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Seller Inventory APC Never used! This item is printed on demand. Seller Inventory William M. Hamner Edited by Julia K. Publisher: Cambridge University Press , Radakov estimated herring schools in the North Atlantic can occupy up to 4. Herring schools in general have very precise arrangements which allow the school to maintain relatively constant cruising speeds. Herrings have excellent hearing, and their schools react very rapidly to a predator. The herrings keep a certain distance from a moving scuba diver or a cruising predator like a killer whale, forming a vacuole which looks like a doughnut from a spotter plane.

Many species of large predatory fish also school, including many highly migratory fish , such as tuna and some oceangoing sharks. Cetaceans such as dolphins, porpoises and whales, operate in organised social groups called pods. Emergent properties give an evolutionary advantage to members of the school which non members do not receive. Support for the social and genetic function of aggregations, especially those formed by fish, can be seen in several aspects of their behaviour. For instance, experiments have shown that individual fish removed from a school will have a higher respiratory rate than those found in the school.

This effect has been attributed to stress, and the effect of being with conspecifics therefore appears to be a calming one and a powerful social motivation for remaining in an aggregation. Even with the best facilities aquaria can offer they become fragile and sluggish compared to their quivering energy in wild schools. It has also been proposed that swimming in groups enhances foraging success. This ability was demonstrated by Pitcher and others in their study of foraging behaviour in shoaling cyprinids. The number of fishes in the groups was varied, and a statistically significant decrease in the amount of time necessary for larger groups to find food was established.

Further support for an enhanced foraging capability of schools is seen in the structure of schools of predatory fish. Partridge and others analysed the school structure of Atlantic bluefin tuna from aerial photographs and found that the school assumed a parabolic shape, a fact that was suggestive of cooperative hunting in this species.

Fish in shoals "share" information by monitoring each other's behaviour closely. Feeding behaviour in one fish quickly stimulates food-searching behaviour in others. Fertile feeding grounds for forage fish are provided by ocean upwellings. Oceanic gyres are large-scale ocean currents caused by the Coriolis effect.

Wind-driven surface currents interact with these gyres and the underwater topography, such as seamounts , fishing banks , and the edge of continental shelves , to produce downwellings and upwellings. The result can be rich feeding grounds attractive to the plankton feeding forage fish.

In turn, the forage fish themselves become a feeding ground for larger predator fish. Most upwellings are coastal, and many of them support some of the most productive fisheries in the world. Copepods , the primary zooplankton , are a major item on the forage fish menu. They are a group of small crustaceans found in ocean and freshwater habitats. Copepods are typically one millimetre 0. Some scientists say they form the largest animal biomass on the planet. They have large antennae see photo below left. When they spread their antennae they can sense the pressure wave from an approaching fish and jump with great speed over a few centimeters.

If copepod concentrations reach high levels, schooling herrings adopt a method called ram feeding. In the photo below, herring ram feed on a school of copepods. They swim with their mouth wide open and their opercula fully expanded. This copepod has its antenna spread click to enlarge.

The antenna detects the pressure wave of an approaching fish. Copepods are a major food source for forage fish like this Atlantic herring. School of herrings ram-feeding on a school of copepods, with opercula expanded so their red gills are visible. Animation showing how herrings hunting in a synchronised way can capture the very alert and evasive copepod. The fish swim in a grid where the distance between them is the same as the jump length of their prey, as indicated in the animation above right.

In the animation, juvenile herring hunt the copepods in this synchronised way. The copepods sense with their antennae the pressure-wave of an approaching herring and react with a fast escape jump. The length of the jump is fairly constant. The fish align themselves in a grid with this characteristic jump length. A copepod can dart about 80 times before it tires. After a jump, it takes it 60 milliseconds to spread its antennae again, and this time delay becomes its undoing, as the almost endless stream of herrings allows a herring to eventually snap the copepod.

A single juvenile herring could never catch a large copepod. A third proposed benefit of fish groups is that they serve a reproductive function. They provide increased access to potential mates, since finding a mate in a shoal does not take much energy. And for migrating fish that navigate long distances to spawn, it is likely that the navigation of the shoal, with an input from all the shoal members, will be better than that taken by an individual fish. Forage fish often make great migrations between their spawning, feeding and nursery grounds. Schools of a particular stock usually travel in a triangle between these grounds.

For example, one stock of herrings have their spawning ground in southern Norway , their feeding ground in Iceland , and their nursery ground in northern Norway.

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Wide triangular journeys such as these may be important because forage fish, when feeding, cannot distinguish their own offspring. Capelin are a forage fish of the smelt family found in the Atlantic and Arctic oceans. In summer, they graze on dense swarms of plankton at the edge of the ice shelf. Larger capelin also eat krill and other crustaceans. The capelin move inshore in large schools to spawn and migrate in spring and summer to feed in plankton rich areas between Iceland , Greenland , and Jan Mayen.

The migration is affected by ocean currents. Around Iceland maturing capelin make large northward feeding migrations in spring and summer. The return migration takes place in September to November. The spawning migration starts north of Iceland in December or January. The diagram on the right shows the main spawning grounds and larval drift routes. Capelin on the way to feeding grounds is coloured green, capelin on the way back is blue, and the breeding grounds are red.

This theory states that groups of fish may save energy when swimming together, much in the way that bicyclists may draft one another in a peloton. Geese flying in a Vee formation are also thought to save energy by flying in the updraft of the wingtip vortex generated by the previous animal in the formation. It would seem reasonable to think that the regular spacing and size uniformity of fish in schools would result in hydrodynamic efficiencies. Landa argues that the leader of a school constantly changes, because while being in the body of a school gives a hydrodynamic advantage, being the leader means you are the first to the food.

It is commonly observed that schooling fish are particularly in danger of being eaten if they are separated from the school. One potential method by which fish schools might thwart predators is the "predator confusion effect" proposed and demonstrated by Milinski and Heller Milinski and Heller's findings have been corroborated both in experiment [26] [27] and computer simulations.

Since fields of many fish will overlap, schooling should obscure this gradient, perhaps mimicking pressure waves of a larger animal, and more likely confuse the lateral line perception. In order to produce separate signals, individual prey must be about five body widths apart. If objects are too close together to be distinguished, they will form a blurred image. A third potential anti-predator effect of animal aggregations is the "many eyes" hypothesis.

This theory states that as the size of the group increases, the task of scanning the environment for predators can be spread out over many individuals. Not only does this mass collaboration presumably provide a higher level of vigilance, it could also allow more time for individual feeding.

A fourth hypothesis for an anti-predatory effect of fish schools is the "encounter dilution" effect. The dilution effect is an elaboration of safety in numbers , and interacts with the confusion effect. In the attack component, it was thought that an attacking predator is less likely to eat a particular fish when a greater number of fish are present.

In sum, a fish has an advantage if it is in the larger of two groups, assuming that the probability of detection and attack does not increase disproportionately with the size of the group. Schooling forage fish are subject to constant attacks by predators. An example is the attacks that take place during the African sardine run. The African sardine run is a spectacular migration by millions of silvery sardines along the southern coastline of Africa. In terms of biomass, the sardine run could rival East Africa's great wildebeest migration. Adult sardines, about two years old, mass on the Agulhas Bank where they spawn during spring and summer, releasing tens of thousands of eggs into the water.

The adult sardines then make their way in hundreds of shoals towards the sub-tropical waters of the Indian Ocean. Huge numbers of sharks, dolphins, tuna, sailfish, Cape fur seals and even killer whales congregate and follow the shoals, creating a feeding frenzy along the coastline. When threatened, sardines and other forage fish instinctively group together and create massive bait balls. They are short lived, seldom lasting longer than 20 minutes.

The fish eggs, left behind at the Agulhas Banks, drift north west with the current into waters off the west coast, where the larvae develop into juvenile fish. When they are old enough, they aggregate into dense shoals and migrate southwards, returning to the Agulhas banks in order to restart the cycle. The development of schooling behavior was probably associated with an increased quality of perception, predatory lifestyle and size sorting mechanisms to avoid cannibalism.

The development of vison and the OLS would have permitted detection of potential prey. This could have led to an increased potential for cannibalism within the shoal. On the other hand, increased quality of perception would also give small individuals a chance to escape or to never join a shoal with larger fish. It has been shown that small fish avoid joining a group with larger fish, although big fish do not avoid joining small conspecifics. Predators have devised various countermeasures to undermine the defensive shoaling and schooling manoeuvres of forage fish.

The sailfish raises its sail to make it appear much larger so it can herd a school of fish or squid. Swordfish charge at high speed through forage fish schools, slashing with their swords to kill or stun prey. They then turn and return to consume their "catch". Thresher sharks use their long tails to stun shoaling fishes. Before striking, the sharks compact schools of prey by swimming around them and splashing the water with its tail, often in pairs or small groups. Threshers swim in circles to drive schooling prey into a compact mass, before striking them sharply with the upper lobe of its tail to stun them.

The shark's momentum at the end of these spiralling runs often carries it into the air. Some predators, such as dolphins, hunt in groups of their own. One technique employed by many dolphin species is herding , where a pod will control a school of fish while individual members take turns ploughing through and feeding on the more tightly-packed school a formation commonly known as a bait ball.

Corralling is a method where fish are chased to shallow water where they are more easily captured. In South Carolina , the Atlantic bottlenose dolphin takes this one step further with what has become known as strand feeding, where the fish are driven onto mud banks and retrieved from there. Common bottlenose dolphins have been observed using another technique. One dolphin acts as a "driver" and herds a school of fish towards several other dolphins who form a barrier. The driver dolphin slaps its fluke which makes the fish leap into the air.

As the fish leap, the driver dolphin moves with the barrier dolphins and catches the fish in the air. During the sardine run , as many as 18, dolphins, behaving like sheepdogs, herd the sardines into bait balls, or corral them in shallow water. Once rounded up, the dolphins and other predators take turns ploughing through the bait balls, gorging on the fish as they sweep through. Seabirds also attack them from above, flocks of gannets , cormorants , terns and gulls.

They have air sacs under their skin in their face and chest which act like bubble-wrap , cushioning the impact with the water. Subsets of bottlenose dolphin populations in Mauritania are known to engage in interspecific cooperative fishing with human fishermen. The dolphins drive a school of fish towards the shore where humans await with their nets.

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In the confusion of casting nets, the dolphins catch a large number of fish as well. Intraspecific cooperative foraging techniques have also been observed, and some propose that these behaviours are transmitted through cultural means. Some whales lunge feed on bait balls. This generates the water pressure required to expand its mouth and engulf and filter a huge amount of water and fish. Lunge feeding by the huge rorquals is said to be the largest biomechanical event on Earth. Fish schools swim in disciplined phalanxes, with some species, such as herrings, able to stream up and down at impressive speeds, twisting this way and that, and making startling changes in the shape of the school, without collisions.

It is as if their motions are choreographed, though they are not. The schooling behaviour develops instinctively and is not learnt from older fish. To school the way they do, fish require sensory systems which can respond with great speed to small changes in their position relative to their neighbour.


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Most schools lose their schooling abilities after dark, and just shoal. This indicates that vision is important to schooling. The importance of vision is also indicated by the behaviour of fish who have been temporarily blinded. Schooling species have eyes on the sides of their heads, which means they can easily see their neighbours.

Also, schooling species often have "schooling marks" on their shoulders or the base of their tails, or visually prominent stripes, which provide reference marks when schooling, [53] similar in function to passive markers in artificial motion capture. However fish without these markers will still engage in schooling behaviour, [54] though perhaps not as efficiently. Other senses are also used.

Pheromones or sound may also play a part but supporting evidence has not been found so far. The lateral line is a line running along each side of the fish from the gill covers to the base of the tail. In laboratory experiments the lateral lines of schooling fish have been removed. They swam closer, leading to a theory that the lateral lines provide additional stimuli input when the fish get too close.

It uses receptors called neuromasts , each of which is composed of a group of hair cells. The hairs are surrounded by a protruding jelly-like cupula , typically 0. The hair cells in the lateral line are similar to the hair cells inside the vertebrate inner ear, indicating that the lateral line and the inner ear share a common origin. It is difficult to observe and describe the three dimensional structure of real world fish shoals because of the large number of fish involved.

Techniques include the use of recent advances in fisheries acoustics. Boids simulation — needs Java. The observational approach is complemented by the mathematical modelling of schools. The most common mathematical models of schools instruct the individual animals to follow three rules:. An example of such a simulation is the boids program created by Craig Reynolds in For instance, many models implement these three rules through layered zones around each fish.

The shape of these zones will necessarily be affected by the sensory capabilities of the fish. Fish rely on both vision and on hydrodynamic signals relayed through its lateral line. Antarctic krill rely on vision and on hydrodynamic signals relayed through its antennae. In a masters thesis published in , Moshi Charnell produced schooling behaviour without using the alignment matching component of an individuals behaviour. In a paper published in , researchers from Iceland recount their application of an interacting particle model to the capelin stock around Iceland, successfully predicting the spawning migration route for In order to gain insight into why animals evolve swarming behaviour , scientists have turned to evolutionary models that simulate populations of evolving animals.

Typically these studies use a genetic algorithm to simulate evolution over many generations in the model. These studies have investigated a number of hypotheses explaining why animals evolve swarming behaviour, such as the selfish herd theory , [65] [66] [67] [68] the predator confusion effect, [29] [69] the dilution effect, [70] [71] and the many eyes theory. In , building on recent advances in acoustic imaging, [55] [73] a group of MIT researchers observed for "the first time the formation and subsequent migration of a huge shoal of fish.

The researchers imaged spawning Atlantic herring off Georges Bank. They found that the fish come together from deeper water in the evening, shoaling in a disordered way. A chain reaction triggers when the population density reaches a critical value, like an audience wave travelling around a sport stadium. A rapid transition then occurs, and the fish become highly polarised and synchronized in the manner of schooling fish. There they spawn during the night. In the morning, the fish school back to deeper water again and then disband. Small groups of leaders were also discovered that significantly influenced much larger groups.

Fish schools are faced with decisions they must make if they are to remain together. For example, a decision might be which direction to swim when confronted by a predator, which areas to stop and forage, or when and where to migrate. Quorum sensing can function as a collective decision-making process in any decentralised system. A quorum response has been defined as "a steep increase in the probability of group members performing a given behaviour once a threshold minimum number of their group mates already performing that behaviour is exceeded".

The fish did this by a simple quorum rule such that individuals watched the decisions of others before making their own decisions. This technique generally resulted in the 'correct' decision but occasionally cascaded into the 'incorrect' decision. In addition, as the group size increased, the fish made more accurate decisions in following the more attractive fish model.

Such behaviour has also been demonstrated in the shoaling behaviour of threespine sticklebacks. Other open questions of shoaling behaviour include identifying which individuals are responsible for the direction of shoal movement. In the case of migratory movement, most members of a shoal seem to know where they are going. Observations on the foraging behaviour of captive golden shiner a kind of minnow found they formed shoals which were led by a small number of experienced individuals who knew when and where food was available.


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  6. Experimental studies of shoal preference are relatively easy to perform. An aquarium containing a choosing fish is sandwiched between two aquaria containing different shoals, and the choosing fish is assumed to spend more time next to the shoal it prefers. Studies of this kind have identified several factors important for shoal preference. Fish generally prefer larger shoals. Indeed, the preference for larger shoals seems stronger when predators are nearby, [88] [89] or in species that rely more on shoaling than body armour against predation.

    Competition may mean that hungry individuals might prefer smaller shoals or exhibit a lesser preference for very large shoals, as shown in sticklebacks. Fish prefer to shoal with their own species. Sometimes, several species may become mingled in one shoal, but when a predator is presented to such shoals, the fish reorganize themselves so that each individual ends up being closer to members of its own species.

    Fish tend to prefer shoals made up of individuals that match their own size. Some fish may even prefer shoals of another species if this means a better match in current body size. In golden shiner , large satiated fish prefer to associate with other large individuals, but hungry ones prefer smaller shoalmates. Fish prefer to shoal with individuals with which the choosing fish is already familiar. This has been demonstrated in guppies , [99] [] threespine stickleback , [] banded killifish , [] the surfperch Embiotoca jacksoni , [] Mexican tetra , [] and various minnows.