Shoaling and schooling

Groups of fish that stay together for social reasons are called shoaling. When these groups swim in the same direction in a coordinated way, they are schooling. About one quarter of fish species shoal all their lives, and about one half shoal for part of their lives.

Fish have many reasons for staying together. Being in a group helps them spot danger more easily, makes it less likely any single fish will be caught, and improves their chances of finding food and mates. Staying together can also help fish save energy by moving more efficiently through the water.

Fish choose their group members carefully. They usually prefer larger groups, members of their own species, and others that look similar to themselves in size and appearance. Healthy fish are also preferred, as they make the group stronger. This careful selection helps keep the group safe and well-organized.

Overview

An aggregation of fish is any group of fish that have come together in one place. These groups can be loose or very organized. When fish come together in a social way, they are shoaling. Shoaling fish stay near each other but can swim and find food somewhat on their own.

When a shoal becomes more organized, with all the fish swimming together at the same speed and in the same direction, they are schooling. Schooling fish are usually the same type and age. They move together in precise formations and can change direction as a group. Some scientists think schooling helps fish stay safe, find food, and travel better, while others think it can make it harder to get enough food or oxygen.

Social interaction

When fish stay together in a group, it helps them feel safer and less stressed. Experiments show that a fish taken away from its group becomes more nervous and breathes faster. Being with other fish of the same kind seems to calm them down, which is why they prefer to stay in groups. For example, herring become very upset if they are alone and are rarely shown in aquaria because they don’t do well without their group.

Foraging advantages

Further information: Forage fish § Hunting copepods

Fish that stay together in groups may find food more easily. Studies show that larger groups of fish, like minnows and goldfish, can find food faster than smaller groups. Bigger groups of fish can work together to hunt, making it easier to catch their food.

Ocean currents and special areas near the coast, such as where deep and shallow waters meet, create rich feeding spots for small fish that eat tiny sea creatures called copepods. These copepods are small but very quick, and fish like herring have special ways to catch them by working together in groups.

Reproductive advantages

Further information: Fish migration

Fish groups can help with finding mates because it is easier to find one in a group. For fish that travel long distances to have babies, moving together as a group may help them navigate better than traveling alone.

Some small fish, like capelin, travel between places where they have babies, eat, and raise their young. For example, some herring travel from southern Norway to Iceland and back. These trips help them find food and places to have their babies.

Hydrodynamic efficiency

When fish swim together in groups, they might save energy, similar to how cyclists ride closely to save effort. Geese flying in a V-shape also benefit from the air movements created by the birds in front of them. Scientists believe fish schools and even groups of Antarctic krill could become more efficient swimmers this way.

Experiments show that fish in groups can use up to 20% less energy than when they swim alone. The fish at the front of the group often get to food first but then move to the back after eating, likely because digestion makes it harder for them to keep up with the group.

Predator avoidance

Fish that stay together in groups, called shoals, and those that swim in the same direction in a coordinated way, called schools, often do this to stay safe from animals that want to eat them. When fish are separated from their group, they are more likely to be caught.

One idea is that when many fish move together quickly, it confuses animals that want to eat them. It is hard for these animals to pick out just one fish from a big, moving group. Another idea is that with more fish in a group, each fish can help watch for danger, giving the whole group more time to eat and stay safe. Also, when a group of fish is large, any animal that tries to eat them will catch fewer fish, so there is safety in numbers.

An amazing example of this happens during the African sardine run. Millions of sardines swim together along the southern coastline of Africa. They travel in huge groups, sometimes 7 kilometres long, and many animals like sharks, dolphins, and seals follow them. When threatened, the sardines quickly group together into big balls of fish called bait balls, which can be up to 20 metres across. These bait balls only last a short time but help protect the sardines from being eaten.

Predator countermeasures

Further information: Antipredator adaptations

Predators have many ways to catch fish that stay together in groups. For example, the sailfish uses its sail to look bigger and guide fish into a spot where it can catch them. Swordfish speed through schools of fish, using their long noses to hit and stun them. Thresher sharks swing their tails to hit and daze fish before eating them.

Dolphins also work together to catch fish. Some dolphins act as guides, pushing fish into a tight group while others wait to catch them. In South Carolina, dolphins sometimes push fish onto shore to make catching them easier. During events when many fish gather, called the sardine run, dolphins, birds, and even whales take turns catching fish in these crowded groups. Some whales use a special way of feeding called lunge feed, opening their huge mouths wide to catch lots of fish at once.

How fish school

Fish schools swim together in very organized ways. Some fish, like herrings, can move quickly and change directions without bumping into each other. Even though it looks like they are following a plan, they are not—they just react very fast to each other. Young fish start by practicing with just one friend and then join more as they get better. They are born knowing how to do this, they don’t learn it from older fish.

To stay in a school, fish need to see well. Most fish stop schooling when it gets dark and just stay close together instead. This shows how important sight is. Fish that school usually have special marks or stripes that help them keep their place. Even without these marks, they can still school, but it might not be as smooth.

Fish also use other senses. They have a special line along their sides called the lateral line that helps them feel movements and vibrations in the water. Experiments show that when this line is removed, fish swim closer together, suggesting it helps them stay the right distance apart. This system uses tiny hair cells, similar to those inside our ears, to sense changes in the water.

Describing shoal structure

Further information: Collective animal behavior

It can be hard to study how fish groups look in real life because there are so many fish. Scientists use special tools like fisheries acoustics to learn more.

When scientists study groups of fish, they look at things like:

• Shoal size – This is how many fish are in the group. Some big groups near North America have "tens of millions" of fish like Atlantic herring, scup, hake, and black sea bass, stretching for many kilometers.
• Density – This tells us how close the fish are to each other in the group.
• Polarity – This shows how much the fish are all pointing in the same direction.
• Nearest neighbour distance – This measures the space between each fish and its closest friend in the group.
• Packing fraction – This helps scientists understand how organized the fish are in the group.
• Integrated conditional density – This looks at how evenly spread out the fish are.
• Pair distribution function – This helps describe how the fish are arranged in space.

Modelling school behaviour

Further information: Collective animal behaviour and Swarm intelligence

Scientists study how fish schools behave by using math and computer programs. These programs give fish three simple rules to follow: move in the same direction as others nearby, stay close to them, and avoid bumping into each other. One famous program called boids was made by Craig Reynolds in 1986. Another model, called the self-propelled particle model, was introduced in 1995. Some newer models change these rules a little, using different areas around each fish. Very close by, fish try to move away to avoid crashing. A bit farther, they try to match the direction of their neighbors. And at the farthest distance they can sense, they try to move toward someone.

Some scientists have even created models to understand why fish might have developed schooling behavior over time. These models use special computer programs to simulate how animals change over many generations, helping us learn why schooling might help fish stay safe.

Mapping the formation of schools

In 2009, researchers from MIT watched a large group of fish come together for the first time. They studied Atlantic herring near Georges Bank. The fish gathered in the evening from deeper water. When there were enough fish close together, they suddenly started moving in the same direction, like a big, organized group. These groups then traveled up to 40 kilometres (25 miles) to shallower water to lay eggs at night. In the morning, they returned to deeper water and spread out again. The researchers also found that small groups of fish could guide much larger groups.

Leadership and decision-making

Fish schools need to make choices to stay together, like which way to swim when they see danger or where to find food. They use something called quorum sensing to decide together. This means that once enough fish start doing something, more will follow. Studies show that small groups of fish make group decisions by watching what others do. This usually leads them to the right choice, but sometimes they make the wrong one. Bigger groups make better choices because they can learn from more fish.

Some fish lead the group when looking for food. In tests with golden shiners, a few experienced fish often lead because they know where food is. Even if all fish know the same thing, some still end up in front more often, acting as leaders. Smaller fish might go to the front because they are hungrier, while those worried about danger stay more in the middle.

Shoal choice

Scientists have done simple experiments to learn why fish choose to stay in certain groups. They put a fish in a tank between two other tanks with different groups of fish and watch which group the fish spends more time near.

Fish usually like to join bigger groups because larger groups can help protect them from danger. They especially like bigger groups when danger is nearby or if they depend more on group living than on their own defenses. Bigger groups might also find food faster, but the food must be shared among more fish. Hungry fish might prefer smaller groups because they want to eat more.

Fish also like to stay with fish of their own kind. Even if different kinds of fish are mixed together, they will move closer to their own species when they sense danger. Fish prefer groups where the other fish are about the same size as themselves, as this makes it harder for predators to single them out. Some fish might even join a group of another species if the fish in that group are closer to their own size.

Fish like to stay with fish they already know. This has been shown in many types of fish, like guppies, sticklebacks, and minnows. Fish also prefer groups where the other fish are healthy and well-fed rather than sick or hungry. Some fish, like sticklebacks, prefer bold companions, while others, like angelfish, prefer quieter ones. Fish also like to join groups that are actively looking for food, as this increases their chances of finding meals.

Commercial fishing

Fish that stay together in groups are often caught by big fishing boats. These boats use planes to find groups of fish like tuna and cod. Then, they use special nets to surround and catch the fish quickly.

Further examples

Blacksmith fish live in loose groups. They have a special friendship with senorita fish, which eat parasites off their skin. When they see a group of senorita fish, the blacksmith fish form a tight ball and hang upside down while the senorita fish clean them.

Some groups of fish work together to scare away predators. For example, bluegills sometimes chase away snapping turtles.

Piranha are often thought to hunt in fierce groups, but studies show they are actually quite scared and group together for safety. They join together to protect themselves from bigger animals like cormorants, caimans, and dolphins.

Humboldt squid are large sea animals that move together in big groups of up to 1,200. They swim very fast and hunt together, working as a team to catch smaller fish, crustaceans, cephalopods, and copepod. They are known for eating quickly and sometimes even attack other squid from their own group.