Reading the Magnetic Salmon Map

Story by Mark Hume

New research may have solved the mystery of how salmon migrate across vast stretches of ocean to locate their natal streams.

And the findings raise concerns about how hatcheries and the use of coded-wire tags might be adversely affecting the ability of fish to navigate and find their way home.

Researchers studying the movement of sockeye salmon from British Columbia’s Fraser River say the fish are imprinted with a magnetic map when they are juveniles.

A River Never Sleeps

Juvenile fish trapped in a stream – Photography by Nick Didlick

They use that map to read the earth’s geomagnetic field, which guides them back from the North Pacific, to the river mouth.

“Our paper clearly shows…very small changes in magnetic strength and intensity correspond to changes we see in the migration routes of the fish,” said Nathan Putman, a post-doctoral researcher at the University of Oregon.

“To find their way back home across thousands of kilometers of ocean, salmon imprint on the magnetic field that exists where they first enter the sea as juveniles. Upon reaching maturity, they seek the coastal location with the same magnetic field,” he said.

Dr. Putman’s team looked at Fraser River sockeye because of an unusual trait they have. With Vancouver Island blocking a direct route to the Fraser River from the Gulf of Alaska, migrating fish must choose whether to return through Queen Charlotte Strait, in the North, or through the Strait of Juan de Fuca, in the South.

Scientists and fishermen have long speculated on environmental factors, such as ocean temperatures, that might affect that choice.

But Dr. Putman found there is a strong correlation between the route chosen by the fish and the geomagnetic field, which naturally drifts because of the movement of the Earth’s liquid outer core.

“We found that the proportion of salmon using each route was predicted by geomagnetic field drift: the more the field at a passage entrance diverged from the field at the river mouth, the fewer fish used the passage,” states the paper, published Thursday, in Current Biology.

Dr. Putman said it has long been known that salmon use the Earth’s magnetic field as a sort of compass, to help the determine North and South. But the study shows sockeye are using the magnetic signals in a much more precise manner, like a map or an inner GPS, to assess their location in the open ocean, relative to the mouth of the Fraser River.

Dr. Putman said the findings raise some troubling questions about the long-established practice of putting magnetic, coded-wire tags in the noses of juvenile salmon. Each year on the Pacific Coast more than 45 million salmon and steelhead are tagged, so that managers can determine the origin of fish caught at sea.

A River Never Sleeps

The secret to Sockeye finding their way back to their home streams maybe in the nose – Photography by Nick Didlick

Dr. Putman said it is possible those tags could interfere with the ability of fish to read the geomagnetic field.

“All the signs are pointing to this being a potential problem,” he said, although he added more research is needed on that issue.

Dr. Putman said his findings will help fisheries managers in B.C. predict which migration route Fraser sockeye will follow.And with more research, he said, it should be possible to use the magnetic field to determine where salmon go after they migrate offshore, and vanish into the vastness of the North Pacific.

“As we get more information about how fish are using the information throughout their migrations, what we should be able to do then is make specific predictions about how fish distribute themselves throughout the North Pacific,” he said.

In the following Question and Answer, Mark Hume, editor of, talks with Dr. Putman about his fascinating research into salmon.

Q: How might magnetic tags affect the navigational powers of fish? Is it of concern?

A: The issue is that it’s a tiny little tag. You could fit probably more than a dozen on the tip of your finger. They are so small they don’t seem like they’d be much of a problem and depending on how fish use the magnetic field, they might not be.

Since the late 70’s early 80’s people have known that fish use the magnetic field the way we use compasses, to tell which way is North, which way is South and to keep a straight line bearing.

A River Never Sleeps

Nathan Putman stream side – Photography by Emily Putman

[Fish] don’t need to be very sensitive to the magnetic field to detect that. Presumably such a small thing [as a nose tag] isn’t going to cause problems for that. But if you are using the magnetic field for other things, like as a map to assess your actual location relative to a goal, then it gets problematic because the earth’s magnetic field is very weak and to be able to do that you need to be able to respond to very weak magnetic field. At that level those little tags do distort the strength of the magnetic field that fish might be able to feel or to detect . . . [and] it seems certain the magnetic signal of that tag would likely make it difficult to get a good reading on the earth’s magnetic filed.

Q: That could be significant in terms of how it might affect the ability of fish to return to their home river, or to return on time.

A: Right . . . the big thing for these fish is they need to find the right location and they need to be there at the right time. They need to be able to move efficiently and they need to have energy, once they find the right place, to be able to go up stream. . . [they need] a quick route in and using the magnetic field would allow for that. It is one of the most efficient ways for them to go from some place out in the ocean to the right river . . .if these fish are having trouble doing that, they might not be able to find the river or [they might] show up late and in poor condition.

I mean the fish aren’t just swimming magnetometers. They have other skills and tools. They can smell things, they can see things . . .they have a very high motivation to find the right place and they use a lot of tricks to do that. [But] If you take any one of those away…they might not do it as effectively . . [and] hatchery fish, for whatever reason, seem to be less precise in their homing than their wild counterparts.

A River Never Sleeps

Using all their senses Sockeye race to the spawning grounds – Photography by Nick Didlick

Q: What about the affect wires and pipes in hatcheries might have? Young salmon would be exposed to the influence those materials have at a key point in their development, when the “magnetic map” is being imprinted.

A: These fish have little iron crystals in their noses. And those are synthesized and grown as the fish develops…how those are grown and how they attach to the nerve cells would presumably be affected by the magnetic field they are in, as they are incubated and reared.

In the wild the only thing factoring into that is what the ambient magnetic field is of the earth. But in a hatchery you’ve got electrical wires, you’ve got iron pipes and those are all generating and distorting the magnetic filed. If the fish are using that as the basic starting point of their magnetic navigation, that’s the zero point with which they judge all other magnetic information that they subsequently encounter. So it could lead to problems with them navigating.

Q: Just being raised in a hatchery could damage their magnetic map?

A: It could. Scrambled, distorted. It could . . . Our paper clearly shows . . .very small changes in magnetic strength and intensity correspond to changes we see in the migration routes of the fish.

The relatively large changes that hatchery fish encounter could be disruptive. But we need to do research on that. At this point if you were a hatchery manager…I would [say don’t] start refitting your iron pipes with aluminium, which is non-magnetic. I’d say, we’ll get back to you in a couple of years. We’re doing some experiments now just to see whether there are these effects on the early life stages of fish.

Q: What advice would you give to fisheries managers who might now question the use of coded-wire nose tags?

A: I would say that should potentially be reconsidered. In science you always want to do an experiment to test the question at hand and our experiment was not designed to test that question…but I definitely think it’s something that should be looked into. All the signs are pointing to this being a potential problem. This past summer some researchers…looking at the cellular mechanisms of magnetic detectors in these fish and have identified them as existing in the nose – and that’ right where they are putting those tags…that’s something those folks who are putting these things in fish noses should think about.

Q: Does your research have any other practical management applications?

A: If we take another couple of steps…what it implies is that at this scale we can make predictions about how fish distribute themselves using magnetic models. And so as we get more information about how fish are using the information throughout their migrations what we should be able to do then is make specific predictions about how fish distribute themselves throughout the North Pacific.

Q: Scientists call the North Pacific the black box because so much of what happens out there with salmon is a mystery. They don’t know in any detail where fish go when they move offshore, or exactly what routes they follow to return – but you might have found a way to read the map.

A: Yeah, hopefully so. That’s what we are definitely trying to do with the lab experiments. We’ll be exposing these fish to magnetic fields that exist across the North Pacific…seeing whether or not we can get them to respond to that. Those responses would tell us [where] they would swim.

A River Never Sleeps

The final destination a male and female sockeye arrive on their redd – Photography by Nick Didlick