Quick Answer:Salmon remember odors associated with their home stream and use them during the freshwater part of their return migration. Evidence also supports a role for Earth's magnetic field over longer distances at sea. The sensory mechanism that detects that field is less certain than the familiar story about a built-in magnetite compass suggests.

Two scales of navigation

Finding a river after months or years in the ocean is a different task from choosing the right tributary once inside it. A useful account of salmon homing therefore separates a broad ocean journey from the final approach to a spawning site. One cue need not solve both problems.

USGS describes salmon homingas a combination of magnetic information at sea and the chemical character of the home river closer to the destination. Anadromous salmon hatch in freshwater, migrate to salt water, and later return to freshwater to reproduce. That sequence gives juveniles an opportunity to encounter the cues adults will use.

A remembered chemical signature

According toNOAA's fish olfaction research, young Pacific salmon learn odors associated with their natal stream before migrating seaward. Maturing fish retain those odor memories. Their sense of smell helps guide the homing journey in freshwater.

This is olfactory imprinting: experience during an earlier life stage influences later recognition. It is more specific than simply detecting that water is fresh. The fish is responding to a learned chemical signature. NOAA also reports that juveniles denied access to natal water during appropriate stages are more likely to return to other sites, connecting early experience to later behavior.

What the magnetic evidence shows

A2014 study of pink and sockeye salmon returning to British Columbia's Fraser Rivercompared migration routes with changes in Earth's magnetic field. The field changes gradually over time. If fish remember magnetic conditions near their home region, those changes should affect which route they take around Vancouver Island.

The observed route patterns were consistent with that prediction. This supports geomagnetic imprinting as part of long-distance navigation. It does not identify a particular receptor inside the fish or demonstrate that every salmon species uses an identical sequence of cues. Behavior consistent with magnetic sensing is evidence about navigation; it is not proof of how a magnetic sensor is built.

Homing is precise, but imperfect

Some salmon stray to other streams. NOAA studies straying because it matters to hatchery management and the genetics of wild populations. Homing is therefore a strong tendency supported by sensory information and learning, rather than an infallible return to one exact point.

The remarkable result is a journey assembled from information at different scales: broad orientation toward a home region, then recognition of local water. The remaining uncertainties concern how those cues are detected and combined, not whether salmon can remember their home stream.

Sources & further reading

That’s the mechanism. The wonder is still allowed.