Topics Animals, plants and life

How do salmon find their way home?

Salmon find their way home using two main tools, and scientists think they use them in sequence. In the open ocean, they appear to rely on the Earth's magnetic field to steer toward the right stretch of coast. Once they are near the river mouth, smell takes over. Each stream has a chemical signature, and young salmon learn it before they leave.

What makes this interesting is how much is still unsettled. The smell part is well supported by experiments over many decades. The magnetic part is supported by evidence too, but exactly how a fish senses a magnetic field is debated, and how it picks a target in the vast ocean is not fully worked out. Not every salmon returns to its birth stream either, and some stray into new rivers.

An episode on bre would walk through the whole journey, from gravel nest to ocean and back, and sort what is well established from what is still a good guess. The hosts are AI, so they can get things wrong, and you can press Talk to ask a question of your own while it plays.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. A fish that comes back to the same streamThe life cycle of a salmon: hatching in fresh water, years at sea, then a return to spawn. Why returning to the birth stream is the pattern, even though it is not perfect.
  2. Imprinting: learning the smell of homeYoung salmon take in the chemistry of their home stream as they migrate to the sea. Experiments that blocked or changed the sense of smell showed how much the return depends on it.
  3. The long ocean stretchSalmon spend years far from their birth river. How they cross that open water is less certain, and the hosts say so plainly.
  4. A possible magnetic mapEvidence suggests salmon can detect the Earth's magnetic field and use it to orient. How the sensing works is still debated.
  5. Up the river, one bend at a timeNear the coast, smell guides the fish at each fork in the river. Currents, temperature and obstacles shape the final climb.
  6. When salmon get it wrongSome fish stray into other streams, which helps populations colonize new rivers. Dams and habitat loss complicate the journey for many runs.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Tess. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, picture a salmon that hatched in a gravel creek, swam out to sea, and spent years in open ocean. Then it turns around and finds the same creek. How?

  2. TessAI host

    Honestly, most people's phones can't find their car in a parking garage. So that is impressive, and a little humbling.

  3. breAI host

    Fair. The short version is two tools. Something like a magnetic sense for the big ocean part, and smell for the last stretch.

  4. TessAI host

    Smell? Like it remembers the creek?

  5. breAI host

    Roughly, yes. Young salmon learn the chemical makeup of their home water before they leave. Later, near the coast, that memory acts like a signpost.

  6. TessAI host

    Okay, but real talk. How sure are we about the magnet part?

  7. breAI host

    Less sure. There's evidence fish can sense the field and use it. How their bodies do that is still argued over. I don't know the full answer, and I don't think anyone does.

  8. TessAI host

    Good. I like a mystery that admits it's a mystery. Start with the smell?

Questions people also ask

Do all salmon return to the stream where they were born?
Most do, but not all. A share of salmon stray into other streams, and that straying is thought to help the species settle new rivers, especially after habitats change. The exact share varies by species and population, so it is best treated as a tendency, not a rule.
Can salmon really smell their home stream?
Yes, smell is the best-supported part of the story. Salmon learn the chemical signature of their home water when young, and experiments where scientists disrupted the sense of smell showed that fish had far more trouble finding the right stream.
Do salmon use the Earth's magnetic field to navigate?
Evidence suggests they can detect it and use it as a kind of map in the ocean. Scientists still debate how the sensing works inside the fish and how much it matters compared with other cues, so this part is not fully settled.
Why do salmon die after spawning?
It depends on the species. Pacific salmon typically die soon after spawning, having used up their energy on the upstream journey and reproduction. Atlantic salmon can survive and sometimes spawn more than once, though many do not.

Related topics

More: all 300 topics, animals, plants and life, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.