The Surprising Diversity Among Schools of Fish

It’s all too easy to imagine a school of fish—or even a looser congregation, called a shoal—as a single organism. After all, its members streak past in tandem, and in dazzling patterns. Species such as the three-spined stickleback do seem to cooperate to achieve shared goals, such as sizing up predators. But that’s not to say that shoal mates operate with a watery hive mind. It turns out that the dynamics of these groups are pretty complex—there’s wondrous variation between them, and even within a single swimming cohort.

Researchers at University of Cambridge’s Department of Zoology studied groups of stickleback fish to see how leadership and coordination emerged. For their study, newly published in the Proceedings of the Royal Society B, researchers divided wild-caught fish into 25 groups with five fish apiece. Since the scientists wanted to gauge how individual and collective behaviors emerged, it was important that the members of one of these impromptu shoals didn’t have a chance to huddle up beforehand—they weren’t drawn from shared tanks.

The researchers then studied how the fish behaved in three distinct conditions: an open field (essentially, an empty tank), water speckled with food, and an environment dotted with both food and plant cover. The scientists filmed the shoals from above to measure the behaviors of individual fish—such as how quickly they motor through the water, whether they break ranks, and how far apart they disperse—and to detect patterns that emerged from the larger group.

Marked differences became clear: Some groups darted faster, and with more cohesion, and others showed more clearly defined leadership structures. These differences didn’t seem correlated with factors such as age and size—it wasn’t that the larger, older fish had more leadership qualities, for instance. “Our research reveals that the collective performance of groups is strongly driven by their composition, suggesting that consistent behavioral differences among groups could be a widespread phenomenon in animal societies,” said coauthor Andrea Manica, in a statement.

Humans borrow the image of schooling fish as shorthand for blending in with the crowd, or going with the flow. But studies like these show that scientists have a lot more to learn about the secrets of synchronicity.