How do salmon find their way back to the exact stream where they were born?

The answer

Salmon use a combination of Earth's magnetic field for open-ocean navigation and an extraordinarily sensitive sense of smell for the final leg home. During their early development, young salmon imprint on the unique chemical signature of their natal stream, essentially memorizing its scent. Years later, returning adults can detect these trace chemicals at concentrations as low as one part per billion, following the scent gradient upstream through branching tributaries to the exact gravel bed where they hatched. This remarkable olfactory memory is encoded during a critical imprinting window regulated by thyroid hormones during the parr-to-smolt transition.