Networks Unit
IMT School
for Advanced Studies
Lucca (Italy)


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Rules of interaction of shoaling fish

Some of the most mesmerising sights in the natural world are seen in the collective motion of fish schools and shoals. In the ocean, vast schools of hundreds of thousands of fish can form which then move together in unison. The shoals we see in our local aquaria may be smaller but they nonetheless are impressive examples of how a group can move together without a leader. What are the rules by which individual fish interact with each other to move together and stay cohesive over time?

We have studied groups of a small lake-dwelling fish, known as the mosquito fish (Gambusia holbrooki). The video below shows how the fish are individually tracked to follow their position and direction of movement.

A simple way of studying how a fish responds to its neighbours is illustrated in the video below. The video is exactly the same as the previous one, but here all the frames have been rotated to follow the movement of the fish marked in red. The colours of the other fish reflect their distance from the red fish: the green fish is the nearest neighbour etc. By looking at where the other fish of the shoal are when the fish in red speeds up, slows down, or turns to a particular direction we can learn what "rules of motion" the fish follow when schooling together.

The rules turned out to be strikingly simple but effective. Fish try to catch up with other fish in front of them, but they slow down when they get too close. Contrary to the predictions of some simulations, the fish didn’t turn their bodies to line up with each other. Fish also appear to react primarily to only their nearest neighbour. Through these simple rules, shoals of mosquito fish move in a co-ordinated fashion.

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