In terms of biomass, what is the world's largest synchronous migration?

In terms of biomass, the world's largest synchronous migration is diel vertical migration.

This daily movement occurs mainly in oceans and lakes. Organisms such as copepods and other zooplankton rise toward surface waters at night, where food is plentiful, then descend into deeper, darker water during daylight. Darkness reduces the risk of being seen by visual predators.

The phrase “diel” means a 24-hour period, so this is not a seasonal journey between distant locations. It is a coordinated vertical commute repeated every day by immense numbers of organisms. Examples also include squid, fish and other planktonic animals.

Diel vertical migration is important beyond animal behaviour: it helps move carbon from surface waters into the deep ocean. It is sometimes confused with a single-species migration, but the phenomenon spans many unrelated aquatic groups moving on the same daily cycle.