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What environmental cues help Atlantic salmon return to the river where they were born?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Atlantic salmon rely on a combination of environmental cues to navigate back to their natal rivers, with olfaction (the sense of smell) playing the most critical role in identifying their specific spawning grounds.12

During their early development in fresh water, young salmon undergo a process of imprinting, where they memorize the unique chemical signature of their home stream. These chemical characteristics are derived from the local substrate, vegetation, and the presence of other salmon populations. When the fish return as adults, hormonal changes—specifically involving thyroxine and reproductive hormones—increase the sensitivity of their olfactory system, allowing them to detect and follow these familiar chemical trails.1

While olfaction is essential for pinpointing the exact spawning site, other environmental factors are believed to assist in the broader navigation from the sea to the correct river system. These may include:

Geomagnetic orientation: The use of Earth’s magnetic field to navigate.13
Celestial cues: Steering by the position of the Sun.1
Topographical features: Recognition of physical landscape characteristics.2
Water temperature: Responding to specific thermal signatures within the water.2