Adult American eels are common in northern streams and rivers, yet no one has ever seen their young in fresh water. Baby American eels were eventually discovered thousands of miles from land in the Sargasso Sea. Catadromous fishes, American eels spend most of their lives in fresh water, sometimes taking as long as 40 years before they migrate to the sea to spawn.

Atlantic salmon, shad, and alewives are anadromous fishes that migrate from the ocean to fresh water to spawn. Salmon return to the streams where they themselves were born, which they recognize by smell. Acid rain and other forms of pollution can change the chemistry of their home streams, making it hard for the returning salmon to identify them.