How Schooling Works Among Freshwater Fish

Schools of fish can consist of just two or three fish or over a million. Some species form huge schools, such as the grey mullet in the Caspian Sea, whose schools may extend for an incredible 60 miles (100 km). But irrespective of the species in question or the size of school it forms, the basic principles about schools are the same. What looks to be an extremely complicated and choreographed behavior is in fact beautifully simple and this simplicity underlies its success.

How do fish form schools?

Individual fish gravitate together and are held there by the forces of social attraction. Among fish such as tetras, this attraction exerts an extremely powerful pull; individuals are strongly drawn to conspecifics and even to individuals of other closely related species. If a school of fish is attacked and the members of the school scatter, individuals may find themselves isolated Solitary fish become highly stressed as a result of their segregation and urgently try to locate others by sight or using longer-range chemical cues. If and when they detect these cues, they rapidly home in on their source and rejoin those fish until eventually the entire school reforms. Similarly, schools break up as darkness falls each evening, but reform at dawn every day as individual fish seek out and stick together with conspecifics.

How close do fish within a school get to one another?

From the tiniest tetras to the huge ocean-going tunas, schools of fish almost always maintain a steady distance between themselves and their nearest neighbors. This distance relates to the length of the fish. Under normal circumstances, fish in schools tend to keep between two and three body lengths apart. Thus, a school of 1-inch (2.5-cm) long fish will keep a distance of 1.5 to 2 inches (4 to 5 cm) between themselves and their nearest neighbors. These distances are governed by the situation that the fish find themselves in. Nearest neighbor distances increase when fish are hungry and scouring their habitat for food, and decrease when fish are threatened or swimming against a fast current causing the school to coalesce.

How do fish keep their schools together?

Fish use all their sensory abilities to keep tabs on their schoolmates. The sense of smell is important for recognizing school members and locating the school if it moves out of sight. The sensation of touch, via the lateral line, is essential if fish are to polarize and move coherently with the rest of the school.

However; the most important sensory input comes from the eyes. Although blind fish are capable of schooling, fish primarily use vision to maintain contact with the school. Many schooling species, including X-ray tetras (Pristella maxillaris), signal to each other using bold bars, often sited on their dorsal or caudal fins or along their bodies. In the case of X-ray tetras, the fish flick their banded dorsal fin to help keep the school together and show an increase in their rate of fin-flicking when they feel threatened. Research on this also found that X-ray tetras without the bars on their fins were much less socially attractive to conspecifics, causing schools to be less coherent