When it’s time to spawn, salmon have an uncanny ability to swim from the middle of the Pacific Ocean in almost a straight line directly back to the mouth of the river where they were spawned and make their way upstream.

The mystery of this unflappable internal GPS system has fascinated ichthyologists for centuries.

But now there may be an answer: The fish have tiny magnets in their flesh that allow them to use the Earth’s magnetic field as both a map and a compass, according to new research unveiled last month.

For California, the findings could have positive ramifications for the state’s struggling commercial and recreational salmon fishing industry. In recent years, state and federal officials have relied more and more on fish raised in hatcheries that are hauled downriver via boat or in a truck. The hauling somehow throws salmon GPS systems out of whack.

Researchers have for years suspected that salmon are like other animals such as rodents, bats, birds, sea turtles and lobsters that rely on the crystals of magnetite — a naturally magnetic mineral — in their bodies to find their way around the globe on their migrations.

To test the hypothesis, researchers at the University of North Carolina, Chapel Hill, Oregon State University and a private firm, LGL Ecological Research Associates, used an elaborate electrified coil system to subject juvenile Chinook salmon to magnetic pulses known to reverse the polarity of magnetic particles.

These same pulses have been shown to affect magnetic orientation behavior in other migratory animals.

Testing magnets in salmon flesh

The researchers divided the small salmon they wanted to study into groups.

The control group wasn’t subjected to any pulse; the second was subjected to a typical magnetic field; and the third group was pulsed with a magnetic field whose properties had been tweaked, which the researchers suspected would alter their swimming course.

When placed in the water, the control group and the one pulsed with the regular field oriented in the tank almost identically. But the fish hit with the altered magnetic pulse swam on the same path, while the control group fish zipped around the tank seemingly at random.

While more study needs to be done to confirm the findings, the research appears to prove the theory that the “magnetoreceptors” in salmon play a critical role in their migration.

“We demonstrated from the time they hatched, they can detect the magnetic field and they can respond to it,” said David Noakes, a professor at Oregon State, the director of the Oregon Hatchery Research Center and one of the researchers involved in the study.

Noakes said it’s likely that when a juvenile fish reaches saltwater, it essentially locks in the latitude and longitude of the river mouth, knowing it will need to come back to those coordinates — almost like “dropping a pin” on a smartphone’s GPS mapping program.

Finding their way back home in the vastness of the Pacific is a pretty remarkable feat, given the fish make just one return trip in their life cycle. Salmon die after they spawn. So unlike nature’s other migrants, the fish don’t learn the journey from their parents, he said.

But the salmon’s internal magnetic field sensors only take them so far, Noakes said.

Once the adult fish get to the river itself, they likely rely on the chemical composition of the water to find their way back to the very gravel beds in the tributary in which they hatched.

“(A salmon) goes through a special kind of learning when it’s incubating and just before it leaves the river,” Noakes said. “It pays very close attention to the chemical nature of the water it’s exposed to. The same way you remember your grandmother’s kitchen from the smell of baking bread and chocolate cookies and that kind of stuff.”

Studies are underway to confirm that theory, too, he said.

Helping trucked fish find their way

Aside from answering a question that has perplexed scientists for centuries, the hope is the research could have practical applications to aid the West Coast’s struggling fishing industry.

It’s become a common practice in the Pacific Northwest and in California to haul juvenile fish raised in hatcheries downstream in tanker trucks or in barges. Hauling has become increasingly popular in drought years when fish numbers plummet.

Under these hauling programs, hatchery managers take the fish closer to the mouth of the river, keeping them safe from predators. This gives them a better chance of survival and provides more fish for anglers to catch. Efforts also are underway to attempt to “trap and haul” endangered winter-run Chinook salmon to bring them back to their traditional spawning grounds, which were blocked when Shasta Dam was built in the 1930s and 40s.

The problem with hauling fish, Noakes said, is the salmon tend to lose their way back when they reach adulthood.

Noakes said it’s likely the salmon’s magnetic maps and compasses go haywire from the electromagnetic fields generated by the engines of the trucks and the barges.

The hope is maybe someday, the fish before they’re released could get pulsed with a magnetic field that could restore their internal GPS systems.

“That’s the next experiment we’re going to do,” he said.