Animals

Here’s How Schooling Fish Coordinate Their Graceful, Mesmerizing Movements

The seamless, collective movement of schooling fish, as well as flocking birds, is advantageous to a group, protecting it from predators and helping it find food.

A school of herring migrate at high speed to their spawning grounds in the Baltic Sea. | Wikimedia Commons
A school of herring migrate at high speed to their spawning grounds in the Baltic Sea. | Wikimedia Commons

Humans tend to live in hierarchies, which rely on good leadership that ideally benefits the collective whole. There is little doubt that such systems are subject to frequent flaws, given that leaders may be vulnerable, corrupt, or self-interested. Even the most popular leaders have their detractors.

But most species of schooling fish and flocking birds do not have such problems. New research published in the journal PLOS Computational Biology finds that schooling fish switch their attention from neighbor to neighbor for seamless collective movement. The findings are helping scientists to design leaderless, self-organizing systems, such as swarms of drones.

The study focused on a common aquarium fish known as the rummy-nose tetra. When schooling, they can almost look like a single organism, given their coordinated movements. Each individual fish, however, has its own unique personality that can lead to relationship drama.

"While the movement of the fish gives the impression of the school as being very similar to a single organism, there are nonetheless conflicts within these groups: Rummy-nose tetras will still fight over food, or when mating," co-author Andrea Perna of the University of Roehampton in London told Seeker.

Despite the problems, fish do not drop out of "school."

"Schooling is advantageous to each individual fish because they are less likely to be eaten by predators when they are in a school and because they can get information about food and about the environment from their neighbors,” she continued. “This is probably the main reason why they each individually decide to keep staying in the school."

A spontaneous change of swimming direction by a single fish triggers a collective U-turn in a group of rummy-nose tetras (Hemigrammus rhodostomus). | Jiang et al.

Schools of these fish can zip all over the place, making them challenging to study. To overcome that obstacle, the scientists had small groups of tetras swim in a ring-shaped tank so that the fish's directional shifts would be obvious. The tetras could only go in two directions: clockwise or counterclockwise.

The researchers paid special attention to what happened during U-turns — whenever the fish would suddenly switch their direction of movement. They noted how each fish reacted to the movement of its neighbors and measured the brief time delay that occurs when a fish detects what its neighbors are doing and its application of that information to its own swimming behavior.  

"Those delays define whether and when a fish has responded and thus has been influenced by another individual," co-author Luca Giuggioli of the University of Bristol explained to Seeker.

The scientists further developed a computational model to rule out any individual fish movements that might have just occurred by chance.

The analysis revealed that tetras follow a limited number of influential neighbor fish that are not necessarily the ones closest to them.

Giuggioli indicated that humans often behave in a similar way. When people are skiing downhill, for example, one skier may be faster, yet farther away from others. In this instance, he said some of the skiers who may unwittingly collide with the faster one could benefit from paying attention to the speedy individual.

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Senior author Guy Theraulaz of Université Paul Sabatier in Toulouse, France, added that no single fish consistently makes decisions that the group follows. As a result, schooling fish have no long-term, defined leader. 

"When a fish is incidentally a leader," Theraulaz said, "the average number of its neighbors decreases significantly and, after a certain period of time, this lack of perception of other fish triggers a U-turn, suggesting that each fish wants to stay in contact with other fish."

The same phenomenon happens in flocks of starlings: Birds that initiate changes in collective traveling direction are found at the edges of the flock. It’s true then that it can be lonely at the top — and edges — given that those in the middle have plenty of others to follow and rely upon.

The researchers believe that fish and birds are mostly responding in a conscious way to the movement of others. Their quick adjustments, in other words, are not just reflex actions. Should an even faster response be needed, however, a reflex response could occur.

This is similar to a person driving, paying basic attention to the road and other drivers, and then suddenly having a rock hit the windshield. In that moment, a reflex action could cause the driver's head to move and eyes to close.

Although approximately 50 percent of all fish species exhibit some form of schooling, it is rare to find groups that can coordinate their movements as smoothly as tetras do. Even "properly swarming insects are rare, including bees and locusts," Perna said, "and their swarming rarely depends on rapid alignment among close neighbors."

This ability to align is part of "swarm intelligence," which researchers are working to apply to robotics.

Dozens of small off-the-shelf drones sit idle while program officials conduct system checks to prepare them for flight preceding an evaluation. | US Army photo by John Hamilton

Theraulaz explained: "It is a major challenge in collective robotics to design swarms of autonomous drones that are able to self-organize, sense their environment, coordinate their movements and cooperate to perform collective tasks in real-world situations such as, for instance, monitoring forest fires or rescuing in emergency situations."

He said the behavioral interactions used by fish schools and bird flocks to coordinate their movements and to collectively process information are inspiring methods to control collective motion in swarms of drones. In particular, the information may be applied to drone sensors and communication systems.

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Drone swarms — whether aerial, terrestrial, and aquatic — could be used detect enemy planes or vessels and even wage an unmanned attack. They might also be used to search and destroy land mines or bombs or deliver supplies. The US Navy and the Pentagon's Strategic Capabilities Office are just two groups that are exploring related technologies.

The intelligence of drone systems lags behind the impressive, coordinated skills of schooling fish and flocking birds.

Theraulaz said that while he and his team have characterized the individual-level interactions of schooling members and the influence on them by neighbors, they still do not precisely know what sensory-motor mechanisms individual fish are using to decide their movements.

"As our knowledge on the way these biological systems collectively process information becomes more precise,” he said, “it could be used to design new distributed control algorithms in swarm robotics."

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