Fish may not be known for their educational achievements but they are very big on schools. Not the type we are familiar with of course, but their reliance on schooling literally saves their life. The terms schooling and shoaling are common terms when referring to groupings of fish swimming together. But shoaling describes the social behaviour of fishes that swim in groups, and schooling is the specific coordinated manner in which they swim. The terms are used interchangeably and quite loosely, but there are important differences in their meaning. We have all seen large fish groups swimming and seamlessly changing direction together, moving like mercury. But just why and how do they do it?
Around a quarter of fish, shoal all their lives and a half for part of their lives. Some groups can form mixed species. Although they relate to each other in the group, they may feed independently. As a shoal becomes larger, more organised and grouping are tighter; fish synchronise their swimming positions so that the shoal is moves as one, and this is what is meant by the term “schooling”.
Shoaling With Benefits
Shoaling is a form of collective animal behaviour and there are costs and benefits of such group membership. And it is not just fishes that use shoaling, it happens in flocking birds, herding cattle, pods of dolphins and whales. But just why do they do it? There is of course the theory of safety in numbers, and indeed this is perhaps the largest draw card, but there is a host of other benefits to a group living together in a shoal. Research has shown that fish removed from them shoal suffer higher stress levels. In much the same way that humans like to socialise. And this is a powerful motivation for remaining in such coordinated groupings. Shoals may serve further serve a reproductive function as they increase the chances of finding mates for individuals.
The safety in numbers theory relies on the predator confusion effect. When faced with a large number of animals together, it is extremely difficult for a predator to choose a prey. Couple this with the undulating and alternating movement of the groups and there is clear visual confusion. And a large shoal may look more like a large animal than lots of smaller tasty bites to a would be predator. The next part of the safety in numbers theory is what is called; the more eyes effect. In large groups the job of keeping watch for danger is spread amongst many individuals and is therefore more effective. Not only is there better vigilance for all members of the group, but it then provides more feeding time for individual fish, safe in the knowledge that there are many eyes still on the lookout for danger.
Studies also suggest that the shoal assists in saving energy, by fish dragging each other along in their wakes. The tiny currents set up as they use their fins, reduces water friction of their shoal mates. But there is a down side to such close group living, and build-up of excreta and oxygen depletion in the immediate environment as well as increased competition for food, are a cost to the fish. The considerable advantages though outweigh them.
How Do Fish Stay Together?
For a fish living in a shoal there are three rules; move in the same direction as your neighbour, stay close and avoid collisions. This is quite an achievement for any animal. For a fish, there are two critical senses; vision and signals relayed through its sensitive lateral line. Sight is critical for fish and having eyes at the side of the head allows them to see the position and behaviour of other fish. The lateral line; acoustico lateralis, is a specialised group of cells running down each side of the body. It is incredibly sensitive to movements and water displacement. Some species lack this lateral line and rely on eyesight.
How fish decide what direction to follow is still up for debate. Fish appear to use what is termed a “consensus decision making model” where fish watch the decision of others before making their own decisions, as group size increases, say the model, the fish make more accurate decisions. But shoaling and then schooling behaviour is not something fish appear to do immediately. As young fish, the majority of fish species do not appear to form shoals. As they grow and mature, fish swim in pairs and then graduate up to increasingly larger groups. It seems that in some ways the coordinated schooling behaviour is learned, although studies show that as young fish grow, the sense organs that are involved in schooling behaviour strengthen and enable fish to participate in shoals and to school.
Shoals of fish can be immense. Herring for example can form schools up to an estimated 4.5 cubic kilometres, about 3 billion fish to one school, all moving together across oceans. We are though more likely to encounter smaller groups. Schooling fish are commonly encountered and taken for granted, but the science behind their remarkable behaviour is still not fully understood.
Sarah Curran-Ragan - Science journalist and marine biologist, Sarah Curran Ragan, keeps you informed about our Oceans.
