How many fish may be found in a single school of herring or anchoveta?

Schools of clupeiform fish, such as herring and anchoveta, are remarkable for their size and coordination. It is estimated that a single school may contain many millions of individuals. Some authorities specifically assert that a single school can consist of as many as three million fish.12

These schools function as a single, highly coordinated organism, providing significant survival advantages, particularly through protection from predators. When threatened, these massive groups can contract into tight, dense spheres to thwart attacks. The individuals within these schools are typically of a similar size, as fish that fall outside of a specific size range will break away to form their own separate schools.12