Why are clupeiform fish in a school usually similar in size?

Clupeiform fish, such as herrings and anchovies, naturally form schools consisting of individuals of roughly similar size, which are often of the same age group.12

This uniformity occurs because fish that fall outside the size range of a particular school will break away to form or join other schools composed of individuals more similar to themselves. While the exact adaptive value of this behavior is not fully understood, the primary benefit of schooling is the safety of the individual. By swimming in a tight, coordinated group, these fish can act as a single, larger organism, which helps thwart predators that might otherwise target a lone fish.12

Additionally, this natural sorting process is a matter of convenience for the fishing and canning industries, as the fish effectively organize themselves by size within their schools.2