Shoaling and schooling

Lore & Background

Fish derive many benefits from shoaling, including better predator detection, dilution of individual capture risk, enhanced foraging success, and higher success in finding a mate. Schooling additionally increases hydrodynamic efficiency, allowing fish to move faster and expend less energy. Fish choose shoalmates based on traits such as larger shoal size, same species, similar size and appearance, health, and kin recognition. The oddity effect posits that any shoal member that stands out in appearance will be preferentially targeted by predators, thus homogenising shoals. Shoaling fish can shift into disciplined, coordinated schools and back to amorphous shoals within seconds, triggered by changes in activity such as feeding, resting, traveling, or avoiding predators. When schooling fish stop to feed, they break ranks and become shoals, which are more vulnerable to predator attack. The shape of a shoal or school depends on the type of fish and activity; traveling schools can form long thin lines, squares, ovals, or amoeboid shapes, while fast-moving schools usually form a wedge shape. Forage fish, such as herring, anchovies, and menhaden, compensate for their small size by forming schools. These schools can become huge, with herring schools in the North Atlantic estimated to occupy up to 4.8 cubic kilometers, containing about three billion fish. Herring schools have very precise arrangements that maintain constant cruising speeds, and they react rapidly to predators, forming a vacuole around a moving threat. Many large predatory fish, including tuna and some oceangoing sharks, also school.

Reader's Guide

Shoaling and schooling are fundamental behaviors in fish biology, shaping the ecology of both prey and predator species. These behaviors represent a trade-off between anti-predator benefits and increased foraging competition. The cumulative advantages of shoaling, including better predator detection, dilution of capture risk, enhanced foraging success, and improved hydrodynamic efficiency, provide strong selective inducements for fish to join groups. Schooling is a classic example of emergence, where properties possessed by the school are not possessed by individual fish, giving evolutionary advantages to members. The social and genetic functions of aggregations are supported by observations such as increased respiratory rate in isolated fish, indicating stress and a calming effect from conspecifics. Herring, for instance, become agitated when isolated and are rarely displayed in aquaria. The foraging advantages of shoaling are demonstrated by experiments showing that larger groups find food faster, attributed to many eyes searching and information sharing through behavior monitoring. The structure of schools, such as the parabolic shape of Atlantic bluefin tuna schools, suggests cooperative hunting. These behaviors are critical for the ocean food web, as immense gatherings of forage fish fuel marine predators.

Did You Know?

• About one quarter of fish species shoal all their lives, and about one half shoal for part of their lives.
• Herring schools in the North Atlantic can occupy up to 4.8 cubic kilometers, totalling about three billion fish in a single school.
• Fish removed from a school have a higher respiratory rate than those in the school, attributed to stress.
• The oddity effect posits that any shoal member that stands out in appearance will be preferentially targeted by predators.