Which fish is famous for swimming upstream to lay eggs?

Salmon, which hatch in fresh water, migrate to the sea, and then return years later to the exact stretch of the exact river where they were born - often after thousands of kilometres in open ocean. Pacific species spawn once and die; Atlantic salmon can survive to do it again, though most do not.

They navigate in two stages. In the open sea they appear to use the Earth's magnetic field, imprinting on the magnetic signature of the river mouth as juveniles and steering back to it. In the last stretch they switch to smell, recognising the specific chemical signature of their home stream - an ability accurate enough that fish will pass the right tributary junction in the dark.

The return trip is a one-way conversion of body into eggs. They stop feeding on entering fresh water, their bodies reabsorb muscle and organs for energy, males grow hooked jaws, and by spawning they are visibly disintegrating. Their carcasses are the point ecologically: nitrogen from the ocean, carried inland in the bodies of dying fish and spread by bears and eagles, turns up measurably in the trees along salmon rivers. The forest is partly made of fish.