Editing Swarm behaviour (section)

===Marine life===

====Fish====
{{Main|Shoaling and schooling}}
[[File:Moofushi Kandu fish.jpg|thumb|[[Shoaling and schooling|Schooling]] predator fish size up schooling [[anchovy|anchovies]]]]
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|image1=[https://www.youtube.com/watch?v=kI1aU1RVILY Foraging efficiency]<ref>Pitcher et al. 1982.</ref>
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The term "shoal" can be used to describe any group of fish, including mixed-species groups, while "school" is used for more closely knit groups of the same species swimming in a highly synchronised and polarised manner.

Fish derive many benefits from shoaling behaviour including defence against predators (through better predator detection and by diluting the chance of capture), enhanced [[foraging]] success, and higher success in finding a mate.<ref name="Pitcher 1993">Pitcher TJ and Parish JK (1993) [https://books.google.com/books?id=10lElCT7v5wC&dq=%22Behaviour+of+teleost+fishes%22+%22%22Functions+of+shoaling+behaviour+in+teleosts%22%22&pg=PA363 "Functions of shoaling behaviour in teleosts"] In: Pitcher TJ (ed) ''Behaviour of teleost fishes''. Chapman and Hall, New York, pp 363–440</ref> It is also likely that fish benefit from shoal membership through increased [[hydrodynamic]] efficiency.<ref name="Hoare et al. 2000">Hoare DJ, Krause J, Peuhkuri N and Godin JGJ (2000) [https://archive.today/20130105143046/http://www3.interscience.wiley.com/journal/119004845/abstract?CRETRY=1&SRETRY=0 ''Body size and shoaling in fish''] Journal of Fish Biology, 57(6) 1351-1366.</ref>

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 recognised). 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 them. The oddity effect would thus tend to homogenise shoals.<ref>{{cite journal |vauthors= Snekser JL, Ruhl N, Bauer K, McRobert SP |year= 2010 |title= The Influence of Sex and Phenotype on Shoaling Decisions in Zebrafish |url= http://comparativepsychology.org/2010ijcpissue1/05.Snekser_etal_Final.pdf |journal= International Journal of Comparative Psychology |volume= 23 |pages= 70–81 |doi= 10.46867/IJCP.2010.23.01.04 |url-status= dead |archive-url= https://web.archive.org/web/20110725190329/http://comparativepsychology.org/2010ijcpissue1/05.Snekser_etal_Final.pdf |archive-date= 2011-07-25}}</ref>

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, a form of [[imprinting (psychology)|imprinting]].<ref>{{cite journal |vauthors=Engeszer RE, Ryan MJ, Parichy DM |year= 2004 |title= Learned Social Preference in Zebrafish |url= http://www.sbs.utexas.edu/ryan/Publications/2004/2004CurrBiol14%20881.pdf |journal= Current Biology |volume= 14 |issue= 10 |pages= 881–884 |doi= 10.1016/j.cub.2004.04.042 |pmid= 15186744|s2cid= 18741014 |doi-access= free |bibcode= 2004CBio...14..881E }}</ref>

Other open questions of shoaling behaviour include identifying which individuals are responsible for the direction of shoal movement. In the case of [[fish migration|migratory]] movement, most members of a shoal seem to know where they are going. In the case of foraging behaviour, captive shoals of [[golden shiner]] (a kind of [[minnow]]) are led by a small number of experienced individuals who knew when and where food was available.<ref>{{cite journal |doi=10.1006/anbe.1999.1314 |author=Reebs, S.G. |title=Can a minority of informed leaders determine the foraging movements of a fish shoal? |journal=Animal Behaviour |volume=59 |issue=2 |pages=403–9 |year=2000 |pmid=10675263|s2cid=4945309 }}</ref>

Radakov estimated herring schools in the North Atlantic can occupy up to {{convert|4.8|km3}} with fish densities between 0.5 and 1.0 fish/cubic metre, totalling several billion fish in one school.<ref>Radakov DV (1973) ''Schooling in the ecology of fish.'' Israel Program for Scientific Translation, translated by Mill H. Halsted Press, New York. {{ISBN|978-0-7065-1351-6}}</ref>

{{See also|Eel life history}}
* Partridge BL (1982) [https://web.archive.org/web/20110703035553/https://137.146.28.140/biology/BI358j/Readings/Partridge%201982.pdf "The structure and function of fish schools"] ''Scientific American'', June:114–123.
* {{cite journal |vauthors= Parrish JK, Viscido SV, Grunbaum D |year= 2002 |title= Self-Organized Fish Schools: An Examination of Emergent Properties |url= http://faculty.washington.edu/random/Reprints/Parrishetal02.pdf |archive-url= https://web.archive.org/web/20060911165104/http://faculty.washington.edu/random/Reprints/Parrishetal02.pdf |url-status= dead |archive-date= 11 September 2006 |journal= Biol. Bull. |volume= 202 |issue= 3 |pages= 296–305 |doi= 10.2307/1543482 |pmid= 12087003 |jstor= 1543482 |citeseerx= 10.1.1.116.1548 |s2cid= 377484 }}

====Fish migration====
{{Main|Fish migration}}
{{See also|Sardine run|Salmon run}}
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|image1=[https://www.youtube.com/watch?v=xYl4m0xFcCU Video clip of the "Sardine run"]<ref>Photographer: Mark van Coller</ref>
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Between May and July huge numbers of [[sardine]]s spawn in the cool waters of the [[Agulhas Bank]] and then follow a current of cold water northward along the east coast of South Africa. This great migration, called the [[sardine run]], creates spectacular feeding frenzies along the coastline as marine predators, such as dolphins, sharks and gannets attack the schools.

====Krill====
[[File:Krill swarm.jpg|right|thumb|Swarming [[krill]]]]
Most [[krill]], small shrimp-like [[crustacean]]s, form large swarms, sometimes reaching densities of 10,000–60,000 individual animals per cubic metre.<ref name="H+83">{{cite journal |last1=Hamner |first1=WM |last2=Hamner |first2=PP |last3=Strand |first3=SW |last4=Gilmer |first4=RW |title=Behavior of Antarctic Krill, ''Euphausia superba'': Chemoreception, Feeding, Schooling and Molting' |journal=[[Science (journal)|Science]] |volume=220 |pages=433–5 |year=1983 |doi=10.1126/science.220.4595.433 |pmid=17831417 |issue=4595|bibcode= 1983Sci...220..433H|s2cid=22161686 }}</ref><ref>{{cite book |author1=U. Kils |author2=P. Marshall |chapter=Der Krill, wie er schwimmt und frisst – neue Einsichten mit neuen Methoden ("''The Antarctic krill – how it swims and feeds – new insights with new methods''") |editor1=I. Hempel |editor2=G. Hempel |title=Biologie der Polarmeere&nbsp;– Erlebnisse und Ergebnisse (''Biology of the Polar Oceans Experiences and Results'') |publisher=[[Fischer Verlag]] |year=1995 |pages=201–210 |isbn=978-3-334-60950-7}}</ref><ref>{{cite book |author=R. Piper |title=Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals |url=https://archive.org/details/extraordinaryani0000pipe |url-access=registration |publisher=Greenwood Press |year=2007 |isbn=978-0-313-33922-6}}</ref> Swarming is a defensive mechanism, confusing smaller predators that would like to pick out single individuals. The largest swarms are visible from space and can be tracked by satellite.<ref>Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. {{ISBN|978-0-565-09243-6}}.</ref> One swarm was observed to cover an area of 450 square kilometres (175 square miles) of ocean, to a depth of 200 meters (650 feet) and was estimated to contain over 2 million tons of krill.<ref name="Hoare, Ben 2009 p. 107">Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. {{ISBN|978-0-565-09243-6}}</ref> Recent research suggests that krill do not simply drift passively in these currents but actually modify them.<ref name="Hoare, Ben 2009 p. 107"/> Krill typically follow a [[diurnality|diurnal]] [[diel vertical migration|vertical migration]]. By moving vertically through the ocean on a 12-hour cycle, the swarms play a major part in mixing deeper, nutrient-rich water with nutrient-poor water at the surface.<ref name="Hoare, Ben 2009 p. 107"/> Until recently it has been assumed that they spend the day at greater depths and rise during the night toward the surface. It has been found that the deeper they go, the more they reduce their activity,<ref>{{cite journal |author1=J.S. Jaffe |author2=M.D. Ohmann |author3=A. de Robertis |url=http://jaffeweb.ucsd.edu/files/pubs/Sonar%20estimates%20of%20daytime%20activity%20levels%20of%20Euphausia%20pacifica%20in%20Saanich%20Inlet.pdf |title=Sonar estimates of daytime activity levels of ''Euphausia pacifica'' in Saanich Inlet |journal=Canadian Journal of Fisheries and Aquatic Sciences |volume=56 |issue=11 |pages=2000–10 |year=1999 |doi=10.1139/cjfas-56-11-2000 |s2cid=228567512 |url-status=dead |archive-url=https://web.archive.org/web/20110720075623/http://jaffeweb.ucsd.edu/files/pubs/Sonar%20estimates%20of%20daytime%20activity%20levels%20of%20Euphausia%20pacifica%20in%20Saanich%20Inlet.pdf |archive-date=2011-07-20}}</ref> apparently to reduce encounters with predators and to conserve energy.

Later work suggested that swimming activity in krill varied with stomach fullness. Satiated animals that had been feeding at the surface swim less actively and therefore sink below the mixed layer.<ref>{{cite journal |journal=Current Biology |year=2006 |volume=16 |pmid=16461267 |issue=3 |pages=83–4 |doi=10.1016/j.cub.2006.01.044 |title=Satiation gives krill that sinking feeling |author1=Geraint A. Tarling |author2=Magnus L. Johnson |name-list-style=amp|doi-access=free |bibcode=2006CBio...16..R83T }}</ref> As they sink they produce faeces which may mean that they have an important role to play in the Antarctic carbon cycle. Krill with empty stomachs were found to swim more actively and thus head towards the surface. This implies that vertical migration may be a bi- or tri-daily occurrence. Some species form surface swarms during the day for feeding and reproductive purposes even though such behaviour is dangerous because it makes them extremely vulnerable to predators.<ref name="howard">Howard, D.: "[http://pubs.usgs.gov/circ/c1198/chapters/133-140_Krill.pdf Krill]", pp.&nbsp;133–140 in Karl, H.A. et al. (eds): ''[http://pubs.usgs.gov/circ/c1198/ Beyond the Golden Gate – Oceanography, Geology, Biology, and Environmental Issues in the Gulf of the Farallones]'', [[U.S. Geological Survey|USGS]] Circular 1198, 2001. URLs last accessed 2010-06-04.</ref> Dense swarms may elicit a [[feeding frenzy]] among fish, birds and mammal predators, especially near the surface. When disturbed, a swarm scatters, and some individuals have even been observed to [[ecdysis|moult]] instantaneously, leaving the [[exuvia]] behind as a decoy.<ref>{{cite web |author=D. Howard |url=http://oceanexplorer.noaa.gov/explorations/02quest/background/krill/krill.html |title=Krill in Cordell Bank National Marine Sanctuary |publisher=[[National Oceanic and Atmospheric Administration|NOAA]] |access-date=15 June 2005}}</ref> In 2012, Gandomi and Alavi presented what appears to be a [[Swarm intelligence#Krill herd algorithm|successful stochastic algorithm]] for modelling the behaviour of krill swarms. The algorithm is based on three main factors: " (i) movement induced by the presence of other individuals (ii) foraging activity, and (iii) random diffusion."<ref name=kha2012>{{cite journal |first1=A.H. |last1= Gandomi |first2=A.H. |last2=Alavi |title= Krill Herd Algorithm: A New Bio-Inspired Optimization Algorithm |journal= Communications in Nonlinear Science and Numerical Simulation |doi=10.1016/j.cnsns.2012.05.010|year=2012 |bibcode= 2012CNSNS..17.4831G |volume=17 |issue=12 |pages=4831–4845}}</ref>

====Copepods====
[[File:Copepodkils.jpg|thumb|right|This copepod has its antenna spread (click to enlarge). The antenna detects the pressure wave of an approaching fish.]]
{{See also|Hunting copepods}}

[[Copepod]]s are a group of tiny [[crustacean]]s found in the sea and lakes. Many species are [[plankton]]ic (drifting in sea waters), and others are [[benthos|benthic]] (living on the ocean floor). Copepods are typically {{convert|1|to|2|mm|in|2}} long, with a teardrop shaped body and large [[antenna (biology)|antennae]]. Although like other crustaceans they have an armoured [[exoskeleton]], they are so small that in most species this thin armour, and the entire body, is almost totally transparent. Copepods have a compound, median single eye, usually bright red, in the centre of the transparent head.

Copepods also swarm. For example, monospecific swarms have been observed regularly around [[coral reef]]s and [[sea grass]], and in lakes. Swarms densities were about one million copepods per cubic metre. Typical swarms were one or two metres in diameter, but some exceeded 30 cubic metres. Copepods need visual contact to keep together, and they disperse at night.<ref>{{cite journal |last1= Hamner |first1= WM |last2= Carleton |first2= JH |year= 1979 |title= Copepod swarms: Attributes and role in coral reef ecosystems |journal= Limnol. Oceanogr. |volume= 24 |issue= 1 |pages= 1–14 |doi= 10.4319/lo.1979.24.1.0001 |bibcode= 1979LimOc..24....1H|doi-access= free }}</ref>

Spring produces [[algal bloom|blooms]] of swarming [[phytoplankton]] which provide food for copepods. Planktonic copepods are usually the dominant members of the [[zooplankton]], and are in turn major food organisms for many other marine animals. In particular, copepods are prey to [[forage fish]] and [[jellyfish]], both of which can assemble in vast, million-strong swarms. Some copepods have extremely fast [[escape response]]s when a predator is sensed and can jump with high speed over a few millimetres (see animated image below).

<gallery>
File:Herringramkils.jpg|Photo: School of herrings [[ram feeding]] on a swarm of copepods.
File:Synchropredation.gif|Animation showing how herrings hunting in a synchronised way can capture the very alert and evasive copepod (click to view).
File:Jelly cc4.jpg|Swarms of [[jellyfish]] also prey on copepods
</gallery>

Planktonic copepods are important to the [[carbon cycle]]. Some scientists say they form the largest animal [[biomass]] on earth.<ref>{{cite web |author1=Johannes Dürbaum |author2=Thorsten Künnemann |name-list-style=amp |date=November 5, 1997 |title=Biology of Copepods: An Introduction |url=http://www.uni-oldenburg.de/zoomorphology/Biologyintro.html |publisher=[[Carl von Ossietzky University of Oldenburg]] |access-date=December 8, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20100526164720/http://www.uni-oldenburg.de/zoomorphology/Biologyintro.html |archive-date=May 26, 2010}}</ref> They compete for this title with [[Antarctic krill]]. Because of their smaller size and relatively faster growth rates, however, and because they are more evenly distributed throughout more of the world's oceans, copepods almost certainly contribute far more to the [[secondary productivity]] of the world's oceans, and to the global ocean [[carbon sink]] than [[krill]], and perhaps more than all other groups of organisms together. The surface layers of the oceans are currently believed to be the world's largest carbon sink, absorbing about 2 billion tonnes of carbon a year, the equivalent to perhaps a third of [[greenhouse gas|human carbon emissions]], thus reducing their impact. Many planktonic copepods feed near the surface at night, then sink into deeper water during the day to avoid visual predators. Their moulted exoskeletons, faecal pellets and respiration at depth all bring carbon to the deep sea.

====Algal blooms====
Many single-celled organisms called [[phytoplankton]] live in oceans and lakes. When certain conditions are present, such as high nutrient or light levels, these organisms reproduce explosively. The resulting dense swarm of phytoplankton is called an [[algal bloom]]. Blooms can cover hundreds of square kilometres and are easily seen in satellite images. Individual phytoplankton rarely live more than a few days, but blooms can last weeks.<ref>Lindsey R and Scott M (2010) [http://earthobservatory.nasa.gov/Features/Phytoplankton/page1.php What are phytoplankton] [[NASA Earth Observatory]].</ref><ref name=NOAA_HAB>[http://www.glerl.noaa.gov/res/Centers/HumanHealth/docs/habs.pdf Harmful algal blooms in the Great Lakes] {{Webarchive|url=https://web.archive.org/web/20100616173236/http://www.glerl.noaa.gov/res/Centers/HumanHealth/docs/habs.pdf |date=2010-06-16}} 2009, [[NOAA]], Center of Excellence for Great Lakes and Human Health.</ref>