Salmon return from the sea to the rivers where they hatched in order to spawn.

The longer version

Chemical traces carried by a river give each catchment its own signature, blending geology, soil and vegetation. Juveniles learn that signature during their freshwater stage and retain the memory for years, then use it to steer upstream when adults return from distant feeding grounds.

Large mouths sample water on both sides, letting the fish compare concentrations and hold to the stronger scent. Sun angle and the magnetic field give coarse direction during the open-ocean portion. Feeding generally stops before freshwater entry, spawning follows, and spent adults supply nutrients to riverbanks and forests. Some populations travel far inland, while others spawn close to the estuary.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.