Which jawless fish migrates up rivers to spawn, after which the adults die?

The jawless fish that migrates up rivers to spawn, after which the adults die, is the sea lamprey.

Sea lampreys (Petromyzon marinus) are anadromous: they spend part of their lives in the sea or large lakes, then move into freshwater rivers to reproduce. Females lay eggs in gravel nests, or redds, prepared by males in streams with suitable currents. After spawning, both adults die naturally.

Their life cycle is unusually divided between habitats. Larvae burrow into soft stream sediment and filter-feed on plankton and organic debris for several years. After metamorphosis, young lampreys migrate downstream and develop a feeding stage in which they attach to fish with a suction-cup mouth, rasp tissue with their tongue and consume blood and body fluids.

A common mix-up is calling lampreys fish in the ordinary jawed sense. They are fish, but they belong to an ancient jawless lineage. Their post-spawning death also differs from the life cycle of many familiar migratory fish, which may survive and spawn repeatedly.