WATER COLUMN By Douglas David Seifert SILKY SKILLS: silky sharks corral a ball of mackerel against the surface at Cocos Island All photography by Douglas Seifert 26 FINISHINGschool Why do fish gather in bait balls when it seems they are just setting themselves up as a living buffet for the predators’ ball? DOUGLAS SEIFERT explains all www.divemagazine.co.uk 27 FISH SUPPER: the bait ball is an easy target when there are plenty of sharks attacking it 28 WATER COLUMN Finishing school hen you are sense of touch gives to animals with antennae, a small fish, whiskers, legs, arms, fingers and the like, but the best you without actual physical contact. can do is The defensive strategy of schooling fish lies avoid the in mesmerising movement: the prey hiding in bigger fish: full view but unable to be isolated for more they tend to than an instant amid a backdrop of incessant try to eat activity designed to distract. The larger the you. It’s school, the more mesmerising the effect, and nothing the more difficult it is for a single predator or personal; it’s just the nature of being a fish. a number of them to prey on individuals. The The vast majority of bony fish reproduce target is constantly shifting. Schools grow Wexternally, via broadcast spawning, where the larger by recruiting a continual influx of female releases a humongous number of eggs, newcomers as the population swells from a male or many males simultaneously release hundreds to thousands by the effectiveness a tremendous volume of sperm and a number of this protective strategy. of these eggs will be fertilised. Each egg contains Throughout the seas of the world, large a droplet of oil that will provide nutrients to formations occur predictably at distinct times enable the development of the embryo within of the year. The occurrence of these schools is the egg capsule, as well as providing buoyancy, driven by seasonality and the growth rates and to carry the egg as it floats on the current to the life-cycle stage of the particular species. relative safety of the open ocean. These larval Commercial fisheries target the pelagic species fish drift on currents, gradually developing that aggregate to create the largest schools: into miniature versions of their parents. Their sardines, menhaden, anchovies, pilchards, strategy for survival is to disperse into the herring and the like. Fishing fleets of the world vastness of the ocean, where encounters between converge on these massive schools and harvest animals occur infrequently. as many from the sea as their nets can catch. As these embryos develop, they consume The irony is that although the fish form schools their fat store and hatch from the egg case. supposedly as protection from predators, so The offspring begin their lives as miniature predators themselves and they must eat or die. Eventually, in their search for food, they will The larger the come across others of their own kind, and these fish will form aggregations. school, the more This is a survival strategy. While reef and shore fish have some advantages in their nooks difficult it is for and crannies to provide a degree of protection – predators to prey places such as coral or kelp forests filled with hidey-holes and obstacles to give smaller fish “ on individuals the chance to evade larger predators – pelagic fish are not so fortunate. The open sea offers protection only by means of boundless distance, space and volume of a universe of relatively much food in a concentrated area is actually just empty blue water – or by a unique form of as likely to attract predators. camouflage in plain sight: hiding in a shifting One of the largest and best-known schooling mass of similar-looking neighbours of the same aggregations of sardines occurs every July off size and same species. the coast of South Africa and is known as the It is believed that large groups of identical Sardine Run. Millions – possibly billions – of animals moving in synchronised formations can sardines migrating down the coast face an overwhelm a predator’s ability to fixate on an overwhelming onslaught, not only from individual target. The majority of fish schools fishermen awaiting the schools to approach comprise silvery fish of the same species in the the shore but also from sharks, dolphins, tuna, same size class for this very reason. The fish whales, sea birds and sea lions. All the predation maintain a constant spacing between individuals, activity that the sardine school undergoes, day near enough to close the ranks but not so close in and day out, as it continues down the coast that they collide with each other. The fish details the carnage and unrelenting onslaught maintain their formation and constant speed on a mind-boggling scope. by a combination of eyesight and a remote On a smaller scale, in other parts of the world, sense called the ‘lateral line’. similar predations on schools of aggregating Their eyes are positioned on the sides of their fishes of the open ocean are witnessed with heads and their field of vision gives them an regularity. These smaller schools do not comprise unobstructed and accurate view of the world millions of individuals, but a few thousand. around them in all dimensions. Their lateral line These mini-schools are tightly formed and are detects changes in pressure and motion through migratory by season. Though not made up of the water, giving the same information that the overwhelmingly large numbers of individuals, » www.divemagazine.co.uk 29 WATER COLUMN Finishing school even relatively small fish The cast of predatory schools are effective at characters varies depending deterring the occasional on location. Off the east predator: the odd shark, coast of Florida, cigar a small pod of dolphins, minnows run the gauntlet a few billfish. through goliath grouper The problem occurs when that aggregate to mate over the predators aggregate in winter. Around Isla Mujeres, large numbers. Predators off Mexico’s Yucatán don’t school: they gather in Peninsula, aggregations response to a concentrated of Atlantic sailfish gather abundance of food. And, in the winter months to when a predator is faced feed on the abundance with another predator in the of sardines. On Western pursuit of feeding, they become highly FISH BALLS: Australia’s Ningaloo Reef, silvery jacks attracted competitive. When this occurs, the phenomenon bait balls like by the small animals feeding on coral spawn run is referred to as a ‘bait ball’. this occur in the risk of becoming the food of whale sharks. open ocean A bait ball is the product of schooling fish being [main photo]; Often, sea birds will opportunistically join in corralled by predators into a tightly defined area, Atlantic sailfish the predation. The predatory fish (sharks, tuna, basically surrounded on all sides and trapped harry sardines jacks and so on) and/or marine mammals against the surface. The predators do not work off Mexico (dolphins, sea lions) do all the work and drive together per se, but they coordinate their efforts [inset] the prey to the surface; the sea birds (gannets, to achieve the result of forcing the schooling fish cormorants, boobies, shearwaters) dive-bomb into a closely packed assembly, in which their the school and pluck unwary fish, sometimes movements become frantic. from as deep as 20m. » 30 WATER COLUMN Finishing school In the tropical eastern Pacific, in the waters around Cocos Island and Malpelo Island, the spring brings schools of jack mackerel through their migratory route. Unfortunately for them, the remote islands and sea mounts they must pass serve as a gathering point for wide-ranging predators, such as silky sharks, Galápagos sharks, bottlenose dolphins, yellowfin tuna, rainbow runners and jacks. When the predators locate the schools passing through their domain, bait balls may erupt several times a day for weeks on end. It starts out with a few predators locating schooling fish in the depths, then systematically driving them through a series of rushes and attacks to concentrate the school and eventually force it to the surface, where it will have one less direction in which to flee. The sounds and movement and smell of blood in the water attracts more predators, and soon there is a carousel of them, each engaged in its own predatory strategies, working the school into a manageable target. The behaviour of the schooling fish changes from well-defined order into absolute chaos. The collective protection strategy of polarity and spacing is abandoned into an obvious strategy of every fish for itself. The predators reap the rewards of panic. Every time a fish breaks rank and attempts to flee from the melee, it is quickly singled out and devoured. There is a definite frenzy as the predators attempt The behaviour of the schooling fish changes from order to “ absolute chaos to out-compete each other, and attacks become GROUP HUG: fast and furious. The water disperses the blood cigar minnows but the scales and scraps drift down slowly, a stream around a goliath grouper silver snow of little fish playing their role in off Florida the food chain. [main photo]; Gradually, the bait ball begins to thin, and as jack mackerel the school diminishes in size and number, the migrate through easier it is for the predators to target individuals the Eastern Pacific in spring and decimate what is left of it. A few members [right] of the school survive and perhaps find and join another one. But chances are those fortunate surviving individuals will face the predators another day, when the season for bait balls once again gets underway.