Things You Didn't Know About Schooling Fish
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A huge number of different fish school. It is estimated that over half of the 25,000 or so known fish species school at some point during their lives. The incredibly widespread use of this strategy among fish species is an indication of its value.
Schooling provides individual fish with significant benefits. It reduces each individual fish's risk of predation, helping it to find food more easily and to save energy through the hydrodynamic advantages of swimming in a group.
Is there a difference between a shoal and a school?
Both words refer to a group of fish. Some people try to make a distinction between them, saying that in a school the individuals tend to be more closely knit as a unit, more polarized -- all facing or swimming the same way -- as opposed to shoaling, where the group is much less cohesive. Fish, however; do not respect this distinction, and often the division is quite blurry. In fact, the appropriate term can change from moment to moment. Thus, it has no useful biological reality, and it is unimportant for aquarists. Throughout this site we use the term "school" to indicate a group of fish of any type.
Who schools and who doesn't in the fish world?
Different fish use the schooling strategy to different extents. A few species, such as herring, are known as "obligate" schoolers, meaning they are compelled to live in the social group and will rapidly die if isolated. Most species are "facultative" schoolers, using the advantages of schooling when it suits them and disbanding at other times, say, when food arrives. Many species use schooling at some stage in their life cycle, usually when they are small and vulnerable to predators, before abandoning the strategy as they grow larger and older. Cichlids are a good example. As juveniles they live in groups, but when they mature they leave the safety of the school to establish themselves in their own territories. Most familiar aquarium fish species school in early life. Bitterling (temperate fish) lay their eggs inside freshwater mussels and the larvae school as soon as they emerge from the mantle cavity of the mollusc.
When should fish school?
Schooling works extremely well as an anti-predator strategy, but it also has costs. Living in a group means fighting many competitors to gain a share of any resource. Fish tend more to form schools when they feel threatened, for example, when a predator is around, when they move into a new and unfamiliar environment or when they are in open water with no nearby hiding places to dash into. Schooling only works against
visual predators -- those that hunt using sight. At night, fish schools often tend to break up because concentrated schools of fish actually attract hunters that use smell to locate their prey.