite—the same mineral used in compass needles.

The researchers in the salmon experiment, reporting in the journal Current Biology, could reconstruct precisely how the homing fish sorted themselves from year to year because those that returned via the northward route around Vancouver Island were caught by Canadian fishermen while those that traveled the southern route around the island were caught by U.S. fishermen.

In recent years, the overall numbers of sockeye salmon returning to the Fraser River—North America's longest dam-free river—have fluctuated wildly, from the lowest to the highest yields on record. "There is a huge amount of interest in predicting the proportion that comes in one way or another," said Dr. Putman.

In fact, the researchers found that subtle shifts in Earth's natural magnetic map during recent decades have steered greater numbers of the homing salmon northward into Canadian waters and away from U.S. fishing boats—a trend expected to continue for the foreseeable future, the researchers said.