Freshwater Fish Must Not Only Find, But Often Fight for Food

Although schools work mostly to defeat predators, the combined senses of many individuals in the group also prove to be excellent at locating food patches in their environment. The result is that fish in schools find food a great deal faster than if they were hunting alone. And there are other benefits to schooling. For example, fish in traveling schools also save considerable amounts of energy by swimming in the slipstream of leading fish. But set against all these advantages, there is one major cost: competition. Fish in schools live among their greatest competitors. Although schools are good at locating food, there is rarely enough to go around for all the school members, and this means that all but the most dominant are left hungry. This can be a significant cost and, as a result, fish must weigh the pros and cons of schooling according to the situation.

How do fish decide when to school and when to go it alone?

In each situation they experience, schooling fish are fairly well able to assess the risks. If danger is lurking, fish very seldom actively choose to leave the safety of the school. If a situation does not appear to be especially risky, and if they are hungry, fish may venture away from the group to try to find some food for themselves. A great deal of research has been done on the habitats occupied by guppies in their home streams of Trinidad in the Caribbean. Some of these streams are also home to pike cichlids and blue acaras, which like nothing more than to feast on the small livebearers. In such streams the guppies form extremely tight, cohesive schools as a defensive measure. Other populations of guppies that live free from the depredations of the cichlids have all but abandoned schooling behavior.

How can fish get an advantage over their schoolmates in the scramble for food?

To find food, schooling fish can either seek it out for themselves or keep a very close eye on the other fish in the school. If they detect that another fish has found a food patch, they will often home in and stake their claim to a share. This kind of exploitation of others is sometimes referred to as “informational parasitism.” And it is not just members of the school that look out for feeding fish; predators keep close tabs on them as well and often concentrate their attacks on the feeders who may be far less vigilant because they are distracted by their feeding.

Do schooling fish ever become aggressive with each other?

Schooling would be a poor strategy if the fish in a group spent all their time fighting. As a result schooling fish are usually extremely peaceable. But this apparent harmony can occasionally vanish. If a school comes across a rich and concentrated food source, it is a case of every fish for itself. Zebra danios are among the least aggressive of all community fish, but studies have shown that their peaceful nature can be easily disrupted when there is a scramble for food. Slow-motion film shows that they nip, block, and charge one another as they try to obtain the greatest possible share for themselves.