How Do Salmon Find Their Way Home? Smell and Magnetism

How young salmon learn the smell of home

Every river has its own smell. The rocks, soil, plants and animals in a river’s catchment give its water a chemical signature that a salmon’s very sensitive nose can detect in tiny amounts.

Young salmon memorize this smell, a process called imprinting. The most important learning happens when parr turn into smolts, the silvery stage that is about to swim to sea. At this point, a surge of thyroid hormone seems to switch on a strong period of memory, and the fish record the scent of the water around them.

Smolts may also learn a sequence of smells as they travel downstream, from the stream where they hatched to the main river and then the estuary. Years later, adults can follow this chain of scents back in reverse, choosing the right branch at each fork.

The nose-plugging experiments

The idea that salmon home by smell goes back to the 19th century, but it was tested properly by the American biologist Arthur Hasler and his students at the University of Wisconsin from the 1950s onward.

In one famous experiment in Issaquah Creek, Washington, the team caught returning coho salmon in two branches of the stream, plugged the nostrils of some fish and released them all below the fork. Fish that could still smell mostly went back to the branch where they had been caught, while fish with plugged noses chose a branch at random.

In the 1970s, the same group raised young coho salmon in water scented with a harmless synthetic chemical called morpholine and released them into Lake Michigan. When the adults returned, they were drawn to streams that had been scented with the same chemical, showing that salmon really do learn a smell when young and seek it out later.

Do salmon use Earth’s magnetic field?

Smell cannot explain how salmon find their way across thousands of kilometers of open sea. For that, the best evidence points to magnetoreception, the ability to sense Earth’s magnetic field.

Earth’s magnetic field varies in strength and angle from place to place, so a coastline has its own magnetic signature. Scientists think young salmon record the magnetic signature of the coast where they enter the sea, and later steer back toward it.

Two well-known studies by Nathan Putman and colleagues support this. A 2013 study of Fraser River sockeye in British Columbia found that returning fish chose their route around Vancouver Island in a way that tracked slow, natural drift in the magnetic field over decades. A 2014 experiment exposed young hatchery Chinook salmon, which had never been to sea, to magnetic fields matching different parts of the ocean. The fish turned in directions that would lead them toward their feeding grounds, suggesting they inherit a basic magnetic map. A 2018 study found similar responses in young Atlantic salmon.

Tiny crystals of magnetite, a magnetic iron mineral, have been found in the heads of salmon and may act as the compass needle, but exactly how the sense works is still not known.

Other clues salmon may use

Salmon may combine several other cues. Ocean currents carry young Atlantic salmon toward their feeding grounds and back, the sun may help them keep a heading, and water temperature and saltiness change in predictable ways near land.

Another idea is that salmon recognize the smell of their own population. Young salmon release chemical signals into the water, and returning adults may be drawn to rivers that smell of their relatives. This would help explain why salmon can find rivers that have changed since they left.

Navigation of this kind is not unique to fish. Sea turtles and many migrating birds also sense the magnetic field; for birds, see How do migrating birds navigate?

How accurate is salmon homing?

Very accurate, but not perfect. Tagging studies show that the great majority of wild salmon return to the river where they hatched, and many go back to the same tributary or reach. Usually only a few percent stray into another river, most often one nearby.

Straying is not simply a mistake. Without strays, salmon could never have recolonized rivers freed from ice at the end of the last ice age, or rivers that have been cleaned up after pollution. Many salmon rivers in Europe and North America have been repopulated this way, alongside restocking.

Salmon raised in hatcheries and released far from where they were reared tend to stray more, because they never imprinted on the right river. This is one reason modern restoration programs try to release young fish in the river where they are meant to return.

Why homing matters

Returning home gives salmon an advantage. Their parents spawned successfully in that river, so it is likely to have the right gravel, flow and temperature. Over thousands of years, each river’s salmon have adapted to local conditions, from the timing of their runs to the size of their eggs.

This is why so many separate populations exist, and why losing one river’s run is a real loss. To see what this journey involves, read Why do salmon swim upstream?, or compare it with other travelers in our list of the longest animal migrations.