Shoaling and schooling - AbsoluteAstronomy.com

In biology, any group of fish that stay together for social reasons are said to be shoaling , and if, in addition, the group is swimming in the same direction in a coordinated manner, they are said to be schooling . In common usage, the terms are sometimes used rather loosely. About one quarter of fishes shoal all their lives, and about one half of fishes shoal for part of their lives.

Fish derive many benefits from shoaling behaviour including defence against predators (through better predator detection and by diluting the chance of individual capture), enhanced foraging

Foraging

- Definitions and significance of foraging behavior :Foraging is the act of searching for and exploiting food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce...

success, and higher success in finding a mate. It is also likely that fish benefit from shoal membership through increased hydrodynamic efficiency.

Fish use many traits to choose shoalmates. Generally they prefer larger shoals, shoalmates of their own species, shoalmates similar in size and appearance to themselves, healthy fish, and kin (when recognized).

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 would thus tend to homogenize shoals.

Overview

An aggregation of fish is the general term for any collection of fish that have gathered together in some locality. Fish aggregations can be structured or unstructured. An unstructured aggregation might be a group of mixed species and sizes that have gathered randomly near some local resource, such as food or nesting sites.

If, in addition, the aggregation comes together in an interactive, social way, they are said to be shoaling. Although shoaling fish can relate to each other in a loose way, with each fish swimming and foraging somewhat independently, they are nonetheless aware of the other members of the group as shown by the way they adjust behaviour such as swimming, so as to remains close to the other fish in the group. Shoaling groups can include fish of disparate sizes and can including mixed-species subgroups.

If, as a further addition, the shoal becomes more tightly organised, with the fish synchronising their swimming so they all move at the same speed and in the same direction, then the fish are said to be schooling. Schooling fish are usually of the same species and the same age/size. Fish schools move with the individual members precisely spaced from each other. The schools undertake complicated manoeuvres, as though the schools have minds of their own.

Shoaling is a special case of aggregating, and schooling is a special case of shoaling. While schooling and shoaling mean different things within biology, they are often treated as synonym

Synonym

Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...

s by non-specialists, with speakers of British English

British English

British English, or English , is the broad term used to distinguish the forms of the English language used in the United Kingdom from forms used elsewhere...

tending to use "shoaling" to describe any grouping of fish, while speakers of American English

American English

American English is a set of dialects of the English language used mostly in the United States. Approximately two-thirds of the world's native speakers of English live in the United States....

tend to use "schooling" just as loosely.
The intricacies of schooling are far from fully understood, especially the swimming and feeding energetics. Many hypotheses to explain the function of schooling have been suggested, such as better orientation, synchronized

Synchronization

Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....

hunting, predator confusion and reduced risk of being found. Schooling also has disadvantages, such as excretion buildup in the breathing media and oxygen and food depletion. The way the fish array in the school probably gives energy saving advantages, though this is controversial.

Fish can be obligate

Obligate

Obligate means "by necessity" and is used mainly in biology in phrases such as:* Obligate aerobe, an organism that cannot survive without oxygen* Obligate anaerobe, an organism that cannot survive in the presence of oxygen...

or facultative

Facultative

Facultative means "optional" or "discretionary" , used mainly in biology in phrases such as:* Facultative anaerobe, an organism that can use oxygen but also has anaerobic methods of energy production...

shoalers. Obligate shoalers, such as tuna

Tuna

Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

s, herring

Herring

Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...

s and anchovy

Anchovy

Anchovies are a family of small, common salt-water forage fish. There are 144 species in 17 genera, found in the Atlantic, Indian, and Pacific Oceans. Anchovies are usually classified as an oily fish.-Description:...

, spend all of their time shoaling or schooling, and become agitated if separated from the group. Facultative shoalers, such as Atlantic cod

Atlantic cod

The Atlantic cod, Gadus morhua, is a well-known demersal food fish belonging to the family Gadidae. It is also commercially known as cod, codling or haberdine....

, saiths

Pollachius virens

Pollachius virens is a species of marine fish in the Pollachius genus. Together with Pollachius pollachius it is generally referred to in the U.S. as Pollock. Other names include the Boston blues , coalfish and saithe in the UK.This species can be separated from P...

and some carangids, shoal only some of the time, perhaps for reproductive purposes.

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, travelling or avoiding predators.

When schooling fish stop to feed, they break ranks and become shoals. Shoals are more vulnerable to predator attack. The shape a shoal or school takes depends on the type of fish and what the fish are doing. Schools that are travelling can form long thin lines, or squares or ovals or amoeboid shapes. Fast moving schools usually form a wedge shape, while shoals that are feeding tend to become circular.

Forage fish

Forage fish, also called prey fish or bait fish, are small fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the food chain on plankton, often by filter feeding...

are small fish which are preyed on by larger predators for food. Predators include other larger fish, seabird

Seabird

Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...

s and marine mammal

Marine mammal

Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...

s. Typical ocean forage fish are small, filter feeding

Filter feeder

Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

fish such as herring

Herring

, anchovies and menhaden

Menhaden

Menhaden, also known as mossbunker, bunker and pogy, are forage fish of the genera Brevoortia and Ethmidium, two genera of marine fish in the family Clupeidae.-Description:...

. Forage fish compensate for their small size by forming schools. Some swim in synchronised grids with their mouths open so they can efficiently filter feed on plankton

Plankton

Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

. These schools can become huge, moving along coastlines and migrating

Fish migration

Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...

across open oceans. The shoals are concentrated food resources for the great marine predators.

These sometimes immense gatherings fuel the ocean food web. Most forage fish are pelagic fish

Pelagic fish

Pelagic fish live near the surface or in the water column of coastal, ocean and lake waters, but not on the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, and reef fish which are associated with coral reefs.The marine pelagic...

, which means they form their schools in open water, and not on or near the bottom (demersal fish). Forage fish are short-lived, and go mostly unnoticed by humans, apart from an occasional support role in a documentary about a great ocean predator. The predators are keenly focused on the shoals, acutely aware of their numbers and whereabouts, and make migrations

Fish migration

Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...

themselves, often in schools of their own, that can span thousands of miles to connect with, or stay connected with them.

Herring

Herring

are among the more spectacular schooling fish. They aggregate together in huge numbers. The largest schools are often formed during migrations by merging with smaller schools. “Chains” of schools one hundred kilometres long have been observed of mullet

Mugil

Mugil is one genus among 17 genera containing all together about 80 species of ray-finned fish of the family Mugilidae commonly known as mullet...

migrating in the Caspian Sea

Caspian Sea

The Caspian Sea is the largest enclosed body of water on Earth by area, variously classed as the world's largest lake or a full-fledged sea. The sea has a surface area of and a volume of...

. Radakov estimated herring schools in the North Atlantic can occupy up to 4.8 cubic kilometres with fish densities between 0.5 and 1.0 fish/cubic metre. That's about three billion fish in one school. These schools move along coastlines and traverse the open oceans. 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

Tuna

and some ocean going shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s. Cetaceans such as dolphins, porpoises and whales, operate in organised social groups called pods.

"Shoaling behaviour is generally described as a trade-off between the anti-predator benefits of living in groups and the costs of increased foraging competition." Landa (1998) argues that the cumulative advantages of shoaling, as elaborated below, are strong selective inducements for fish to join shoals. Parrish et al. (2002) argue similarly that schooling is a classic example of emergence

Emergence

In philosophy, systems theory, science, and art, emergence is the way complex systems and patterns arise out of a multiplicity of relatively simple interactions. Emergence is central to the theories of integrative levels and of complex systems....

, where there are properties that are possessed by the school but not by the individual fish. Emergent properties give an evolutionary advantage to members of the school which non members do not receive.

Social interaction

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. Herring, for instance, will become very agitated if they are isolated from conspecifics. Because of their adaptation to schooling behaviour they are rarely displayed in aquaria

Aquaria

Aquaria may refer to:*Aquarium, in the plural*Aquaria , a computer game*Aquaria , a Brazilian metal band*Aquaria, one of the Twelve Colonies in Battlestar Galactica...

. Even with the best facilities aquaria can offer they become fragile and sluggish compared to their quivering energy in wild schools.

Foraging advantages

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 cyprinid

Cyprinid

The family Cyprinidae, from the Ancient Greek kyprînos , consists of the carps, the true minnows, and their relatives . Commonly called the carp family or the minnow family, its members are also known as cyprinids...

s. In this study, the time it took for groups of minnows and goldfish to find a patch of food was quantified. 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.

"The reason for this is the presence of many eyes searching for the food. 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

Coriolis effect

In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

. Wind-driven surface currents interact with these gyres and the underwater topography, such as seamount

Seamount

A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...

s, fishing banks, and the edge of continental shelves

Continental shelf

The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

, to produce downwellings and upwelling

Upwelling

Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...

s. These can transport nutrients which plankton thrive on. 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. Regions of notable upwelling include coastal Peru

Peru

Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

, Chile

Chile

Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...

, Arabian Sea

Arabian Sea

The Arabian Sea is a region of the Indian Ocean bounded on the east by India, on the north by Pakistan and Iran, on the west by the Arabian Peninsula, on the south, approximately, by a line between Cape Guardafui in northeastern Somalia and Kanyakumari in India...

, western South Africa

South Africa

The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

, eastern New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

and the California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

coast.

Copepod

Copepod

Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s, the primary zooplankton

Zooplankton

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

, are a major item on the forage fish menu. They are a group of small crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s found in ocean and freshwater

Fresh Water

Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

habitats

Habitat (ecology)

A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...

. Copepods are typically one millimetre (0.04 in) to two millimetres (0.08 in) long, with a teardrop shaped body. Some scientists say they form the largest animal biomass

Biomass

Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

on the planet. Copepods are very alert and evasive. They have large antennae

Antenna (biology)

Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

(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 copepod

Copepod

s. They swim with their mouth wide open and their opercula

Operculum (fish)

The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....

fully expanded.
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 copepod

Copepod

s in this synchronised way. The copepods sense with their antennae

Antenna (biology)

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.

Reproductive advantages

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

Fish migration

Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...

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

Norway

Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

, their feeding ground in Iceland

Iceland

Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

, and their nursery ground in northern Norway. Wide triangular journeys such as these may be important because forage fish, when feeding, cannot distinguish their own offspring.

Capelin

Capelin

The capelin or caplin, Mallotus villosus, is a small forage fish of the smelt family found in the Atlantic and Arctic oceans. In summer, it grazes on dense swarms of plankton at the edge of the ice shelf. Larger capelin also eat a great deal of krill and other crustaceans...

are a forage fish of the smelt family found in the Atlantic

Atlantic Ocean

The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

and Arctic

Arctic Ocean

The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...

oceans. In summer, they graze on dense swarms of plankton

Plankton

at the edge of the ice shelf. Larger capelin also eat krill

Krill

Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...

and other crustacean

Crustacean

s. The capelin move inshore in large schools to spawn and migrate in spring and summer to feed in plankton rich areas between Iceland

Iceland

, Greenland

Greenland

Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...

, and Jan Mayen

Jan Mayen

Jan Mayen Island is a volcanic island in the Arctic Ocean and part of the Kingdom of Norway. It is long and 373 km2 in area, partly covered by glaciers . It has two parts: larger northeast Nord-Jan and smaller Sør-Jan, linked by an isthmus wide...

. The migration is affected by ocean current

Ocean current

An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...

s. 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

Spawn (biology)

Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called 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.

Hydrodynamic efficiency

This theory states that groups of fish may save energy when swimming together, much in the way that bicyclists may draft

Drafting (racing)

Drafting or slipstreaming is a technique where two vehicles or other moving objects are caused to align in a close group reducing the overall effect of drag due to exploiting the lead object's slipstream...

one another in a peloton

Peloton

The peloton , field, bunch or pack is the large main group of riders in a road bicycle race. Riders in a group save energy by riding close near other riders...

. Geese flying in a Vee formation are also thought to save energy by flying in the updraft of the wingtip vortex

Wingtip vortices

Wingtip vortices are tubes of circulating air that are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure...

generated by the previous animal in the formation. Increased efficiencies in swimming in groups have been proposed for schools of fish and Antarctic krill

Antarctic krill

Antarctic krill, Euphausia superba, is a species of krill found in the Antarctic waters of the Southern Ocean. It is a shrimp-like crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic metre...

.

It would seem reasonable to think that the regular spacing and size uniformity of fish in schools would result in hydrodynamic efficiencies. However, experiments in the laboratory have failed to find any gains from the hydrodynamic lift created by the neighbours of a fish within a school, though it is still thought that efficiency gains do occur in the wild. Landa (1998) 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.

Predator avoidance

It is commonly observed that schooling fish are particularly in danger of being eaten if they are separated from the school. Several anti-predator functions of fish schools have been proposed.

Confusion effect – One potential method by which fish schools may thwart predators is the ‘predator confusion effect’ proposed and demonstrated by Milinski and Heller (1978). This theory is based on the idea that it becomes difficult for predators to pick out individual prey from groups because the many moving targets create a sensory overload of the predator's visual channel. "Shoaling fish are the same size and silvery, so it is difficult for a visually oriented predator to pick an individual out of a mass of twisting, flashing fish and then have enough time to grab its prey before it disappears into the shoal."
Schooling may confuse other sensory channels than vision. Schooling behaviour confuses the lateral line organ (LLO) as well as the electrosensory system (ESS) of predators.

Fin movements of a single fish will act as a point-shaped wave source, emitting a gradient by which predators might localize it. 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. The LLO is essential in the final stages of a predator attack.

Electro-receptive animals may localize a field source by using spatial non-uniformities. 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. Based on this it was suggested that schooling may confuse the ESS of predators

Many eyes effect – A second 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
Mass collaboration
Mass collaboration is a form of collective action that occurs when large numbers of people work independently on a single project, often modular in its nature...

presumably provide a higher level of vigilance, it could also allow more time for individual feeding.
Dilution effect – A third hypothesis for an anti-predatory effect of fish schools is the "encounter dilution" effect. The dilution effect is an elaboration of safety in numbers
Safety in numbers
Safety in numbers is the hypothesis that, by being part of a large physical group or mass, an individual is proportionally less likely to be the victim of a mishap, accident, attack, or other bad event...

, and interacts with the confusion effect. A given predator attack will eat a smaller proportion of a large shoal than a small shoal. Hamilton proposed that animals aggregate because of a "selfish" avoidance of a predator and was thus a form of cover-seeking. Another formulation of the theory was given by Turner and Pitcher and was viewed as a combination of detection and attack probabilities. In the detection component of the theory, it was suggested that potential prey might benefit by living together since a predator is less likely to chance upon a single group than a scattered distribution. 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

Sardine run

The sardine run of southern Africa occurs from May through July when billions of sardines – or more specifically the Southern African pilchard Sardinops sagax – spawn in the cool waters of the Agulhas Bank and move northward along the east coast of South Africa...

. 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

Blue Wildebeest

The Blue Wildebeest , also called the Common Wildebeest, is a large antelope and one of two species of wildebeest. It grows to 115–145 cm shoulder height and attains a body mass of 168–274 kg. They range the open plains, bushveld and dry woodlands of Southern and East Africa, living for...

Sardine

Sardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines are named after the Mediterranean island of Sardinia, around which they were once abundant....

s have a short life-cycle, living only two or three years. Adult sardines, about two years old, mass on the Agulhas Bank

Agulhas Bank

The Agulhas Bank is a broad, shallow part of the southern African continental shelf which extends south of Cape Agulhas before falling away steeply away to the abyssal plain....

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

Indian Ocean

The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

. A larger shoal might be 7 kilometres (4 mi) long, 1.5 kilometres (1 mi) wide and 30 meters (100 ft) deep. Huge numbers of sharks, dolphins, tuna, sailfish, Cape fur seal

Cape Fur Seal

The brown fur seal , also known as the Cape fur seal, South African fur seal and the Australian fur seal is a species of fur seal.-Description:...

s and even killer whales congregate and follow the shoals, creating a feeding frenzy

Feeding frenzy

In ecology, a feeding frenzy is a situation where oversaturation of a supply of food leads to rapid feeding by predatory animals. For example, a large school of fish can cause nearby sharks to enter a feeding frenzy. This can cause the sharks to go wild, biting anything that moves, including each...

along the coastline.

When threatened, sardines instinctively group together and create massive bait ball

Bait ball

A bait ball, or baitball, occurs when small fish swarm in a tightly packed spherical formation about a common centre. It is a last ditch defensive measure adopted by small schooling fish when they are threatened by predators...

s. Bait balls can be up to 20 meters (70 ft) in diameter. 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 evolution of schooling – The development of schooling behavior was probably associated with an increased quality of perception, predatory lifestyle and size sorting mechanisms to avoid cannibalism. In filter feeding ancestors, before vision and the OLS had developed, the risk of predation would have been limited and mainly due to invertebrate predators, hence, at that time, safety in numbers was probably not a major incentive for gathering together in shoals or schools. The development of the OLS, and vision, 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. This sorting mechanism based on increased quality of perception could have resulted in homogeneity of size of fish in shoals, which would increase the capacity for moving in synchrony.

Predator countermeasures

Predators have devised various countermeasures to undermine the defensive shoaling and schooling manoeuvres of forage fish. Some of these predators, such as dolphins, hunt in groups of their own. One technique employed by many dolphin species is herding

Herding

Herding is the act of bringing individual animals together into a group , maintaining the group and moving the group from place to place—or any combination of those. While the layperson uses the term "herding", most individuals involved in the process term it mustering, "working stock" or...

, 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

Bait ball

.) Corralling is a method where fish are chased to shallow water where they are more easily captured. In South Carolina

South Carolina

South Carolina is a state in the Deep South of the United States that borders Georgia to the south, North Carolina to the north, and the Atlantic Ocean to the east. Originally part of the Province of Carolina, the Province of South Carolina was one of the 13 colonies that declared independence...

, the Atlantic bottlenose dolphin

Bottlenose Dolphin

Bottlenose dolphins, the genus Tursiops, are the most common and well-known members of the family Delphinidae, the family of oceanic dolphins. Recent molecular studies show the genus contains two species, the common bottlenose dolphin and the Indo-Pacific bottlenose dolphin , instead of one...

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.

During the sardine run

Sardine run

, as many as 18,000 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 gannet

Gannet

Gannets are seabirds comprising the genus Morus, in the family Sulidae, closely related to the boobies.The gannets are large black and white birds with yellow heads. They have long pointed wings and long bills. Northern gannets are the largest seabirds in the North Atlantic, with a wingspan of up...

s, cormorant

Cormorant

The bird family Phalacrocoracidae is represented by some 40 species of cormorants and shags. Several different classifications of the family have been proposed recently, and the number of genera is disputed.- Names :...

s, tern

Tern

Terns are seabirds in the family Sternidae, previously considered a subfamily of the gull family Laridae . They form a lineage with the gulls and skimmers which in turn is related to skuas and auks...

s and gull

Gull

Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...

s. Some of these seabirds plummet from heights of 30 metres (100 feet), plunging through the water leaving vapour-like trails, similar to that of fighter planes. Gannet

Gannet

s plunge into the water at up to 100 kilometres per hour (60 mph). They have air sacs under their skin in their face and chest which act like bubble-wrap, cushioning the impact with the water.

The sailfish

Sailfish

'Sailfish' are two species of fish in the genus Istiophorus, living in warmer sections of all the oceans of the world. They are predominately blue to gray in color and have a characteristic erectile dorsal fin known as a sail, which often stretches the entire length of the back...

raises its sail to make it appear much larger so it can herd a school of fish or squid. Swordfish

Swordfish

Swordfish , also known as broadbill in some countries, are large, highly migratory, predatory fish characterized by a long, flat bill. They are a popular sport fish of the billfish category, though elusive. Swordfish are elongated, round-bodied, and lose all teeth and scales by adulthood...

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 shark

Thresher shark

Thresher sharks are large lamniform sharks of the family Alopiidae. Found in all temperate and tropical oceans of the world, the family contains three species all within the genus Alopias.-Taxonomy:...

s 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.

Spinner shark

Spinner shark

The spinner shark is a species of requiem shark, family Carcharhinidae, named for the spinning leaps it makes as a part of its feeding strategy. This species occurs in tropical and warm temperate waters worldwide, except for in the eastern Pacific Ocean...

s charge vertically through the school, spinning on their axis with their mouths open and snapping all around. The shark's momentum at the end of these spiralling runs often carries it into the air.

Some whales lunge feed on bait balls. Lunge feeding is an extreme feeding method, in which the whale accelerates from below a bait ball to a high velocity and then opens its mouth to a large gape angle. 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 rorqual whales is said to be the largest biomechanical

Biomechanics

Biomechanics is the application of mechanical principles to biological systems, such as humans, animals, plants, organs, and cells. Perhaps one of the best definitions was provided by Herbert Hatze in 1974: "Biomechanics is the study of the structure and function of biological systems by means of...

event on Earth.

How fish school

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. There must be very fast response systems to allow the fish to do this. Young fish practise schooling techniques in pairs, and then in larger groups as their techniques and senses mature. 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. 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, similar in function to passive markers in artificial motion capture. However fish without these markers will still engage in schooling behaviour, 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

Lateral line

The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

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. The lateral-line system is very sensitive to changes in water currents and vibration in the water. 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

Cupula

The cupula is a structure in the vestibular system, providing proprioception.The cupula is located within the ampullae of each of the three semicircular canals...

, typically 0.1 to 0.2 mm long. 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. However, many schooling fish, such as herrings and anchovies, do not possess lateral lines.

Describing shoal structure

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

Fisheries acoustics

Fisheries acoustics includes a range of research and practical application topics using acoustical devices as sensors in aquatic environments. Acoustical techniques can be applied to sensing aquatic animals, zooplankton, and physical and biological habitat characteristics.-Basic Theory:Biomass...

.

Parameters defining a fish shoal include:

Shoal size
Group size measures
Many animals, including humans, tend to live in groups, herds, flocks, bands, packs, shoals, or colonies of conspecific individuals. The size of these groups, as expressed by the number of participant individuals, is an important aspect of their social environment...

– The number of fish in the shoal.
Density – The density of a fish shoal is the number of fish divided by the volume occupied by the shoal. Density is not necessarily a constant throughout the group. Fish in schools typically have a density of about one fish per cube of body length.

Polarity – The group polarity describes the extent to which the fish are all pointing in the same direction. In order to determine this parameter, the average orientation of all animals in the group is determined. For each animal, the angular difference between its orientation and the group orientation is then found. The group polarity is the average of these differences (Viscido 2004).
Nearest neighbour distance – The nearest neighbour distance (NND) describes the distance between the centroid of one fish (the focal fish) and the centroid of the fish nearest to the focal fish. This parameter can be found for each fish in an aggregation and then averaged. Care must be taken to account for the fish located at the edge of an fish aggregation, since these fish have no neighbour in one direction. The NND is also related to the packing density. For schooling fish the NND is usually between one-half and one body length.
Nearest neighbour position – In a polar coordinate system, the nearest neighbour position describes the angle and distance of the nearest neighbour to a focal fish.
Packing fraction – The packing fraction is a parameter borrowed from physics to define the organization (or state i.e. solid, liquid, or gas) of 3D fish groups. It is an alternative measure to density. In this parameter, the aggregation is idealized as an ensemble of solid spheres, with each fish at the center of a sphere. The packing fraction is defined as the ratio of the total volume occupied by all individual spheres divided by the global volume of the aggregation (Cavagna 2008). Values range from zero to one, where a small packing fraction represents a dilute system like a gas.
Integrated conditional density – This parameter measures the density at various length scales and therefore describes the homogeneity of density throughout an animal group.
Pair distribution function – This parameter is usually used in physics to characterize the degree of spatial order in a system of particles. It also describes the density, but this measure describes the density at a distance away from a given point. Cavagna et al. found that flocks of starlings exhibited more structure than a gas but less than a liquid.

Modelling school behaviour

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:

Move in the same direction as your neighbour
Remain close to your neighbours
Avoid collisions with your neighbours

An example of such a simulation is the boids

Boids

Boids is an artificial life program, developed by Craig Reynolds in 1986, which simulates the flocking behaviour of birds. His paper on this topic was published in 1987 in the proceedings of the ACM SIGGRAPH conference...

program created by Craig Reynolds

Craig Reynolds (computer graphics)

Craig W. Reynolds , is an artificial life and computer graphics expert, who created the Boids artificial life simulation in 1986. Reynolds worked on the film Tron as a scene programmer, and on Batman Returns as part of the video image crew. He is the author of the OpenSteer library.-External...

in 1986. Another is the self-propelled particle model introduced by Vicsek and Couzin et al in 1995 Many current models use variations on these rules. For instance, many models implement these three rules through layered zones around each fish.

In the zone of repulsion very close to the fish, the focal fish will seek to distance itself from its neighbours in order to avoid a collision.
In the slightly further away zone of alignment, a focal fish will seek to align its direction of motion with its neighbours.
In the outmost zone of attraction, which extends as far away from the focal fish as it is able to sense, the focal fish will seek to move towards a neighbour.

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

Lateral line

. Antarctic krill rely on vision and on hydrodynamic signals relayed through its antennae

Antenna (biology)

.

In a masters thesis published in 2008, Moshi Charnell produced schooling behaviour without using the alignment matching component of an individuals behaviour. His model reduces the three basic rules to the following two rules:

Remain close to your neighbours
Avoid collisions with your neighbours

In a paper published in 2009, researchers from Iceland recount their application of an interacting particle model to the capelin stock around Iceland, successfully predicting the spawning migration route for 2008.

Mapping the formation of schools

In 2009, building on recent advances in acoustic

Acoustics

Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

imaging, a group of MIT researchers observed for "the first time the formation and subsequent migration of a huge shoal of fish." The results provide the first field confirmation of general theories about how large groups behave, from locust swarms to bird flocks

Flocking (behavior)

Flocking behavior is the behavior exhibited when a group of birds, called a flock, are foraging or in flight. There are parallels with the shoaling behavior of fish, the swarming behavior of insects, and herd behavior of land animals....

.

The researchers imaged spawning

Spawn (biology)

Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...

Atlantic herring

Atlantic herring is a fish in the family Clupeidae. It is one of the most abundant fish species on earth. Herring can be found on both sides of the Atlantic Ocean, congregating in large schools. They can grow up to in length and weigh more than...

off Georges Bank

Georges Bank

Georges Bank is a large elevated area of the sea floor which separates the Gulf of Maine from the Atlantic Ocean and is situated between Cape Cod, Massachusetts and Cape Sable Island, Nova Scotia ....

. 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

Audience wave

The wave or the Mexican wave is an example of metachronal rhythm achieved in a packed stadium when successive groups of spectators briefly stand and raise their arms...

travelling around a sport stadium. A rapid transition then occurs, and the fish become highly polarised and synchronized in the manner of schooling fish. After the transition, the schools start migrating, extending up to 40 kilometres (25 mi) across the ocean, to shallow parts of the bank. 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.

Making decisions

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. How are these decisions made? Do more experienced 'leaders' exert more influence than other group members, or does the group make a decision by consensus? A recent investigation showed that small groups of fish used consensus decision-making when deciding which fish model to follow. 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. Consensus decision-making, a form of collective intelligence

Collective intelligence

Collective intelligence is a shared or group intelligence that emerges from the collaboration and competition of many individuals and appears in consensus decision making in bacteria, animals, humans and computer networks....

, thus effectively uses information from multiple sources to generally reach the correct conclusion.

Other open questions of shoaling behaviour include identifying which individuals are responsible for the direction of shoal movement. In the case of migratory

Fish migration

Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...

movement, most members of a shoal seem to know where they are going. Observations on the foraging behaviour of captive golden shiner

Golden shiner

The golden shiner is a cyprinid fish native to eastern North America. It is the sole member of its genus. Much used as a bait fish, it is probably the most widely pond-cultured fish in the United States.-Description:...

(a kind of minnow

Minnow

Minnow is a general term used to refer to small freshwater and saltwater fish, especially those used as bait fish or for fishing bait. More specifically, it refers to small freshwater fish of the carp family.-True minnows:...

) found they formed shoals which were led by a small number of experienced individuals who knew when and where food was available. In a herd of sheep the same lead sheep are always in the front row. But in a school of migrating fish, if the school changes direction, the fish previously at the flank become the lead fish.

One puzzling aspect of shoal selection is how a fish can choose to join a shoal of animals similar to themselves, given that it cannot know its own appearance. Experiments with zebrafish have shown that shoal preference is a learned ability, not innate. A zebrafish tends to associate with shoals that resemble shoals in which it was reared (that is, a form of imprinting

Imprinting (psychology)

Imprinting is the term used in psychology and ethology to describe any kind of phase-sensitive learning that is rapid and apparently independent of the consequences of behavior...

Commercial fishing

The schooling behaviour of fish is exploited on an industrial scale by the commercial fishing

Commercial fishing

Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...

industry. Huge purse seiner vessels use spotter planes to locate schooling fish, such as tuna, cod, mackerel and forage fish. They can capture huge schools by rapidly encircling them with purse seine nets with the help of fast auxiliary boats and sophisticated sonar, which can track the shape of the shoal.

Further examples

Blacksmith
Chromis punctipinnis
The blacksmith is a fish native to the eastern Pacific Ocean. Its range is from Monterey Bay, California, to central Baja California, Mexico. The blacksmith reaches a maximum size of 25 cm and is dark blue.The blacksmith is symbiotic with the senorita fish...

fish live in loose shoals. They have a symbiotic relationship with the parasite eating senorita fish. When they encounter a shoal of senorita fish, they stop and form a tight ball and hang upside down (pictured), each fish waiting its turn to be cleaned. The senorita fish pick dead tissues and external parasites, like parasitic copecods and isocods, from the skin of other fishes.
Subsets of bottlenose dolphin
Bottlenose Dolphin
Bottlenose dolphins, the genus Tursiops, are the most common and well-known members of the family Delphinidae, the family of oceanic dolphins. Recent molecular studies show the genus contains two species, the common bottlenose dolphin and the Indo-Pacific bottlenose dolphin , instead of one...

populations in Mauritania
Mauritania
Mauritania is a country in the Maghreb and West Africa. It is bordered by the Atlantic Ocean in the west, by Western Sahara in the north, by Algeria in the northeast, by Mali in the east and southeast, and by Senegal in the southwest...

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. 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. Rendell & Whitehead have proposed a structure for the study of culture in cetaceans,
Some fish shoals engage in mobbing behaviour. For example, bluegill
Bluegill
The Bluegill is a species of freshwater fish sometimes referred to as bream, brim, or copper nose. It is a member of the sunfish family Centrarchidae of the order Perciformes.-Range and distribution:...

s form large nesting colonies and sometimes attack snapping turtles. This may function to advertise their presence, drive the predator from the area, or aid in cultural transmission of predator recognition.
Piranha
Piranha
A piranha or piraña is a member of family Characidae in order Characiformes, an omnivorous freshwater fish that inhabits South American rivers. In Venezuela, they are called caribes...

have a reputation as fearless fish that hunt in ferocious packs. However, recent research, which "started off with the premise that they school as a means of cooperative hunting", discovered that they were in fact rather fearful fish, like other fish, who schooled for protection from their predators, such as cormorants, caimans and dolphins. Piranhas are "basically like regular fish with large teeth".
Humboldt squid
Humboldt Squid
The Humboldt squid , also known as jumbo squid, jumbo flying squid, pota or diablo rojo , is a large, predatory squid found in the waters of the Humboldt Current in the Eastern Pacific Ocean. They are most commonly found at depths of , from Tierra del Fuego to California...

are large carnivorous marine invertebrates that move in schools of up to 1,200 individuals. They swim at speeds of up to 24 kilometres per hour (15 mph (12.3 kn)) propelled by water ejected through a siphon
Hyponome
A siphon is an anatomical structure which is part of the body of aquatic molluscs in three classes: Gastropoda, Bivalvia and Cephalopoda. In other words, a siphon is found in some saltwater and freshwater snails, in some clams, and in octopus, squid and relatives.Siphons in molluscs are tube-like...

and by two triangular fins. Their tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s bear suckers lined with sharp teeth with which they grasp prey and drag it towards a large, sharp beak. During the day the Humboldt squid are mesopelagic fish, living at depths of 200 to 700 m (656.2 to 2,296.6 ft). Electronic tagging has shown that they also undergo diel vertical migration
Diel vertical migration
Diel vertical migration, also known as diurnal vertical migration, is a pattern of movement that some organisms living in the ocean and in lakes undertake each day. Usually organisms move up to the epipelagic zone at night and return to the mesopelagic zone of the oceans or to the hypolimnion zone...

s which bring them closer to the surface from dusk to dawn. They hunt near the surface at night, taking advantage of the dark to use their keen vision to feed on more plentiful prey. The squid feed primarily on small fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, crustaceans, cephalopods, and copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

, and hunt for their prey in a cooperative fashion, the first observation of such behaviour in invertebrates. The Humboldt squid is also known to quickly devour larger prey when cooperatively hunting in groups. Humboldt squid are known for their speed in feasting on hooked fish, sharks, and squid, even from their own species and shoal, and have been known to attack fishermen and divers. Their colouring and aggressive reputation has earned them the nickname diablos rojos (red devils) from fishermen off the coast of Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

as they flash red and white when struggling with the fishermen.
Other collective nouns used for fish include a draught of fish, a drift of fish, or a scale of fish. Collective nouns used for fish species groups include a grind of blackfish, a troubling of goldfish, glean of herrings, bind or run of salmon, shiver of sharks, fever of stingrays, taint of tilapia, hover of trouts and pod of whales.

External links

Collective Animal Behavior website organized around David Sumpter's book (2008) by the same name
STARFLAG project: Description of starling flocking project
Center for Biologically Inspired Design at Georgia Tech
David Sumpter's research website
Iain Couzin's research website
Website of Julia Parrish, an animal aggregation researcher
Pelagic Fisheries Research Program (2002) Current status and new directions for studying schooling and aggregation behavior of pelagic fish
Clover, Charles (2008) Fish can count to four - but no higher Telegraph Media Group.
Herring Migratory Behaviour

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Overview

Social interaction

Foraging advantages

Reproductive advantages

Hydrodynamic efficiency

Predator avoidance

Predator countermeasures

How fish school

Describing shoal structure

Modelling school behaviour

Mapping the formation of schools

Making decisions

Commercial fishing

Further examples

Further reading

External links