It can be disturbingly disorientating to find yourself in the middle of a dense shoal of fish, as Mark Webster knows. Why do fish go in for formation dancing, anyway?
Just the thought of a dive at Ras Mohammed in the Egyptian Red Sea is enough to send the pulses of most divers racing, but when you are actually hanging over that blue abyss, the feeling is greatly magnified. I come here regularly in the summer to connect with the vast shoals of fish which congregate as the water temperature rises Today, I am swimming away from the famous wall towards another which is composed entirely of fish. These are red snappers, not a small fish, and they all seem to be glowering at me as I approach. When I am within reach, the nearest fish reluctantly begin to part and suddenly I am engulfed by this massive shoal which closes behind me, while each individual fish just keeps its distance. As I begin to compose pictures, I realise that I am beginning to lose my sense of direction within the school. I am moving with the fish and also with the current, and the only indication of an increase or decrease in depth is the pressure on my ears. It's not an entirely pleasant sensation, and as I check my computer I realise that the fish on which I am trying to focus is actually taking me deeper. I become concerned at the distance I might be from the wall - but which way to swim? I start by heading across what appears to be the current, but the fish just appear to be swimming with me. After a few anxious moments, I realise that this is an optical illusion created by the schooling fish. Suddenly I burst out of the bubble, and there is the wall, within sight but still a hard swim across the current. Heading back, I begin to think that this is just how a predator must feel when it is trying to single out a lone fish for dinner. I begin to understand the logic of the safety-in-numbers approach to life. So, exactly why do fish school together, and how do they achieve the incredible discipline and communication required to behave as a single entity?
madding crowd Research has shown that about a quarter of all fish school throughout their lives and that at least half of all species do so for at least part of their existence. The reasons are numerous - it helps them to grow, travel, feed, rest, breed and, mostly of course, to avoid predators. Although it is mostly fish of the same species and size that you find schooling together, you will occasionally come across a rogue species within a shoal using it as protection. The most amazing skill that schooling fish possess is the ability to move in a totally co-ordinated fashion, always seeming to maintain the same distance from the other fish in the shoal. They use a technique defined as the 'optomotor reaction', which is the simultaneous use of eyes, nerves and muscles to communicate and sense movement between individual fish. Often fish feature a dark stripe or spot in their livery and it is this which enables each individual in a shoal to zero in on the movements of its neighbours, and thus achieve an astonishing chain reaction of movement.
shape-changing Confusing potential predators is the school's single most important activity. Most predators want to home in on an individual fish which displays the shape and behaviour of prey from previous dining encounters. So the objective of the school is to confuse this identification process and prevent the predator singling out an individual. As a predator approaches, the shape of the shoal typically contracts to present a tighter, smaller volume and perhaps a different shape. This is followed by quick changes in direction and speed to confuse the enemy. These movements also alter the coloration of the school - one moment dark, the next bright and silvery as the light is reflected from their scales. All this activity masks the shape of the individual fish. A mass of glimmering silver makes it extremely difficult for the predator to decide where to attack.
bringing up the rear Not every fish in the shoal can keep up with this operation, and often single fish or small groups get left behind, possibly with fatal consequences. However, there seems to be a game plan for this as well and you can observe schools splitting into two to further confuse the predator, with one part sweeping back to pick up the stragglers. Fish which do not normally school together often do so when the breeding season starts, to increase the chances of successful fertilisation. The sheer number and proximity of males and females is a boost, although some species do break away temporarily from the school in mating pairs when the moment to spawn arrives. Encountering normally solitary reef fish in big concentrations can be spectacular. For me, the most striking include the bright yellow-masked butterflyfish and those cute-looking masked pufferfish, which swim up and down the reef with such purpose when they shoal.
top predator Sadly, the schooling behaviour that protects fish from their natural predators so well has been the downfall of many species which are targeted by the most efficient predator of all - man. Modern oceanographic and fisheries technology means that determined fleets can track a species from its spawning and breeding grounds to feeding grounds, where the target fish can be scooped from the sea with frightening ease. Stocks of many species around British coastal waters are already in terminal decline, as fleets compete to net as much as they can before they are prohibited by quota constraints, or the stocks become uneconomical to hunt. The quota systems appear to be too little too late in some areas, while in others the rules are flouted. When you consider this, and the illogical notion of discarding a dead bycatch or target species which is too small, to conserve stocks, it seems it is only a matter of time before certain favourites are gone forever. Some tropical regions have not yet been subjected to this technological assault , so we can still witness the amazing sight of schooling fish. One can only hope that these areas will remain until the fishing industry has developed better fish-farming methods or stock management techniques that allow species to regenerate in step with the hunter's demands. Until then, enjoy the spectacle whenever you encounter it.