How do salmon find their way back to the river where they were born?
― A memory of smell, and Earth as a "map"
Salmon swim down the river where they were born and out to sea. Then, after years roaming the open ocean, they return to that very same river. With no map or landmarks out on the water, how do they find their way? The answer seems to come in two stages: Earth's magnetic field for the long haul, and the smell of the water up close. Two very different signposts, thought to be used one after the other.
An autumn river. In the shallows, big fish line up with their dorsal fins breaking the surface. Their bodies are covered in scars, their colour turned a dark reddish-black. These are salmon, come upriver to spawn.
These fish were born in this very river, years ago. As tiny fry they headed out to sea and roamed thousands of kilometres across the northern ocean.
Out of that vast ocean, they come back to this one particular river — not just any nearby river, but the exact one they were born in. That is no coincidence.
There are broadly two reasons for this
Earth is a giant magnet. The strength and tilt of its magnetic field vary slightly from place to place. Salmon are thought to sense these differences and use them to work out roughly where they are and which way to head.
River water has its own dissolved ingredients, unique to that river. Salmon memorise this smell as young fry, and as adults they swim into the river whose smell matches that memory.
The magnetic field can only tell them "roughly around here." Smell only works up close. Neither one alone is enough — only by using the two in sequence does a single path home take shape. See Figure 1.
The smell of the river is learned as a young fry
River water is not just water. It carries faint traces of substances dissolved out of the surrounding soil, rock, fallen leaves and plants. That mix differs from river to river, giving each one its own particular "scent signature."
Salmon fry are thought to memorise this scent strongly around the time they head down to the sea. An experience from that early, narrow window in life stays with them ever after. This is called imprinting.
When an adult salmon nears the coast, water from many different rivers is flowing out to sea. The salmon is thought to sniff its way through them one by one, picking out the river that matches its memory — and swimming past any that don't match.
The magnetic field works as a "rough map"
A magnetic force acts all around the Earth — it's why a compass needle points north. But that force isn't the same strength everywhere. The angle at which the needle tips downward from horizontal also grows larger the further north you go.
Put "strength of the field" and "angle of tilt" together, and you get a rough fix on your position, even out at sea. Salmon are thought to use exactly this kind of cue.
But the magnetic field can only narrow things down to a rough area — nowhere near precise enough to tell neighbouring rivers apart. That final, fine-grained step is left to smell.
Salmon are thought to keep using smell as a guide even after entering the river. The dissolved substances in the water shift subtly between upstream and downstream, and between tributary and main channel. By following that trail, the fish are thought to make their way back close to their birthplace.
Homing to the birth river isn't perfect. Some salmon end up entering a different river. At first glance that looks like a failure, but it actually benefits the population as a whole: if the original river gets blocked by sediment, the stragglers that scattered into other rivers keep the line going.
Summary
A salmon swaps tools partway through its journey home. Out in the open ocean, it relies on Earth's magnetic field for a rough sense of direction and position. Closer to shore, it picks out its own river by the smell it memorised as a fry. A coarse map plus a fine-grained landmark together tie a long journey back to a single river.
For the long distance: Earth's magnetism.
For the final stretch: the smell of the water.
For another creature that travels just as far, see How do migrating birds never lose their way?; for more on Earth being a magnet, see Why does a compass needle point north?; on the link between water and smell, see Why does rain have that distinctive smell when it starts falling?; and for what happens inside a salmon's body as it moves from sea to river, see Why can sea fish drink salt water and be fine?.
- Take a little water from a nearby river or pond, from the tap, and from a puddle after rain, each into its own separate cup.
- Close your eyes, have a family member mix up the order, and see if you can tell them apart by smell alone.
- Compare water from the same spot on a sunny day and after rain — you should notice the smell has changed.
Just smell it, don't drink it. If even a human nose can tell the difference, a fish that senses smell underwater must experience it far more distinctly.
Want to know more? ― Terms, formulas, and how this connects to the curriculumLabels show whether each part is roughly middle-school, high-school, or university level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology
- High school+Advanced high-school content, or a textbook sidebar topic
- UniversityNot covered in high school — university-level content (animal behaviour, geophysics)
- ResearchNot yet settled even at university level — an active research question
Middle schoolTerms: this phenomenon has a name
- Natal homing: the trait of returning to the river where one was born, in order to spawn there. Well known among salmon species.
- Imprinting: a mechanism by which an experience during a fixed, early window in life becomes a lasting memory.
- Inclination (dip angle): the angle at which a compass needle tips downward from horizontal when viewed from the side. It grows larger further north.
Middle schoolHigh schoolWorking it out with a formula: how long is the ocean journey?
Let's estimate the length of the journey from swimming speed and number of days. The symbols below use metres per second for speed, seconds and days for time, and metres and kilometres for distance.
| In symbols | d = v × t |
| In words | distance travelled = swimming speed × time spent swimming |
| Where it comes from | This is just the definition of speed (distance covered per second). It's an estimate that assumes constant speed the whole way |
| Rough distance a salmon covers per second | take it as about 1 metre |
| Seconds in a day | 86400 seconds |
| Metres in a kilometre | 1000 metres |
| Number of days assumed swimming | 90 days |
| Distance per day, in metres | 1 × 86400 = 86400 |
| Convert that to kilometres | 86400 ÷ 1000 = 86.4 |
| Scale up to 90 days | 86.4 × 90 = 7776 |
Swimming non-stop, that works out to roughly 7776 kilometres over three months. In reality, ocean currents carry the fish along or hold it back, so it won't be exactly this. Still, it shows this is roughly the scale of a full circuit of the northern ocean for a salmon. Even a slight error in direction would land it somewhere very different — which is exactly why it needs signposts.
High schoolHigh school+When does the memory of smell get fixed?
High schoolSmell is sensed when cells deep in the nose pick up molecules dissolved in water. In fish, it's water rather than air that passes straight through the nostrils. The information picked up travels along nerves to the smell-processing part of the brain.
High school+The memory is thought to become fixed around the time the fry are preparing to head down to sea. During this period the body's hormonal activity changes dramatically, remodelling the fish to survive in salt water. The memory of smell is thought to strengthen during this very same period of change.
UniversityThe magnetic "map" drifts over time
Earth's magnetic field is generated by flows deep inside the planet. Because those flows shift over time, the pattern of field strength and tilt across the globe also drifts slowly, over the course of years. If salmon are memorising the magnetic field as a landmark, then after a few years that landmark itself will have shifted. In Pacific salmon, researchers have reported a correspondence between this drift in the magnetic field and year-to-year changes in the routes the fish take home. It seems the magnetic field is being used not as a precise address, but as a moving reference point.
This trait is called natal homing. The idea that fish memorise a river's smell is called the olfactory imprinting hypothesis; the idea that they memorise the magnetic field of their birthplace is called the geomagnetic imprinting hypothesis; and the slow drift of the magnetic field itself is called secular variation of the geomagnetic field.
📖 Further reading: Salmon (natal homing trait, Wikipedia, Japanese) ／ Geomagnetism (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- What the smell actually is The exact substances that give each river its distinct smell are still not fully identified. Alongside compounds from soil and plants, one idea is that the smell of fellow salmon already living there also plays a role.
- How the magnetic sense works Where in the body the magnetic field is actually detected is still unresolved. One hypothesis points to tiny magnetic particles inside the body; another points to a chemical reaction taking place in the eye.
- The switch between the two Exactly when the fish switches its reliance from the magnetic field to smell is also unclear. Sea water temperature and day length have been suggested as possible triggers.
In other words, everything in this article is "the best explanation given what we currently know." How salmon find their way home remains a subject of ongoing observation and experiment.
Connections to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: current and magnetic fields / life science | Earth being a magnet, animal behaviour |
| High school | Biology: animal responses and behaviour | How smell is detected, imprinting |
| High school+ | Biology: maintaining the internal environment | Bodily changes when heading out to sea |
| University | Animal behaviour / geophysics | Distribution and year-to-year drift of the magnetic field |
| Research | Fish physiology / sensory biology | Composition of the smell, organs that sense magnetism |
| ― | Everyday connections | How changes in river environments affect the fish that return |
- Japan Fisheries Research and Education Agency, Fisheries Resources Institute (水産研究・教育機構 水産資源研究所) (salmon research division), commentary on "The ecology and migration of salmon"
- Arthur D. Hasler, Allan T. Scholz, Olfactory Imprinting and Homing in Salmon, Springer-Verlag, 1983
- Nathan F. Putman et al., "Evidence for Geomagnetic Imprinting as a Homing Mechanism in Pacific Salmon," Current Biology, 2013
- Fisheries Agency of Japan (水産庁), Fisheries White Paper (水産白書), sections on salmon resources and hatchery release
※This article is a general-audience science explainer. The figures given are approximate, meant to aid understanding of the underlying mechanism. If observing at a riverside, please follow local guidance and the instructions of river authorities.