The mouth is equipped with moderately developed canine-like teeth which are lost as the fish approaches maturity, so by spawning time it usually has no teeth at all. They spawn and hatch in fresh water and grow to maturity in the sea where as juveniles and adults they feed mainly on euphasids, a small shrimp-like crustacean sometimes called krill. As the spawning season approaches, eulachon gather in large schools off the mouths of their spawning streams and rivers. The upstream migration is closely keyed to the water temperature of the stream. May. Some streams have two separate but overlapping migrations.
Males usually outnumber the females during the spawning migration.
Eulachon generally spawn in the lower reaches of the river or stream. The eggs hatch in 21 to 40 days, depending on the water temperature. Newly hatched young are carried to the sea with the current where they feed mainly on copepod larvae and other plankton. After three to four years at sea, they return as adults to spawn. After spawning, the majority of eulachon die.