Shoaling and schooling

Lore & Background

Fish aggregations can be unstructured, such as mixed-species groups gathered near resources, or structured as shoals where fish interact socially and adjust behavior to stay close. Shoals can include disparate sizes and species. When a shoal becomes tightly organized with synchronized swimming at the same speed and direction, it is termed a school. Schooling fish are usually the same species and age/size, with precise spacing, and can perform complex maneuvers. Fish can shift between shoaling and schooling within seconds, triggered by changes in activity like feeding, resting, traveling, or avoiding predators. Forage fish, such as herring and anchovies, form immense schools that can contain billions of individuals, moving along coastlines and across open oceans. These schools serve as concentrated food resources for larger predators, including tuna, seabirds, and marine mammals. Herring schools have precise arrangements and react rapidly to predators, forming vacuoles around threats.

Reader's Guide

Shoaling and schooling are fundamental to the ecology of many fish species, shaping predator-prey dynamics and energy flow in aquatic ecosystems. The behaviors offer a trade-off between anti-predator benefits and increased foraging competition. Fish derive advantages such as better predator detection, dilution of individual capture risk, enhanced foraging success through information sharing, and higher success in finding mates. Schooling specifically provides hydrodynamic efficiency, allowing fish to move faster with less energy. The oddity effect homogenizes shoals by making standout individuals more vulnerable to predation. These behaviors are considered a classic example of emergence, where the group possesses properties not present in individuals, giving evolutionary advantages to members. Despite extensive study, the intricacies of schooling, especially swimming and feeding energetics, remain not fully understood, and some hypotheses, such as energy saving from array formation, are controversial.

Did You Know?

• About one quarter of fish species shoal all their lives, and about half shoal for part of their lives.
• Herring schools in the North Atlantic can occupy up to 4.8 cubic kilometers, with densities of 0.5 to 1.0 fish per cubic meter, totaling about three billion fish in a single school.
• Fish removed from a school show a higher respiratory rate due to stress, indicating a calming social effect of being with conspecifics.
• The oddity effect posits that any shoal member that stands out in appearance will be preferentially targeted by predators.