Salmon travel thousands of miles - out to the open ocean to feed and mature. Then after a few years they head home – back to the exact river where they hatched – to spawn the next generation.
Scientists don’t fully understand how the fish find their way back, but a new study provides some answers.
EarthFix’s Ashley Ahearn talked with Nathan Putman, a postdoctoral researcher at Oregon State University and lead author of the study, which appeared in Current Biology.
EarthFix: Ok, spoiler alert: the answer is “magnets” – earth’s molten iron core, the same thing that protects this planet from cosmic rays helps salmon and sailors find their way home. Tell me how you explored this question.
Putman: So, first thing, the earth’s magnetic field is predictable for determining your special location so you can use the earth’s magnetic field as a proxy for geographic location, and that’s because the earth’s magnetic field is strongest, it has the strongest force at the poles, and then it gradually grows weaker as you get towards the equator. So if you’re capable of detecting the strength of the magnetic field then you have an onboard system for assessing, at least crudely, your latitudinal position.
EarthFix: And that’s what these salmon are doing.
Putman: Exactly. So what we think is happening is that these fish leave their rivers as juveniles, they hit the salt water, they remember the strength of the magnetic field where they first encounter the salty water and then they spend two years or so feeding, not paying much attention to anything except where to find food to get big and fat. And then they have to make it back to their home river and they need to do this efficiently and precisely and that’s because, well where’s a good place to lay your eggs? It’s hard to guess from out in the ocean so the fish seem to have hit on a strategy of saying to themselves, “well it was good enough for me to survive and grow to maturity so it will be good enough for my offspring”.
EarthFix: So tell me a little more about how it works. They hit salt water, what’s happening then?
Putman: Salt water serves as a cue that they are no longer in their river and are entering a new environment and so if they remember what the magnetic field is like where they first encounter salt water that would necessarily lead them back to their river some two years later. So the salt water isn’t particularly exciting or interesting to them but it serves as a cue for knowing at what point they need to start paying attention to the magnetic field.
EarthFix: So these fish “imprint” the magnetic field. Do we know how, where, what part of their biology allows them to do this?
Putman: So what we do know, from this summer actually, some researchers found tiny magnetic crystals – these little pieces of iron – in the noses of fish and these appear to be associated with the nervous system and what these little iron crystals appear very well suited to do is respond to changes in the direction and the strength of the earth’s magnetic field.
So these little pieces seem to be little magnetometers, little devices that help them measure what the magnetic field is like.
EarthFix: So that’s the onboard compass you were talking about?
EarthFix: Wow, that is so cool. So tell me how you conducted your research? Why did you focus on Vancouver Island and the way fish navigated around it to get to the Fraser River?
Putman: Sure, so the Vancouver fish are no more special or less special than any other pacific salmon. They start their lives in rivers, they swim out to sea and then they come back. The only trouble for these fish is when they try to come back there’s a big island, 350 or so kilometers long, that’s blocking their access back to their home river and what that allows us to do is sort of treat it like it’s a natural experiment. Fish are given a two-choice test they can either come in through the northern route around Vancouver Island, or they can come in through the southern route and this has been monitored for the past 56 years or so since the early 1950s and there’s a good data set for the proportion of fish following each route.
And it’s remarkably variable. The fish change their minds over the course of years as to how many come in each route. You might have some years where more than 80 percent of the fish are coming in from the Northern route and other years where less than 3 percent of the fish are coming in from the northern route. And it’s been a bit of a question for people why that is and what we were able to do is relate the proportion of fish following each route to the changes in the earth’s magnetic field through time.
So the earth’s magnetic field is not constant and that’s because it’s this molten liquid outer core that’s swirling around which causes the field to be generated but all this swirling is somewhat chaotic so the field gradually drifts and I think a lot of people are aware the earth’s magnetic north pole is not the same as the earth’s geographic north pole and the magnetic north pole moves around a little bit and this little bit of movement causes the magnetic field’s gradient and intensity to wiggle around a little bit to.
Now this isn’t very much - the wiggles are on the order of tens of kilometers, maybe, over the course of a couple of years, but because we have this big obstacle for the fish they have to make a larger geographic detour and that detour, the proportion following each route, is predicted by how well the magnetic field at the entryway to each of those routes corresponds to what the magnetic field was like at the mouth of the Fraser River two years before.
EarthFix: So how exactly does that work? The magnetic field is stronger on the western side of the island than the eastern side and that tells them to go west?
Putman: So it’s more of a North to South sort of thing. In general the magnetic field is stronger towards the north and weaker towards the south, at least in the northern hemisphere and so what we think the fish are doing is when they’re in a magnetic field that is much stronger than their home magnetic field then that means they need to swim south. When the field is weaker then they need to swim north if they’re going to get back to their home site.
In a perfect world if the magnetic field didn’t wiggle around or change at all then you would expect the fish to always have one preferred entry way, depending on how that gradient of magnetic strength was aligned. But that’s not the case, the earth’s magnetic field wiggles around a little bit so the similarity between the magnetic field at the Fraser River and the northern route also varies. 7:40 And so what we’re able to take advantage of is if the fish want to find home and they’re using the magnetic field as a proxy for home and the fish are coming to Vancouver Island they’re faced with a two choice test – they can either go to the north or they can go to the south - and they seem to make that decision based on whichever inlet is more similar to the Fraser River magnetic field in a given year is the one most fish come to.
EarthFix: So what happens when the earth’s magnetic field reverses, as it does every several hundred thousand years or so?
Putman: That’s a great question and it’s something that we’re looking into conceptually building a model with some geophysicists at Oregon State University. It’s one of those things where it depends on how quickly the earth’s field reverses and how much the fish are paying attention to the magnetic field.
So it could cause, depending on how quickly this occurs, major disruption, or it could be largely innocuous.
EarthFix: Nathan, what else do we know about how salmon find their way home, it’s not just the magnetic field, right?
Putman: Definitely not. The fish are endowed with lots of senses and lots of tricks up their fins for finding their way around. So there are lots of schemes they could use to help guide their migrations. And since they only get one chance to do this they probably have evolved a lot of mechanisms for helping refine that homeward migration.
One of these seems to be pretty clearly the earth’s magnetic field, using it as a sort of map. Other things they do to stay on course could be using a sun compass, using odors, the characteristic scent of their particular birth site might be a good way to locate on a fine scale, where they should spawn.
But a lot of those are a question of scale. So you might be able to find your way to your kitchen if you’re baking cookies from say upstairs or out on the porch. Or if the wind conditions are right you might be able to smell your kitchen from down the street. But whether I could smell your kitchen from here in Corvallis, that seems unlikely and so some other larger scale cue would be needed to get into the vicinity of your kitchen before I could smell my way in.
EarthFix: For a salmon that “other larger scale cue” could be the strength of the magnetic field.
Nathan Putman is a postdoctoral researcher at Oregon State University and lead author of the study, which appeared in Current Biology.
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