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How do salmon locate the stream where they developed as young fish?

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A herd of wildebeest, including calves, moves across a dusty savanna in Africa with scattered trees visible in the background. — Migration A herd of common wildebeest (*Connochaetes taurinus*) migrating across a dusty savanna in Africa. The animal is a keystone species (i.e., a species with a disproportionately large effect on its biological community) in plains and acacia savanna ecosystems from southeastern Africa to central Kenya.

Salmon locate the specific stream where they developed through a remarkable process of imprinting on the unique chemical signature of their home waters during their early life stages.12

Once a salmon enters its river system from the ocean, olfaction plays the primary role in guiding the fish to its natal site. This chemical signature is derived from the stream's substrate, surrounding vegetation, and the presence of other salmon. While the fish is young, hormonal activity—specifically involving thyroxine—facilitates this imprinting process in the olfactory epithelium.132

The salmon’s sensitivity to these specific environmental odors increases significantly when it is ready to return to spawn, a change regulated by reproductive hormones. While the mechanisms used to navigate from the open ocean to the mouth of the river system remain less understood, researchers hypothesize that salmon may utilize geomagnetic orientation or celestial cues, such as the position of the Sun, to assist in their long-distance migration.134