Schooling as a strategy for survival
In biology, the gregarious behaviors of fishes such as shoaling and schooling mean different things. Fishes that stay together for social reasons are shoaling, and if the group is swimming in the same direction in a coordinated manner, they are schooling (*). Schools may, however, vary in density and polarity (direction) with the ‘bait ball’, a densely packed three-dimensional structure as an extreme example. Shoaling fish can shift into a disciplined and coordinated school, then shift back to an amorphous shoal within seconds. Such shifts are triggered by changes of activity from feeding, resting, traveling, or avoiding predators. Schooling fish are usually of the same species and the same age/size while shoaling aggregations can also be mixed.
Examples of schools Many species of fish of the coral reefs, such as jacks, snappers, sweetlips, rabbitfishes, bannerfishes, silversides, and glassy sweepers form schools of different densities and structures. Normally such fish inhabit open water with little opportunity to hide in rocks and weeds or even in the sand. Schools of barracudas can be seen taking shape as a tornado above a sandy plateau. Jacks like big eye trevallies (Caranx sexfaxiatus) and snappers congrgate in dense schools taking the form of balls: a tight spherical formation of numerous fish, or spirals containing more than 1000 individuals. On shallow coral reefs, the silversides often form enormous clouds that move in the same direction and dart away with lightning speed if a dangerous intruder like a coral grouper comes too close. The hardhead silverside occurs commonly in large schools along sandy shorelines and reef margins. It is reported to be a largely nocturnal fish that forms schools numbering from several hundred individuals to aggregations that may be over 100m long and 20m wide. Tight school-like bait-balls are a favorite target for UW photographers, the general principle being the larger en denser the better, the golden rule: get as close as possible to the school, preferably equipped with a fish-eye lens and using natural light. Unfortunately for the schools, modern fishing boats can now easily detect dense schools of herrings or sardines on their ultrasound fishfinders and scoop up thousands in one catch.
Why schooling? Why fishes school is still a matter of debate and controversy. Schools may serve several reproductive functions, like spawning (Bohar snappers are a good example; see insert above), efficient foraging of social groups, and hydrodynamic advantages. Input coming from all the school members in scanning for food or threats will be better than that from an individual fish. Gregarious behavior is also considered as a form of cover-seeking in which each animal tries to reduce its chance of being caught by a predator.* The ‘’oddity effect" posits that any shoal member that stands out in appearance will be preferentially targeted by predators. This may explain why fish prefer to shoal with individuals that resemble themselves. The oddity effect thus tends to homogenize shoals, and even more so by forming dense schools. Accordingly, Hamilton proposed the selfish-herd theory, stating that individuals can reduce the risk of predation by moving to specific positions, in particular toward one another, within the group. In a bait-ball, centripetal instincts in already gregarious species are manifested in choosing the safest positions in the center rather than more risky positions at the outside. From this point of view, schooling is mostly seen as an adaptive strategy against predators developed by natural selection of individuals seeking protection by staying close to its congeners, *** Another important benefit of schooling is increased vigilance (the ‘many eyes’ effect) to detect potential predators. The factor of communal alertness, makes life more difficult for a predator and safer for a gregarious prey, especially if the predator is one that relies on stealth rather than speed.
Is schooling also survival effective? Predators may indeed be inhibited to attack dense schools since they are used to target out single individuals. But some have learned to use countermeasures to undermine the defensive shoaling and schooling maneuvers of prey fish. The sailfish, for example, raises its sail to make it appear much larger so it can herd and scatter a tight school of fish. Swordfish charge at high speed through fish schools, slashing with their swords to kill or stun prey, immobilizing large numbers with blows from its sharp-edged sword. Similarly, thresher sharks use their long tail as powerful whips to frighten and to spread out the group. They then turn and return to consume their "catch". Worse still for the school, an effective strategy of predators against loosely schooling fish may be to first scare them into forming a bait ball, somewhat similar to the way a sheepdog forces a loose group of sheep into a compact flock. A bait ball works much better if the fish school is first brought into a compact form. Dense schools, in particular bait balls of sardines, are easier to detect by fishes like sharks, tunas, and dolphins using their sonar. Dolphins are known to use bubble curtains to isolate and herd groups of sardines. Gannets detect a shallow bait ball of sardines much easier from the sky than individual species and then dart down through the surface to penetrate and consume their catch. Notice that predators also work together to ensure maximum efficiency in attacking the bait balls.
Cost-benefit principle The paradox is although the predators are the greatest winner when they attack bait-balls and cause the most damage in the number of fishes killed, the sheer greater number of fishes or safety in numbers remains a powerful tool. A given predator attack will also eat a smaller proportion of a large shoal than a small shoal. The evolution of the gregarious tendency as a defense against predators may thus still be the best strategy in the long term for prey fishes, even though the result is a considerable lowering of the overall mean fitness during an occasional attack of certain predators.
In sum, the ‘decision’ of fishes to form a school or not depends on the balance between the risk of capture and the benefit of foraging or mating. Guppies are known to engage in high-risk courtship behavior even when a predator is lurking in the neighborhood. Alternatively, fishes may decide to switch from polarized dense grouping to more loose aggregations depending on the conditions of the danger zone.
*Magurran, Anne E. “The Adaptive Significance of Schooling as an Anti-Predator Defence in Fish.” Annales Zoologici Fennici, vol. 27, no. 2, Finnish Zoological and Botanical Publishing Board, 1990, pp. 51–66