A bait ball, or baitball, occurs when small fish

swarm Swarm behaviour, or swarming, is a collective behaviour exhibited by entities, particularly animals, of similar size which aggregate together, perhaps milling about the same spot or perhaps moving ''en masse'' or migrating in some direction. ...

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. Small schooling fish are eaten by many types of

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

, and for this reason they are called

bait fish 300px, Feeder Goldfish are common baitfish. Bait fish (or baitfish) are small-sized fish caught and used by anglers as bait to attract larger predatory fish, particularly game fish. Baitfish species are typically those that are common and br ...

forage fish Forage fish, also called prey fish or bait fish, are small pelagic 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 ...

. For example,

sardine "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the It ...

s group together when they are threatened. This instinctual behaviour is a defence mechanism, as lone individuals are more likely to be eaten than an individual in a large group. Sardine bait balls can be 10–20 metres in diameter and extend to a depth of 10 metres. The bait balls are short-lived and seldom last longer than 10 minutes. However, bait balls are also conspicuous, and when schooling fish form a bait ball, they can draw the attention of many other predators. As a response to the defensive capabilities of schooling fish, some predators have developed sophisticated countermeasures. These countermeasures can be spectacularly successful, and can seriously undermine the defensive value of forming bait balls.

Background

Small

pelagic fish Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that do live on or near the bottom, and reef fish that are associated with coral reef ...

live in the open water, so unlike

demersal The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer ...

or reef fish, they cannot hide among kelp, or in crevices in coral, or under rocks on the bottom. This leaves them vulnerable to attack by large predatory fish, as well as other predators, such as

marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their re ...

s and

seabird Seabirds (also known as marine birds) are birds that are 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 envir ...

s. As a result, small pelagic fish usually aggregate in

schools A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is sometimes compulso ...

for protection. Schooling fish have evolved sophisticated evasion techniques. When they school, they have many eyes, making ambush difficult; and their silvery bodies dazzle, making it difficult for predators to pick out individual fish.Megurran, AE (1990
"The adaptive significance of schooling as an anti-predator defense in fish"
''Annales Zooligici Fennici'', 27: 51–66. They react to movements from a predator with lightning reflexes, rhythmically streaming up and down with rapid direction changes. When a predator approaches, they can split and reform behind the predator. During the final stages of an attack, they can explosively disband in all directions and then just as rapidly reform.Partridge BL (1982
"The structure and function of fish schools"
''Scientific American'', 246(6) 114–123. Defensive manoeuvres like these appear to be choreographed, though they are not. Within the school itself, there is no centralized intelligence aware of how the school has configured itself. Rather, the schooling behaviour is the emergent consequence of relatively simple rules followed by each individual fish in the school, such as remaining close together, moving in the same direction, and avoiding collisions with each other. Some species of forage fish, driven by nutrient availability and their life-cycle stage, form vast schools at predictable locations and times of the year. Normally, schooling works well as protection from occasional predators. Fish schooling in vast numbers may attract correspondingly huge numbers of predators, including seabirds, sharks, tuna, billfish, pods of dolphins, and killer and humpback whales. Aggregations of predators on such a scale means the schools can be attacked on all sides and panicked into forming bait balls.Seifert DD (2010
Water Column: Finishing School
November 2010, ''Dive Magazine UK''. A bait ball is a last-ditch defensive measure adopted by fish schools when they are overwhelmed and more effective defensive strategies have broken down. Schooling fish are easier to attack once they abandon their free streaming behaviour and form into a tight bait ball. Many predator species have learned that by interacting cooperatively they can panic schooling fish into forming a bait ball. This cooperative behaviour can occur both intraspecifically (among the individuals within a predator species) and

interspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organis ...

ally (across individuals belonging to more than one predator species).

Formation and dissolution

The process that leads to the formation of a bait ball typically starts when predators locate a fish school deep below the surface. The predators make rushes and use various scare tactics to force the fish school to the surface, herding it at the same time into a compact volume. The alarmed fish, trapped against the surface above and surrounded all about, abandon their coordinated schooling movements and become chaotic. Their graceful and disciplined schooling strategies of uniform spacing and polarity degrade into frenetic attempts by each fish to save itself. In this way, a dense bait ball forms as each fish scrambles to get away from the surface of the ball and hide in the interior. The symmetry of this centripetal action forms a sphere, the shape with the minimum surface area for a given volume, thereby exposing the fewest fish on the surface to the predators. The movement, sound and smell can attract more predators, including different predator species, until there is a carousel of them, each species using its own characteristic predatory strategies. Fish that break loose are singled out and eaten. A

frenzy ''Frenzy'' is a 1972 British thriller film directed by Alfred Hitchcock. It is the penultimate feature film of his extensive career. The screenplay by Anthony Shaffer was based on the 1966 novel '' Goodbye Piccadilly, Farewell Leicester Squa ...

can develop as predators compete. As the bait ball reduces in size and number, it becomes progressively easier for the predators to target the survivors.

Predator strategies

Predators have devised various countermeasures to disrupt the defensive shoaling and schooling manoeuvres of forage fish. Often this involves charging the school or bait ball at high speed. 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 action generates the water pressure required to expand its mouth and engulf and filter a huge amount of water and fish. Lunge feeding by

rorqual Rorquals () are the largest group of baleen whales, which comprise the family Balaenopteridae, containing ten extant species in three genera. They include the largest animal that has ever lived, the blue whale, which can reach , and the fin wha ...

s, a family of huge baleen whales that includes the

blue whale The blue whale (''Balaenoptera musculus'') is a marine mammal and a baleen whale. Reaching a maximum confirmed length of and weighing up to , it is the largest animal known to have ever existed. The blue whale's long and slender body can b ...

, is said to be the largest biomechanical event on Earth.

Swordfish Swordfish (''Xiphias gladius''), also known as broadbills in some countries, are large, highly migratory predatory fish characterized by a long, flat, pointed bill. They are a popular sport fish of the billfish category, though elusive. Swordfi ...

charge at high speeds 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 sharks are large lamniform sharks of the family Alopiidae found in all temperate and tropical oceans of the world; the family contains three extant species, all within the genus ''Alopias''. All three thresher shark species have been ...

s use their long tails to stun shoaling fishes.Seitz, J.C
Pelagic Thresher
''Florida Museum of Natural History.'' Retrieved on December 22, 2008. Spinner sharks charge vertically through schools, 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.

Gannet Gannets are seabirds comprising the genus ''Morus'' in the family Sulidae, closely related to boobies. Gannets are large white birds with yellowish heads; black-tipped wings; and long bills. Northern gannets are the largest seabirds in the ...

s plummet from heights of 30 metres (100 feet), plunging through the water and leaving vapour-like trails behind like fighter planes. They enter the water at speeds up to 86 kilometres per hour (53 mph) and descend to depths of 34 metres (111 feet). Under the skin in their faces and chests, gannets have air sacs which act like bubble-wrap, cushioning the impact with water. File:Xiphias gladius1.jpg, Swordfish slash with their swords File:Alopias vulpinus.jpg, Thresher sharks strike with their tails File:Carcharhinus brevipinna.jpg, Spinner sharks spin on their axis File:Morus serrator.jpg, Gannets "divebomb" at high speed

Predator cooperation

The most effective strategy predators use against schooling fish is first to scare them into forming a bait ball. Strategies such as those outlined in the previous section can work to a degree against freely streaming fish schools, but work much better if the fish school is first compacted into a bait ball. It is difficult for predators working individually to scare a fish school into a bait ball, and they usually work together. *

s compact their prey by swimming in circles around them, splashing the water with their long tails, often in pairs or small groups. They then strike sharply at the bait ball to stun the fish with the upper lobe of their tails. * Schools of forage fish can draw

silky shark The silky shark (''Carcharhinus falciformis''), also known by numerous names such as blackspot shark, gray whaler shark, olive shark, ridgeback shark, sickle shark, sickle-shaped shark and sickle silk shark, is a species of requiem shark, in the ...

s in large numbers. Silky sharks have been documented "herding" such schools into a bait ball trapped against the surface, and then consuming the entire school. When attacking tightly packed fish, silky sharks charge through the ball and slash open-mouthed, catching the prey fish at the corners of their jaws. Although multiple individuals may feed at once, each launches its attack independently.Martin, R.A
Open Ocean: Silky Shark
ReefQuest Centre for Shark Research. Retrieved on September 12, 2009. * Pods of many

dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae ( ...

species commonly herd a school of fish into a bait ball while individual members take turns ploughing through and feeding on the more compacted shoal. ''Corralling'' is a method where fish are chased to shallow water where they are more easily captured. Some dolphins take this further with ''strand feeding'', driving prey until they are stranded on mud banks, where they can be easily accessed. Dolphins have also been observed blowing bubbles to startle and separate individual fish from a bait ball. * The

humpback whale The humpback whale (''Megaptera novaeangliae'') is a species of baleen whale. It is a rorqual (a member of the family Balaenopteridae) and is the only species in the genus ''Megaptera''. Adults range in length from and weigh up to . The hu ...

uses a feeding technique called '' bubble net feeding''. A group of whales swim in a shrinking circle, blowing bubbles below a school of prey fish.Hain JHW, Carter GR, Krau, SD, Mayo CA and Winn HE (1982) "Feeding behaviour of the humpback whale, ''Megaptera novaeangliae'', in the Western North Atlantic". ''Fishery Bulletin'', 80: 259–268. Forage fish show a strong fear of bubbles and can be easily contained within a bubble curtain. The shrinking ring of bubbles encircles the school and confines it in an ever smaller cylinder. The whales then lunge feed, often as a synchronised group, suddenly swimming upward through the "net", mouths agape, swallowing thousands of fish in one gulp. The ring can begin at up to in diameter with perhaps a dozen whales cooperating. The use of a crittercam attached to whale's back has revealed that some whales blow the bubbles while others dive deeper to drive fish toward the surface, and yet others herd the prey into the net by vocalizing. Some humpback whales also scare schooling fish by slapping their tails ( lobtail). Although many whale species lunge feed, only humpbacks use bubble nets. *

Killer whale The orca or killer whale (''Orcinus orca'') is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member. It is the only extant species in the genus '' Orcinus'' and is recognizable by its black-and-white p ...

s usually hunt larger fish, such as salmon, individually or in small group of individuals. However, forage fish, such as

herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Oceans, ...

s, are often caught using '' carousel feeding''. The killer whales force the herrings into a tight ball by releasing bursts of bubbles or flashing their white undersides. They then slap the ball with their tail flukes, either stunning or killing up to 10–15 herring with a successful slap. The herring are then eaten one at a time. Carousel feeding has been documented in only the Norwegian killer whale population and with some oceanic dolphin species. *

Sperm whale The sperm whale or cachalot (''Physeter macrocephalus'') is the largest of the toothed whales and the largest toothed predator. It is the only living member of the genus '' Physeter'' and one of three extant species in the sperm whale fami ...

s may also herd prey.

Mixed species feeding

Fish that school in large numbers can draw the attentions of many different predator species. The attraction of huge numbers of prey fish means that these predator species, which might otherwise be mutually antagonistic, usually cooperate with each other in pursuit of their common goal.

Bryde's whale Bryde's whale ( Brooder's), or the Bryde's whale complex, putatively comprises three species of rorqual and maybe four. The "complex" means the number and classification remains unclear because of a lack of definitive information and research ...

s often track dolphin groups as they herd prey species. As soon as the dolphins corral the prey, the whales lunge feed through the centre. In 2001, Clua and Grosvalet proposed a four stage model to describe mixed species feeding behaviour involving

common dolphin The common dolphin (''Delphinus delphis'') is the most abundant cetacean in the world, with a global population of about six million. Despite this fact and its vernacular name, the common dolphin is not thought of as the archetypal dolphin, wit ...

tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae ( mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max len ...

and

shearwater Shearwaters are medium-sized long-winged seabirds in the petrel family Procellariidae. They have a global marine distribution, but are most common in temperate and cold waters, and are pelagic outside the breeding season. Description These tu ...

sea birds. * In the preparation phase, the dolphins rapidly circle the fish school to compact it. * In the intensification phase, the bait ball is further structured by dolphins circling and foraging the periphery of the ball, while shearwaters forage from the ocean surface. * At the mature phase, the dolphins temporarily scatter as giant tuna lunge vertically through the centre of the ball. At this point the concentration of the bait ball is optimal. * Finally in the dispersion phase, the tuna and sea birds disperse, and some dolphins return to forage on the remnants of the ball. Most years, off southern Africa between May and July, billions of

s (specifically the Southern African

pilchard "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the Ita ...

Sardinops sagax ''Sardinops'' is a monotypic genus of sardines of the family Clupeidae. The only member of the genus is ''Sardinops sagax''. It is found in the Indo-Pacific and East Pacific oceans. Its length is up to . It has numerous common or vernacular names ...

'') spawn in the cool waters of the

Agulhas Bank The Agulhas Bank (, from Portuguese for Cape Agulhas, ''Cabo das Agulhas'', "Cape of Needles") is a broad, shallow part of the southern African continental shelf which extends up to south of Cape Agulhas before falling steeply to the abyssal p ...

and move northward along the east coast of

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...

. This great

fish migration Fish migration is mass relocation by fish from one area or body of water to another. 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 thousa ...

is called the

sardine run The KwaZulu-Natal 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 northwar ...

. Their sheer numbers create a feeding frenzy along the

coastline The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in ...

. The run, containing millions of individual sardines, occurs when a current of cold water heads north from the Agulhas Bank up to

Mozambique Mozambique (), officially the Republic of Mozambique ( pt, Moçambique or , ; ny, Mozambiki; sw, Msumbiji; ts, Muzambhiki), is a country located in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Mal ...

where it then leaves the coastline and goes further east into the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...

. During the 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 Gannets are seabirds comprising the genus ''Morus'' in the family Sulidae, closely related to boobies. Gannets are large white birds with yellowish heads; black-tipped wings; and long bills. Northern gannets are the largest seabirds in the ...

cormorant Phalacrocoracidae is a family of approximately 40 species of aquatic birds commonly known as cormorants and shags. Several different classifications of the family have been proposed, but in 2021 the IOC adopted a consensus taxonomy of seven gen ...

tern Terns are seabirds in the family Laridae that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated as a subgroup of the family Laridae which includes gulls and skimmers and consists of ...

s and

gull Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st centur ...

s. The sardine run is featured in the first episode of the 2001 BBC nature documentary ''

The Blue Planet ''The Blue Planet'' is a British nature documentary series created and produced by the BBC. It premiered on 12 September 2001 in the United Kingdom. It is narrated by David Attenborough. Described as "the first ever comprehensive series on th ...

'' and in the 2008 3D IMAX documentary film '' Wild Ocean''. A similar great migration of

s occurs each year during the summer

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruci ...

bloom along the coast of British Columbia and Alaska. The migration is featured in the final episode of the 2009 BBC wildlife documentary ''

Nature's Great Events ''Nature's Great Events'' is a wildlife documentary series made for BBC television, first shown in the UK on BBC One and BBC HD in February 2009. The series looks at how seasonal changes powered by the sun cause shifting weather patterns and oc ...

''. In winter, the coastal

fjord In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Germany, Greenland, the Faroe Islands, Icel ...

s and inlets are relatively lifeless, and the resident

Steller sea lion The Steller sea lion (''Eumetopias jubatus''), also known as the Steller's sea lion and northern sea lion, is a near-threatened species of sea lion in the northern Pacific. It is the sole member of the genus ''Eumetopias'' and the largest of t ...

s must dive deeper and further from the coast to catch the widely-dispersed

Humpback whale The humpback whale (''Megaptera novaeangliae'') is a species of baleen whale. It is a rorqual (a member of the family Balaenopteridae) and is the only species in the genus ''Megaptera''. Adults range in length from and weigh up to . The hu ...

s overwinter in the warm Pacific waters off

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only ...

, where new mothers suckle their calves. They begin their 3,000 mile journey north in early spring, when the sea lions also give birth to their young. Spring storms disturb nutrients in the water which, together with the strengthening power of the sun, act as the catalysts for the plankton bloom. Huge shoals of herring arrive to spawn, turning the shallows milky white. The herring sift plankton from the water. In their wake come larger predators, including

Pacific white-sided dolphin The Pacific white-sided dolphin (''Lagenorhynchus obliquidens''), also known as the hookfin porpoise, is an active dolphin found in the cool or temperate waters of the North Pacific Ocean. Taxonomy The Pacific white-sided dolphin was named by Sm ...

s and

killer whale The orca or killer whale (''Orcinus orca'') is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member. It is the only extant species in the genus '' Orcinus'' and is recognizable by its black-and-white p ...

Common murre The common murre or common guillemot (''Uria aalge'') is a large auk. It has a circumpolar distribution, occurring in low-Arctic and boreal waters in the North Atlantic and North Pacific. It spends most of its time at sea, only coming to land t ...

s dive under the herring shoals and pick off the fish from below, pinning them to the surface. Their defence is to form a bait ball, but

s gathering on the surface attack them from above. The finale to the programme features unique underwater footage of humpbacks engulfing whole bait balls, and reveals their co-operative hunting behaviour called bubble-netting.

Notes

References

* Burgess EA (2006
"Foraging ecology of common dolphins (Delphinus sp.) in the Hauraki Gulf, New Zealand"
MSc thesis, Institute of Natural Resources, Massey University. * Dement
Species Spotlight: Atlantic Sailfish (''Istiophorus albicans'')
''littoralsociety.org''. Retrieved 28 March 2011. * Leighton TG, PR White and Finfer DC (2009
"Hypotheses on the acoustics of whales, dolphins and porpoises in bubbly water"
''Proceedings of the Third International Conference on Underwater Acoustic Measurements, Technologies and Results''. * Fetecau R (2010
"Collective behavior of biological aggregations in two dimensions: a nonlocal kinetic model"
''Mathematical Models and Methods in Applied Sciences'', August 2010. * Marshall Cavendish Corporation (2004
''Encyclopedia Of The Aquatic World''
Page 164, Marshall Cavendish. . * * Robinson KP, Stevick PT and MacLeod CD (2007
An integrated approach to non-lethal research on minke whales in European waters
European Cetacean Society, Special publication series, No 47. * Robinson KP and Tetley MJ (2007
"Behavioural observations of foraging minke whales (''Balaenoptera acutorostrata'') in the outer Moray Firth, north-east Scotland"
''J. Mar. Biol. Assoc. U. K.'', 87: 85–86. * Sfakiotakisa M and Tsakiris DP (2007
"Neuromuscular control of reactive behaviors for undulatory robots"
''Neurocomputing'', 70(10–12): 1907–1913. * Steiner L (1995
"Rough-toothed dolphin, ''Steno bredanensis'': a new species record for the Azores, with some notes on behaviour"
''Life and Marine Sciences'', 13A: 125–127. * Waters, Hannah (2010
Now in 3-D: The shape of krill and fish schools
''Scientific American''. Invited commentary, 10 November 2010.

External links

Computer simulations of bait balls

''ScienceDaily''. 24 June 2011. {{DEFAULTSORT:Bait ball Ichthyology

Background

Formation and dissolution

Predator strategies

Predator cooperation

Mixed species feeding

See also

Notes

References

External links