WATER COLUMN. By Douglas David Seifert

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1 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

2 Finishing school 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 27

3 fish supper: the bait ball is an easy target when there are plenty of sharks attacking it 28

4 WATER COLUMN Finishing school When you are a small fish, the best you can do is avoid the bigger fish: they tend to try to eat you. It s nothing personal; it s just the nature of being a fish. The vast majority of bony fish reproduce externally, via broadcast spawning, where the female releases a humongous number of eggs, a male or many males simultaneously release a tremendous volume of sperm and a number of these eggs will be fertilised. Each egg contains a droplet of oil that will provide nutrients to enable the development of the embryo within the egg capsule, as well as providing buoyancy, to carry the egg as it floats on the current to the relative safety of the open ocean. These larval fish drift on currents, gradually developing into miniature versions of their parents. Their strategy for survival is to disperse into the vastness of the ocean, where encounters between animals occur infrequently. As these embryos develop, they consume their fat store and hatch from the egg case. The offspring begin their lives as miniature predators themselves and they must eat or die. Eventually, in their search for food, they will come across others of their own kind, and these fish will form aggregations. This is a survival strategy. While reef and shore fish have some advantages in their nooks and crannies to provide a degree of protection places such as coral or kelp forests filled with hidey-holes and obstacles to give smaller fish 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 empty blue water or by a unique form of camouflage in plain sight: hiding in a shifting mass of similar-looking neighbours of the same size and same species. It is believed that large groups of identical animals moving in synchronised formations can overwhelm a predator s ability to fixate on an individual target. The majority of fish schools comprise silvery fish of the same species in the same size class for this very reason. The fish maintain a constant spacing between individuals, near enough to close the ranks but not so close that they collide with each other. The fish maintain their formation and constant speed by a combination of eyesight and a remote sense called the lateral line. Their eyes are positioned on the sides of their heads and their field of vision gives them an unobstructed and accurate view of the world around them in all dimensions. Their lateral line detects changes in pressure and motion through the water, giving the same information that the sense of touch gives to animals with antennae, whiskers, legs, arms, fingers and the like, but without actual physical contact. The defensive strategy of schooling fish lies in mesmerising movement: the prey hiding in full view but unable to be isolated for more than an instant amid a backdrop of incessant activity designed to distract. The larger the school, the more mesmerising the effect, and the more difficult it is for a single predator or a number of them to prey on individuals. The target is constantly shifting. Schools grow larger by recruiting a continual influx of newcomers as the population swells from hundreds to thousands by the effectiveness of this protective strategy. Throughout the seas of the world, large formations occur predictably at distinct times of the year. The occurrence of these schools is driven by seasonality and the growth rates and life-cycle stage of the particular species. Commercial fisheries target the pelagic species that aggregate to create the largest schools: sardines, menhaden, anchovies, pilchards, herring and the like. Fishing fleets of the world converge on these massive schools and harvest as many from the sea as their nets can catch. The irony is that although the fish form schools supposedly as protection from predators, so The larger the school, the more difficult it is for predators to prey on individuals much food in a concentrated area is actually just as likely to attract predators. One of the largest and best-known schooling aggregations of sardines occurs every July off the coast of South Africa and is known as the Sardine Run. Millions possibly billions of sardines migrating down the coast face an overwhelming onslaught, not only from fishermen awaiting the schools to approach the shore but also from sharks, dolphins, tuna, whales, sea birds and sea lions. All the predation activity that the sardine school undergoes, day in and day out, as it continues down the coast details the carnage and unrelenting onslaught on a mind-boggling scope. On a smaller scale, in other parts of the world, similar predations on schools of aggregating fishes of the open ocean are witnessed with regularity. These smaller schools do not comprise millions of individuals, but a few thousand. These mini-schools are tightly formed and are migratory by season. Though not made up of overwhelmingly large numbers of individuals,» 29

5 WATER COLUMN Finishing school even relatively small fish schools are effective at deterring the occasional predator: the odd shark, a small pod of dolphins, a few billfish. The problem occurs when the predators aggregate in large numbers. Predators don t school: they gather in response to a concentrated abundance of food. And, when a predator is faced with another predator in the pursuit of feeding, they become highly competitive. When this occurs, the phenomenon is referred to as a bait ball. A bait ball is the product of schooling fish being corralled by predators into a tightly defined area, basically surrounded on all sides and trapped against the surface. The predators do not work together per se, but they coordinate their efforts to achieve the result of forcing the schooling fish into a closely packed assembly, in which their movements become frantic. 30 FISH BALLS: bait balls like this occur in open ocean [main photo]; Atlantic sailfish harry sardines off Mexico [inset] The cast of predatory characters varies depending on location. Off the east coast of Florida, cigar minnows run the gauntlet through goliath grouper that aggregate to mate over winter. Around Isla Mujeres, off Mexico s Yucatán Peninsula, aggregations of Atlantic sailfish gather in the winter months to feed on the abundance of sardines. On Western Australia s Ningaloo Reef, silvery jacks attracted by the small animals feeding on coral spawn run the risk of becoming the food of whale sharks. Often, sea birds will opportunistically join in the predation. The predatory fish (sharks, tuna, jacks and so on) and/or marine mammals (dolphins, sea lions) do all the work and drive the prey to the surface; the sea birds (gannets, cormorants, boobies, shearwaters) dive-bomb the school and pluck unwary fish, sometimes from as deep as 20m.»

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7 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 fast and furious. The water disperses the blood but the scales and scraps drift down slowly, a silver snow of little fish playing their role in the food chain. Gradually, the bait ball begins to thin, and as the school diminishes in size and number, the easier it is for the predators to target individuals and decimate what is left of it. A few members 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. l GROUP HUG: cigar minnows stream around a goliath grouper off Florida [main photo]; jack mackerel migrate through the Eastern Pacific in spring [right] Douglas encountered bait balls and a dizzying number of silky and Galápagos sharks off Cocos Island, Costa Rica while on board the MV Sea Hunter ( info@ underseahunter.com) and guided by the unflinching Wilson 32