Activity characteristic of clupeiform fish ( herrings , anchovies , and allies) in which many fish swim together, appearing to act as a single organism.
A school of herring may contain many millions of individuals of roughly similar size. Fishes above or below the size limit break away and form schools among themselves. The primary advantage to the fish seems to be safety for the individual. When threatened, a school of thousands of anchovies, spread over several hundred metres, will contract to a writhing sphere only a few metres across, thereby thwarting the attempt of a natural predator to catch a single individual.