Background Essay: Beneath the Waters of Cocos Island

There is safety in numbers, as the saying goes, and fish seem to understand this concept instinctively. Countless species of fish gather and swim in large groups, called schools. Many of the species that congregate around Cocos Island -- far off the western coast of Central America -- are no exception, forming huge schools that move with the coordination of one large individual.

To humans, though, swimming in such amazing synchrony looks like an invitation for trouble. After all, a large school is far more conspicuous and enticing than a single, small fish, and a predator lucky enough to find such a large group could seemingly eat its fill with very little effort. So why do so many species of fish form schools? What are the advantages and disadvantages for the individual and for the species?

Many scientists agree that defense is the most likely reason why fish form schools (and why birds fly in flocks, rodents live in colonies, and so on). There are several explanations for how schooling behavior protects prey fish from their predators. First, large groups inevitably have a better chance of detecting approaching predators, simply because there are more eyes keeping watch. Second, individual fish swimming in a group can effectively use other members of the group as predator shields. And third, although it is impossible to know exactly what predators experience, it seems likely that a predator attacking a large school would find it difficult to focus on any one individual when members of a large group scatter in many different directions.

So even if most fish in a preyed-upon school get eaten, it is likely that some will get away alive and pass the genes that encourage schooling behavior on to the next generation. In this way, schooling helps protect the species, even if it can't always protect the individual.

Despite the protection that schooling provides to prey fish, ocean predators, including large fish and whales, occasionally get the best of schools of prey by hunting in schools of their own. Cooperating with one another, predators can cut off the escape routes of their prey, keeping them in a group, and taking one individual after another until not a single fish is left. In this way, predators have evolved, and in some cases learned, to use their prey species' schooling behavior to their own advantage. Typically, though, a prey fish is far better off than an individual swimming the open ocean alone, and so, the behavior persists.