As an experienced SCUBA diver I come across shoals of fish on virtually every dive, from tiny damselfish to schools of jack and even shoals of hammerhead sharks.
It is well known that small fish form shoals for defence from predators, but how does this work? Surely a mass of prey is more visible and an easy target for predators? Also, why do predatory fish such as jacks and sharks form shoals?
School vs shoal: What is the difference?
I have already used the terms school and shoal to describe groups of fish but there is an important difference. While a shoal is any group of fish, a school is an organised shoal, where all the fish swim in unison, pointing in the same direction.
Shoals of small fish
For small fish the reasons for forming shoals are well documented. The simple reason is natural selection, as with pretty much every phenomenon in the natural world. Fish of shoaling species that have a tendency to shoal are more likely to reach reproductive age than those which do not shoal. Obviously some species do not shoal, and their strategy works for that particular species. For example anemone fish do not form large shoals because they have a mutualistic relationship with an anemone – which is only big enough for one family. The anemone provides the protection that a shoal would otherwise afford.
Other than protection from predators, shoals are also thought to increase mating success, as finding a mate is much easier in a shoal. Think of the mating success of nightclubs compared with wandering the moors alone! Shoals are also known to increase the chance of an individual within the shoal finding food. This sounds counterintuitive because once a source of food is found there are more mouths to feed. However the effectiveness of shoals in finding is such that an individual finds more food within a shoal than outside it.
Fish shoals and mimicry
If you look carefully at some shoals, you might notice that some of the fish look slightly different to others. This can be very difficult to spot as, due to a form of convergent evolution, the two species can look very very similar. The example on this page demonstrates a form of aggressive mimicry. The yellowfin goatfish Mulloidichthys vanicolensis forms schools or shoals that swim around the edge of the reef looking for food. Hidden amongst them are very similar looking Ehrenburg’s snapper Lutjanus ehrenbergi. The latter is a predatory fish which feeds on smaller fish. It uses the mixed shoal as camouflage from which to ambush its prey.
Shoals of predatory fish
Some fish are big enough to look after themselves and don’t need the protection of a shoal. However, a lot of larger species, such as barracuda, jacks and even some sharks still form shoals. Rather than protection, these fish shoals are formed mainly because it improves feeding success. As mentioned previously for small fish, individual fish are more successful at finding food when part of a shoal. There is also the element of increased mating success.
The significance of fish shoals in conservation
As already discussed, the tendency of fish to shoal is often for protection from predators. However, the irony is that this shoaling instinct means that fishing can be incredibly efficient. Modern equipment can detect large shoals of fish and even distinguish the species from the boat. Fishing however is just one of many concerns that face marine conservationists. However, there is no question that Marine Protected Areas (MPAs) are helping many species to reestablish the enormous shoals for which they are known.
More information on fish shoals
A gallery of fish shoal photographs on this website
A gallery of fish shoal photographs on Toby Gibson’s Portfolio Website – Images available to purchase online.