Salmon returning to the stream where they hatched are not following a map anyone drew for them: they are reading the chemical signature of their birth water on the final stretch, a scent memory formed before they ever reached the ocean

Salmon returning to the stream where they hatched are not following a map anyone drew for them: they are reading the chemical signature of their birth water on the final stretch, a scent memory formed before they ever reached the ocean

A salmon returning to spawn is usually described as finding the exact stream where it hatched. Closer to the truth, it is reading water. In the last stretch of the trip, the fish matches the chemical signature of the river in front of it against a scent it learned years earlier, as a juvenile, before it had ever reached the sea. That scent memory is the securely established part of the story. The familiar line about a fish following a map it was…