Follow thy neighbour: how fish move in a pack

By Liz T. Williams 8 November 2011
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Schools of fish move fluidly because each individual respond to its nearest neighbour, new research shows.

LARGE SCHOOLS OF FISH move in a surprisingly coordinated and fluid manner given the number of individuals and a novel study has explained how they do it.

Australian researchers have found evidence that mosquitofish – much like human commuters – follow very simple rules of attraction and repulsion as they navigate a crowd.

The key is how a single fish interacts with its nearest mate.

“We found evidence that individuals are only responding to one neighbour – their closest neighbour – at any one time,” says James Herbert-Read, lead author and PhD student at the University of Sydney. Instead of being concerned about what the group is doing as a whole, the fish only moves in relation to its neighbour.

“They don’t systematically align with their neighbours – we don’t find any evidence of that,” James says.

Schools of fish: collective individuals

This study is the first of its kind to gather data on how fish navigate in schools. While researchers have developed mathematical models to try to understand how animals make collective decisions, most models have assumed that animals move based on the trajectories of several neighbours – not just the closest one.

The new study shows that mosquitofish strive to maintain about two body lengths of space between themselves and their nearest neighbour, James says.

By trying to stay within a fixed pace of their nearest neighbour – not unlike the way most drivers navigate a crowded freeway – an individual naturally responds to collective changes in direction and speed.

In this way, shoals of mosquitofish move almost as one collective unit, reacting in the face of danger or a change in current as if they shared one mind. This collective motion is seen as one of the benefits of travelling with a group.

“We see these patterns occurring every day in nature,” James says. “Understanding how these individuals do this is really quite neat, and now we’re actually starting to learn that these simple rules can drive these seemingly very complicated patterns.”

However, it’s still unknown how the fish at the front make decisions on which direction to go.

Group communication better for everyone

The patterns are a vivid illustration of a very simple form of group communication – one which presents advantages for everyone involved.

“You get these really functional benefits of belonging to a group,” James says. “You can gather information more quickly…Using very simple rules, this information can be passed between group members.”

Prior to this study, a variety of theories about how animals moved collectively existed. But until now, verifying those theories with data has proven difficult.

“In theory, there are many interaction rules that can lead to groups moving in unison, but…it is crucial for us to be able to determine what real animals do,” says Dr Jerome Buhl, a research associate at the University of Sydney who studies collective motion in locusts.

How animals move in packs

As this study has shown, observations of real animals may lead to some unexpected conclusions.

“There are two surprises,” says Professor Jeff Moehlis, an engineer at the University of California, Santa Barbara who studies biological dynamics. “One is that the fish don’t seem to pay much attention to the orientations of their neighbours. The other is that they only seem to respond to their single nearest neighbour. It seems amazing that collective motion is possible with such limited information shared between the fish.”

Jeff says the results of the study are likely to affect the mathematical and computational models used to study the collective behaviour of animals.

While these findings only apply for mosquitofish for now, this research may help shed light on how starlings flock or insects swarm, if only from a theoretical perspective for now. Plans are already in the works to test out some of these ideas on other species of fish.

VIDEO: A flock of starlings crowds the skies above two kayakers. Flock birds may move in the same way do as schools of fish.