Few aquarium scenes are more enjoyable than a group of fish swimming in unison, changing directions in an instant, yet never colliding with each other. How are fish able to swim in such perfect unison? Why do some fish swim alone while others prefer living in schools? Is it necessary to keep aquarium fish in schools?
Not everything is known about schooling behavior, but here is what experts know about how and why fish swim in schools.
Why Fish Swim in Schools
First and foremost, schools protect fish from their enemies. It's the same rule our mothers taught us as youngsters, always stay in a group because there is safety in numbers. Predators find it far easier to chase down and gobble up a fish swimming all alone rather than trying to cut out a single fish from a huge group. The same holds in reverse. Fish can better defend their territory in a group. Bullies will think twice about facing an angry school of dozens or hundreds of fish.
It is also believed that swimming close together reduces friction and allows fish to conserve energy while swimming. When dinner time comes along, food is easier to find as a group. Having 50 sets of eyes and noses gives the school a better chance of locating food. Last but not least, when fish spawn a school ensures that at least some of their eggs will elude predators due to the sheer numbers produced by a large group of fish.
How They Swim So Close Without Colliding
A complex combination of senses allows fish to achieve those smooth schooling movements we marvel at. At one time it was believed a leader in the school directed the movements of the entire school. However, it is now known that each fish responds to the movements of the other fish, as well as stimuli such as pheromones. If one fish moves in a different direction all the others sense it and move accordingly.
The anatomy of fish also factors into the schooling equation. Eye placement on the sides of the head allows the fish to readily see what is next to them and move accordingly. However, sight is not the only factor used in schooling. Fish can establish their placement and direction in a school by using hearing, lateral line, sight, and even the sense of smell.
Do All Fish School?
It is estimated that more than 25 percent of the world’s fish species school throughout their lives and many schooling fishes spend a large portion of their lives in schools. As a rule, smaller fish are more likely to live out their lives in schools, although some large fish will school together.
Furthermore, not all fish that school do so for protection in numbers. Some of the fiercest fish in the world live in schools. Piranha fish live in large schools that they are born into. Although they tolerate their brothers and sisters, a sick or weak piranha in a school will be eaten by the others.
How Many Fish Make a School?
There isn’t a magic number that defines a school. However, in the wild schools of fish are generally quite large, often numbering in the hundreds or even thousands. In captivity, schooling fish need to have at least four to six to create a comfortable school. The adage, the more the merrier, definitely applies to schools of fish. In other words, you can’t have too many fish in a school.
Fish That Prefer Schools
Quite a few popular freshwater fish prefer to live in schools. Barbs, Danios, and most Tetras like black phantom tetras should always be kept in schools. Loaches such as the Clown and Kuhli loach often will pine away if they are not kept in a school of their kind, while the weather loach doesn't need to be in a school. Even larger fish such as the Silver Dollar prefer to live in a school.
It's a good idea to do your homework before choosing and purchasing a new fish, so know you upfront if they are best kept in a school. If the fish you are considering prefers to live in a school you should plan on purchasing at least four of the same species. Generally, it’s best if you can purchase them all at the same time. If not, purchase as many as you can at once and make sure that adding additional fish to the school at a later time is not a source of stress for the species you've chosen.
Also consider the size of the tank required to keep a school, as schools require more space for the forward swimming motion of multiple fish. A good local fish shop will advise you on what is needed.
Marras, Stefano et al. Fish Swimming In Schools Save Energy Regardless Of Their Spatial Position. Behavioral Ecology And Sociobiology, vol 69, no. 2, pp. 219-226, 2014, Springer Science And Business Media LLC. doi:10.1007/s00265-014-1834-4
Gruber, David F. et al. Bioluminescent Flashes Drive Nighttime Schooling Behavior And Synchronized Swimming Dynamics In Flashlight Fish. PLOS ONE, vol 14, no. 8, 2019, Public Library Of Science (Plos). doi:10.1371/journal.pone.0219852