There are few sights more mesmerizing than a large school of fish moving in unison, as if they were one organism. As they all suddenly change direction, you might wonder who is in charge, and who decides where and when to go? How do they stay so close together? Why do some fish school and others prefer solitary life?
Why do fish school?
We do not understand all the reasons fish school, but scientists believe one of the main reasons they do it is for protection – safety in numbers. A predator is less likely to attack several hundred or thousands of fish than one or two individuals. It is also easier to find and secure food with more eyes and mouths devoted to the task, which is why Piranhas are thought to travel in packs and saltwater tangs forage on algae in large groups. Schooling also improves a fish's ability to find a mate and successfully reproduce, especially for species that do not practice any sort of parental care. Eggs are more likely to be fertilized in a large group, and with so many eggs produced, at least some are likely to escape predation by other fish. Finally, schools improve the hydrodynamics of each individual fish, making it easier for them to move through the water, thus conserving energy.
Schooling or shoaling?
You might be wondering if the correct term is schooling or shoaling and if there is a difference. Both terms describe fish behavior, but they are technically two different things. Fish that socialize or hang out in loose groups in the same place are said to be shoaling. They can be different species or all the same. Schooling is when they all move together in the same direction, at the same speed, at the same time. Schools are usually made up of a single species.
A school of fish is usually going somewhere, but if they stop to feed, they become a shoal. Obligate schoolers and shoalers spend all their time in the group and become highly stressed if they become separated. Facultative schoolers or shoalers only come together some of the time for a specific purpose, for example to spawn or feed. They may then go off on their own for periods of time.
How does the school stay together?
Fish use a combination of senses to keep the school together. Vision is thought to be key, although sight, smell, the lateral line and the use of pheromones all help fish to determine where they are in the school and what the school is doing. There is not a leader, rather, each fish stays "in communication" with its neighbor and if one fish turns or speeds up, the others react in kind.
Each fish in a school has two zones: the "zone of repulsion", which helps them keep an equal distance from each other, and the "zone of orientation", where each fish tries to do what the fish next to them is doing. When the school is moving, each fish tries to match their neighbor; when the school is stationary, each fish tries to stay close to its neighbor.
Aquarium schooling fish
While many types of aquarium fish are known to form schools at certain times in their lives – primarily as newborn fry, for safety reasons – tetras, barbs, danios, rasboras and rainbow fish, as well as certain types of loaches and Corydoras catfish, are considered true schooling fish as adults. They should be purchased in groups of at least 6 individuals and even more where tank size permits. Long, wide aquariums that offer plenty of swimming distance are preferred over tall, narrow ones.
Many beginning hobbyists understandably want a variety of fish in their aquariums and will buy one or two individuals of a wide selection of species. In truth, however, fish from the groups mentioned above will be happier, healthier, less stressed and more colorful when purchased in larger numbers. They will be less skittish, less inclined to hide in the shadows, and the visual effect of a large group of fish, all moving in unison, is well worth it!
So, the next time you are at your local aquarium store, instead of buying two or three Neon Tetras or Tiger Barbs, why not get a dozen, or 25 if your tank is big enough? Seeing a wall of color moving about the aquarium as one living organism is a sight you will not get tired of watching!
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