View source for Fish

{{Short description|Gill-bearing non-tetrapod aquatic vertebrates}}
{{for-multi|fish as eaten by humans|Fish as food|the superclass of living bony fishes|Osteichthyes|other uses|Fish (disambiguation)}}
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{{Use dmy dates|date=October 2017}}
{{Paraphyletic group
| name = Fish
| fossil_range = {{fossilrange|535|0}} [[Middle Cambrian]] – [[Recent]]
| image = Balantiocheilos melanopterus - Karlsruhe Zoo 02 (cropped).jpg
| image_caption = [[Bala shark]], a bony fish
| auto = yes
| parent = Vertebrata
| includes = *"[[Agnatha]]"
*{{extinct}}"[[Placodermi]]"
*{{extinct}}"[[Acanthodii]]"
*[[Chondrichthyes]]
*[[Osteichthyes]]
**[[Actinopterygii]]
**[[Sarcopterygii]] (including [[tetrapod]]s)
| excludes = *[[Tetrapod|Tetrapoda]]
}}
A '''fish''' ({{plural form}}: '''fish''' or '''fishes''') is an [[aquatic animal|aquatic]], [[Anamniotes|anamniotic]], [[gill]]-bearing [[vertebrate]] [[animal]] with swimming [[fish fin|fins]] and [[craniate|a hard skull]], but lacking [[limb (anatomy)|limbs]] with [[digit (anatomy)|digits]]. Fish can be grouped into the more [[basal (phylogenetics)|basal]] [[jawless fish]] and the more common [[jawed fish]], the latter including all [[extant taxon|living]] [[cartilaginous fish|cartilaginous]] and [[bony fish]], as well as the extinct [[placoderm]]s and [[acanthodian]]s. In a break to the long tradition of grouping all fish into a single [[Class (biology)|class]] (Pisces), modern [[phylogenetics]] views fish as a [[paraphyletic]] group.

Most fish are [[ectotherm|cold-blooded]], their body temperature varying with the surrounding water, though some large [[nekton|active swimmers]] like [[white shark]] and [[tuna]] can hold a higher [[core temperature]]. Many fish can [[communication in aquatic animals#Acoustic|communicate acoustically]] with each other, such as during [[courtship display]]s. The study of fish is known as [[ichthyology]].

The earliest fish appeared during the [[Cambrian]] as small [[filter feeder]]s; they continued to [[evolution|evolve]] through the [[Paleozoic]], diversifying into many forms. The earliest fish with dedicated respiratory gills and [[paired fins]], the [[ostracoderm]]s, had heavy [[armour (anatomy)|bony plates]] that served as protective [[exoskeleton]]s against [[invertebrate]] [[predator]]s. The first fish with [[jaw]]s, the placoderms, appeared in the [[Silurian]] and greatly diversified during the [[Devonian]], the "Age of Fishes".

Bony fish, distinguished by the presence of [[swim bladder]]s and later [[ossified]] [[endoskeleton]]s, emerged as the [[dominance (ecology)|dominant]] group of fish after the [[end-Devonian extinction]] wiped out the [[apex predator]]s, the placoderms. Bony fish are further divided into the [[lobe-finned fish|lobe-finned]] and [[ray-finned fish]]. About 96% of all living fish species today are [[teleost]]s, a [[crown group]] of ray-finned fish that can [[cranial kinesis|protrude their jaws]]. The [[Tetrapoda|tetrapods]], a mostly [[terrestrial animal|terrestrial]] [[clade]] of vertebrates that have dominated the top [[trophic level]]s in both [[aquatic ecosystem|aquatic]] and [[terrestrial ecosystem]]s since the Late [[Paleozoic]], evolved from lobe-finned fish during the [[Carboniferous]], developing air-breathing [[lung]]s [[homology (biology)|homologous]] to swim bladders. Despite the [[cladistic]] lineage, tetrapods are usually not considered to be fish.

Fish have been an important [[natural resource]] for [[human]]s since [[prehistoric]] times, especially [[fish as food|as food]]. [[Commercial fishing|Commercial]] and [[subsistence fishing|subsistence fishers]] harvest fish in [[wild fisheries]] or [[fish farm|farm]] them in [[fish pond|pond]]s or in [[offshore aquaculture|breeding cage]]s in the ocean. Fish are caught for [[recreational fishing|recreation]], or raised by [[fishkeeping|fishkeepers]] as [[ornamental fish|ornaments]] for private and public exhibition in [[aquarium|aquaria]] and [[garden pond]]s. Fish have had a role in [[human culture]] through the ages, serving as [[Deity|deities]], religious symbols, and as the subjects of art, books and movies.

== Etymology ==

The word ''fish'' is inherited from [[Proto-Germanic language|Proto-Germanic]], and is related to [[German language|German]] {{lang|de|Fisch}}, the [[Latin language|Latin]] {{lang|la|piscis}} and [[Old Irish language|Old Irish]] {{lang|sga|īasc}}, though the exact root is unknown; some authorities reconstruct a [[Proto-Indo-European language|Proto-Indo-European]] root {{lang|ine-x-proto|*peysk-}}, attested only in [[Italic languages|Italic]], [[Celtic languages|Celtic]], and [[Germanic languages|Germanic]].<ref>{{Cite web |title=DWDS – Digitales Wörterbuch der deutschen Sprache |url=https://www.dwds.de/wb/Fisch |access-date=2023-01-21 |website=DWDS |language=de |archive-date=31 July 2020 |archive-url=https://web.archive.org/web/20200731172104/https://www.dwds.de/wb/Fisch |url-status=live}}</ref><ref>Winfred Philipp Lehmann, Helen-Jo J. Hewitt, Sigmund Feist, ''A Gothic etymological dictionary'', 1986, ''s.v.'' ''fisks'' p. 118</ref><ref>{{cite web |title=fish, n.1 |url=https://www.oed.com/view/Entry/70646 |publisher=[[Oxford University Press]] |access-date=2023-01-21 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010607/https://www.oed.com/view/Entry/70646 |url-status=live}}</ref><ref>{{cite book |last=Buck |first=Carl Darling |author-link=Carl Darling Buck |title=A Dictionary of Selected Synonyms in the Principal Indo-European Languages |year=1949 |chapter=section 3.65 |page=184}}</ref>

== Evolution ==

{{main|Evolution of fish}}

=== Fossil history ===

{{further|Timeline of fish evolution}}

[[File:Dunkleosteus profile.jpg|thumb|''[[Dunkleosteus]]'' was a giant [[Devonian]] armoured [[Placodermi|placoderm]], c. 400 [[myr|mya]].]]

About 530 million years ago during the [[Cambrian explosion]], fishlike animals with a [[notochord]] and eyes at the front of the body, such as ''[[Haikouichthys]]'', appear in the [[fossil record]].<ref name="Shu 2003">{{cite journal |last1=Shu |first1=D. G. |title=Head and backbone of the Early Cambrian vertebrate ''Haikouichthys'' |journal=Nature |first2=S. |last2=Conway Morris |first3=J. |last3=Han |first4=Z. F. |last4=Zhang |first5=K. |last5=Yasui |first6=P. |last6=Janvier |first7=L. |last7=Chen |first8=X. L. |last8=Zhang |first9=J. N. |last9=Liu |last10=Li |first10=Y. |last11=Liu |first11=H.-Q. |display-authors=6 |volume=421 |issue=6922 |doi=10.1038/nature01264 |bibcode=2003Natur.421..526S |pmid=12556891 |url=https://www.researchgate.net/publication/10926399 |pages=526–529 |date =2003 |s2cid=4401274 }}</ref> During the late [[Cambrian]], other jawless forms such as [[conodont]]s appear.<ref name="Donoghue Purnell 2009">{{cite journal |last1=Donoghue |first1=Philip C. J. |last2=Purnell |first2=Mark A. |title=The Evolutionary Emergence of Vertebrates From Among Their Spineless Relatives |journal=Evolution: Education and Outreach |volume=2 |issue=2 |date=2009 |issn=1936-6426 |doi=10.1007/s12052-009-0134-3 |pages=204–212 |doi-access=free }}</ref><ref name="Miller Clark 1984">{{cite journal |last1=Miller |first1=James F. |last2=Clark |first2=D. L. |title=Cambrian and earliest Ordovician conodont evolution, biofacies, and provincialism |journal=Geological Society of America Special Paper |series=Geological Society of America Special Papers |issue=196 |year=1984 |volume=196 |pages=43–68 |doi=10.1130/SPE196-p43 |isbn=978-0-8137-2196-5 |url=https://books.google.com/books?id=bGGi7wlAuw4C&pg=PA43|url-access=subscription }}</ref>

[[Gnathostomata|Jawed vertebrates]] appear in the [[Silurian]], with giant armoured [[Placodermi|placoderms]] such as ''[[Dunkleosteus]]''.<ref>{{cite news |url=http://www.smh.com.au/news/science/jaws-of-steel-on-this-fish-tank/2006/11/29/1164777657728.html |title=Monster fish crushed opposition with strongest bite ever |newspaper=[[Smh.com.au]] |date=30 November 2006 |access-date=26 February 2013 |url-status=live |archive-url=https://web.archive.org/web/20130402143409/http://www.smh.com.au/news/science/jaws-of-steel-on-this-fish-tank/2006/11/29/1164777657728.html |archive-date=2 April 2013}}</ref> Jawed fish, too, appeared during the Silurian:<ref name="Choo Min Wenjin 2014">{{cite journal |last1=Choo |first1=Brian |last2=Zhu |first2=Min |last3=Zhao |first3=Wenjin |last4=Jia |first4=Liaotao |last5=Zhu |first5=You'an |title=The largest Silurian vertebrate and its palaeoecological implications |journal=Scientific Reports |date=2014 |volume=4 |pages=5242 |doi=10.1038/srep05242 |pmid=24921626 |pmc=4054400 |bibcode=2014NatSR...4.5242C}}</ref> the cartilaginous [[Chondrichthyes]]<ref name="Andreev Sansom 2022a">{{cite journal |last1=Andreev |first1=Plamen S. |last2=Sansom |first2=Ivan J. |last3=Li |first3=Qiang |last4=Zhao |first4=Wenjin |last5=Wang |first5=Jianhua |last6=Wang |first6=Chun-Chieh |last7=Peng |first7=Lijian |last8=Jia |first8=Liantao |last9=Qiao |first9=Tuo |last10=Zhu |first10=Min |display-authors=5 |date=September 2022 |title=Spiny chondrichthyan from the lower Silurian of South China |url=https://www.nature.com/articles/s41586-022-05233-8 |journal=Nature |volume=609 |issue=7929 |pages=969–974 |doi=10.1038/s41586-022-05233-8 |pmid=36171377 |bibcode=2022Natur.609..969A |s2cid=252570103}}</ref><ref name="Andreev Sansom 2022b">{{Cite journal |last1=Andreev |first1=Plamen S. |last2=Sansom |first2=Ivan J. |last3=Li |first3=Qiang |last4=Zhao |first4=Wenjin |last5=Wang |first5=Jianhua |last6=Wang |first6=Chun-Chieh |last7=Peng |first7=Lijian |last8=Jia |first8=Liantao |last9=Qiao |first9=Tuo |last10=Zhu |first10=Min |display-authors=6 |date=September 2022 |title=The oldest gnathostome teeth |url=http://dx.doi.org/10.1038/s41586-022-05166-2 |journal=Nature |volume=609 |issue=7929 |pages=964–968 |bibcode=2022Natur.609..964A |doi=10.1038/s41586-022-05166-2 |issn=0028-0836 |pmid=36171375 |s2cid=252569771}}</ref> and the bony [[Osteichthyes]].<ref>{{cite book |last1=Berg |first1=Linda R.|last2=Solomon |first2=Eldra Pearl |last3=Martin |first3=Diana W. |title=Biology |year=2004 |publisher=[[Cengage Learning]] |isbn=978-0-534-49276-2 |page=599}}</ref>

During the [[Devonian]], fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes".{{sfn|Benton|2005|loc=p. 35: Fig 2.10, p. 73: Fig 3.25}}<ref name="Dalton 2006">{{cite journal |last=Dalton |first=Rex |title=Hooked on fossils |journal=Nature |date=January 2006 |volume=439 |issue=7074 |pages=262–263 |doi=10.1038/439262a |pmid=16421540 |s2cid=4357313 |doi-access=free }}</ref>

=== Phylogeny ===

Fishes are a [[paraphyly|paraphyletic]] group, since any [[clade]] containing all fish, such as the [[Gnathostomata]] or (for bony fish) [[Osteichthyes]], also contains the clade of [[tetrapod]]s (four-limbed vertebrates, mostly terrestrial), which are usually not considered fish.<ref>{{Cite journal |last=Greene |first=Harry W. |date=1998-01-01 |title=We are primates and we are fish: Teaching monophyletic organismal biology |journal=Integrative Biology |volume=1 |issue=3 |pages=108–111 |doi=10.1002/(sici)1520-6602(1998)1:3<108::aid-inbi5>3.0.co;2-t |issn=1520-6602}}</ref><ref name="Nelson 2016"/> Some tetrapods, such as [[cetacea]]ns and [[Ichthyopterygia|ichthyosaurs]], have [[secondary aquatic adaptation|secondarily acquired]] a fish-like body shape through [[convergent evolution]].<ref>{{cite book |last=Davis |first=R. W. |year=2019 |chapter=Return to the Sea: The Evolution of Marine Mammals |pages=7–27 |editor-last=Davis |editor-first=R. W. |title=Marine Mammals: Adaptations for an Aquatic Life |publisher=[[Springer International Publishing]] |location=New York |isbn=978-3-3199-8278-6}}</ref> On the other hand, ''[[Fishes of the World]]'' comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods".<ref name="Nelson 2016"/> The [[biodiversity]] of extant fish is unevenly distributed among the various groups; [[teleosts]], bony fishes [[Cranial kinesis|able to protrude their jaws]], make up 96% of fish species.{{sfn|Benton|2005|pp=175–184}}<ref name="Nelson 2016"/> The [[cladogram]]<ref name="Friedman & Sallan 2012">{{cite journal |last1=Friedman |first1=Matt |last2=Sallan |first2=Lauren Cole |title=Five hundred million years of extinction and recovery: A Phanerozoic survey of large-scale diversity patterns in fishes |journal=Palaeontology |date=June 2012 |volume=55 |issue=4 |pages=707–742 |doi=10.1111/j.1475-4983.2012.01165.x |bibcode=2012Palgy..55..707F |s2cid=59423401 |doi-access=free }}</ref> shows the [[phylogeny|evolutionary relationships]] of all groups of living fishes (with their respective diversity<ref name="Nelson 2016"/>) and the tetrapods.<ref name=IUCN2023>{{cite web |website=[[IUCN Red List of Threatened Species]] |version=2023.1 |title=Summary Statistics |url=https://www.iucnredlist.org/resources/summary-statistics |access-date=5 February 2024}} Table 1a: [https://nc.iucnredlist.org/redlist/content/attachment_files/2023-1_RL_Table_1a.pdf Number of species evaluated in relation to the overall number of described species, and numbers of threatened species by major groups of organisms]<!--counts 38,595 for tetrapods, using ASW6, RD, Birdlife and MDD--></ref> [[Extinct]] groups are marked with a [[dagger (mark)|dagger]] (†); groups of uncertain placement<ref name="Friedman & Sallan 2012"/> are labelled with a question mark (?) and dashed lines (- - - - -).

{{clade| style=font-size:100%;line-height:100%;
|label1=[[Vertebrata|Vertebrates]]
|1={{clade
   |1=[[Cyclostomi]] (118 species: [[hagfish]], [[lampreys]]) [[File:Petromyzon marinus Linnaeus 1758 Fig 106 (Matschie et al. 1909).svg|100px]]
   |2={{clade
      |label1= ? <!--uncertain placement-->|state=dashed
      |1= †[[Thelodonti]], †[[Conodonta]], †[[Anaspida]] [[File:Sphenonectris turnernae white background.jpg|50 px]] [[File:ConodontZICA.png|90px]] [[File:Lasanius NT small.jpg|70 px]]
      |2={{clade
         |1=†[[Galeaspida]] [[File:A Galeaspid.jpg|50px]]
         |2={{clade
            |1=†[[Osteostraci]] [[File:Cephalaspis Lyellii.jpg|100px]]
            |label2=[[Gnathostomata|Jawed]]|sublabel2=[[Gnathostomata|vertebrates]]
            |2={{clade
               |1=†[[Placodermi]] [[File:Dunkleosteus terrelli 2024 reconstruction.jpg|80 px]]
               |2={{clade
                  |label1 = ? <!--uncertain placement-->|state=dashed
                  |1=†[[Acanthodii]] [[File:Diplacanthus reconstructed.jpg|50px]]
                  |2={{clade
                     |label1=[[Chondrichthyes]]
                     |1=(>1,100 species: [[sharks]], [[ray (fish)|rays]], [[ratfish|chimaeras]]) [[File:Notorynchus cepedianus (Broadnose sevengill shark).gif|140px]]
                     |label2=[[Osteichthyes]]<!--bony fishes INC tetrapods-->
                     |2={{clade
                     |1={{clade
                           |label1=[[Sarcopterygii|Lobe-finned fish]]
                           |1={{clade
                              |label1=[[Actinistia]] |1=(2 species: [[coelacanth]]s) [[File:Coelacanth-bgiu.png|100px]]
                              |label2=[[Rhipidistia]]
                              |2={{clade
                                 |1=[[Dipnoi]] (6 species: [[Lungfish#Extant lungfish|lungfish]]) [[File:Lepidosiren paradoxa2.jpg|140px]]
                                 |2=[[Tetrapoda]] (>38,000 species, not considered fish: amphibians, reptiles, birds, mammals) [[File:Salamender (PSF).png|90px]]
                                 }}
                              }}
                          |label2=[[Actinopterygii|Ray-finned fish]]
                           |2={{clade
                              |label1=[[Cladistia]] |1=(14 species: [[bichirs]], [[reedfish]]) [[File:Polypterus bichir.jpg|130px]]
                              |label2=[[Actinopteri]]
                              |2={{clade
                                 |label1=[[Chondrostei]] |1=(27 species: [[sturgeons]], [[paddlefish]]) [[File:Acipenser gueldenstaedtii.jpg|130px]]
                                 |label2=[[Neopterygii]]
                                 |2={{clade
                                    |1={{clade
                                       |label1=[[Holostei]]
                                       |1={{clade
                                          |1=[[Ginglymodi]] (7 species: [[gar]]s, [[alligator gar]]s) [[File:Lepisosteus platostomus.jpg|130px]]
                                          |2=[[Halecomorphi]] (2 species: [[bowfin]], [[Amia ocellicauda|eyetail bowfin]]) [[File:Amia calva.jpg|130px]]
                                          }}
                                       |label2=[[Teleostei]]
                                       |2=(>32,000 species) [[File:Engraulis ringens.jpg|130px]]
                                       }}
                                    }}
                                 }}
                              }}
                           }}
                        }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}

=== Taxonomy ===

{{main|Taxonomy of fish}}

Fishes (without tetrapods) are a [[paraphyletic]] group and for this reason, the class ''Pisces'' seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three [[extant taxon|extant]] [[class (biology)|classes]] ("[[Agnatha]]", [[Chondrichthyes]], and "[[Osteichthyes]]"), and with extinct forms sometimes classified within those groups, sometimes as their own classes.<ref>{{cite book |last=Benton |first=M.J. |year=1998 |chapter=The quality of the fossil record of vertebrates |pages= 269–303, Fig. 2 |editor1=Donovan, S.K. |editor2=Paul, C.R.C. |title=The adequacy of the fossil record |publisher=[[Wiley (publisher)|Wiley]]}}</ref>

Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within the nine largest families; from largest to smallest, these are [[Cyprinidae]], [[Gobiidae]], [[Cichlidae]], [[Characidae]], [[Loricariidae]], [[Balitoridae]], [[Serranidae]], [[Labridae]], and [[Scorpaenidae]]. About 64 families are [[monotypic]], containing only one species.<ref name="Nelson 2016">{{harvnb|Nelson|2016|p=3}}</ref>

=== Diversity ===

{{main|Diversity of fish}}

Fish range in size from the huge {{convert|16|m|adj=on}} [[whale shark]]<ref name="McClain Balk Benfield Branch 2015">{{cite journal |last1=McClain |first1=Craig R. |last2=Balk |first2=Meghan A. |last3=Benfield |first3=Mark C. |last4=Branch |first4=Trevor A. |last5=Chen |first5=Catherine |last6=Cosgrove |first6=James |last7=Dove |first7=Alistair D.M. |last8=Gaskins |first8=Leo |last9=Helm |first9=Rebecca R. |last10=Hochberg |first10=Frederick G. |last11=Lee |first11=Frank B. |last12=Marshall |first12=Andrea |last13=McMurray |first13=Steven E. |last14=Schanche |first14=Caroline |last15=Stone |first15=Shane N. |last16=Thaler |first16=Andrew D. |display-authors=5 |title=Sizing ocean giants: patterns of intraspecific size variation in marine megafauna |journal=[[PeerJ]] |volume=3 |date=2015-01-13 |issn=2167-8359 |doi=10.7717/peerj.715 |doi-access=free |page=e715 |pmid=25649000 |pmc=4304853 }}</ref> to some tiny teleosts only {{convert|8|mm|sigfig=1|adj=on}} long, such as the cyprinid ''[[Paedocypris progenetica]]''<ref name="Kottelat">{{cite journal |last1=Kottelat |first1=Maurice |last2=Britz |first2=Ralf |last3=Heok Hui|first3=Tan |last4=Witte |first4=Kai-Erik |year=2005 |title=''Paedocypris'', a new genus of Southeast Asian cyprinid fish with a remarkable sexual dimorphism, comprises the world's smallest vertebrate |journal=Proceedings of the Royal Society B |volume=273 |issue=1589 |pages=895–899 |doi=10.1098/rspb.2005.3419 |url=http://www.eurocean.org/np4/file/133/Paedocypris_20__20the_20world_20s_20smal.pdf |accessdate=26 October 2012 |pmid=16627273 |pmc=1560243 |url-status=dead |archive-url=https://web.archive.org/web/20090712211749/http://www.eurocean.org/np4/file/133/Paedocypris_20__20the_20world_20s_20smal.pdf|archive-date=12 July 2009}}</ref> and the [[stout infantfish]].<ref name=fishbase>{{FishBase|genus=Schindleria|species=brevipinguis |year=2017 |month=September}}</ref>

<gallery mode="packed" heights=175>
File:Rhincodon typus fgbnms (cropped).jpg|Largest: [[whale shark]]
File:Paedocypris progenetica 001.jpg|Smallest: e.g. ''[[Paedocypris progenetica]]''
</gallery>

Swimming performance varies from fish such as tuna, [[salmon]], and [[Carangidae|jacks]] that can cover 10–20 body-lengths per second to species such as [[eel]]s and [[Batoidea|rays]] that swim no more than 0.5 body-lengths per second.{{sfn|Helfman|Collette|Facey|1997|p=103}}

<gallery mode="packed" heights=100>
File:Salmo salar.jpg|Fastest: e.g. [[salmon]], 10–20 body lengths/second
File:Anguilla japonica 1856.jpg|Slowest: e.g. [[eel]], 0.5 body lengths/second
</gallery>

A typical fish is [[ectothermic|cold-blooded]], has a [[Streamlines, streaklines and pathlines|streamlined]] body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and a tail fin, jaws, skin covered with [[scale (zoology)|scales]], and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes.{{sfn|Helfman|Collette|Facey|1997|pp=3, 33–36}}

<gallery mode="packed" heights=120>
File:Humpback anglerfish.png|[[Ambush predator]]:<br />[[anglerfish]]
File:Atl mackerel photo3 exp.jpg|[[Streamlines, streaklines and pathlines|Streamlined]], somewhat [[warm-blooded]]:<br />[[mackerel]]
File:Hippocampus hippocampus (cropped).jpg|Tail not [[Fish locomotion|used for swimming]]:<br />[[seahorse]]
File:Phycodurus eques P2023146 (cropped).JPG|[[Camouflage]]d:<br />[[leafy seadragon]]
File:Eastern Cleaner Clingfish (cropped).jpg|No [[fish scale|scales]]:<br />[[clingfish]]
File:Cyphotilapia frontosa mouthbrooding.jpg|[[Mouthbrooder]]: [[Cyphotilapia frontosa|front cichlid]] with [[juvenile fish|young]] in mouth
</gallery>

== Ecology ==

=== Habitats ===

[[File:Fish habitat diversity.svg|thumb|upright=2.5|Different fish species are adapted to a wide variety of freshwater and marine habitats.]]

Fish species are roughly divided equally between [[freshwater]] and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species.<ref name="Manel Guerin Mouillot Blanchet 2020">{{cite journal |last1=Manel |first1=Stéphanie |last2=Guerin |first2=Pierre-Edouard |last3=Mouillot |first3=David |last4=Blanchet |first4=Simon |last5=Velez |first5=Laure |last6=Albouy |first6=Camille |last7=Pellissier |first7=Loïc |title=Global determinants of freshwater and marine fish genetic diversity |journal=Nature Communications |volume=11 |issue=1 |date=2020-02-10 |page=692 |issn=2041-1723 |pmid=32041961 |pmc=7010757 |doi=10.1038/s41467-020-14409-7|bibcode=2020NatCo..11..692M }}</ref> [[Coral reef]]s in the [[Indo-Pacific]] constitute the center of diversity for marine fishes,<ref name="Hubert Meyer Bruggemann Guérin 2012">{{cite journal |last1=Hubert |first1=Nicolas |last2=Meyer |first2=Christopher P. |last3=Bruggemann |first3=Henrich J. |last4=Guérin |first4=Fabien |last5=Komeno |first5=Roberto J. L. |last6=Espiau |first6=Benoit |last7=Causse |first7=Romain |last8=Williams |first8=Jeffrey T. |last9=Planes |first9=Serge |display-authors=5 |title=Cryptic Diversity in Indo-Pacific Coral-Reef Fishes Revealed by DNA-Barcoding Provides New Support to the Centre-of-Overlap Hypothesis |journal=PLOS ONE |volume=7 |issue=3 |date=2012-03-15 |pmid=22438862 |pmc=3305298 |doi=10.1371/journal.pone.0028987 |doi-access=free |page=e28987 |bibcode=2012PLoSO...728987H }}</ref> whereas continental freshwater fishes are most diverse in large [[river basin]]s of [[tropical rainforest]]s, especially the [[Amazon Basin|Amazon]], [[Congo Basin|Congo]], and [[Mekong]] basins.<ref name="van der Sleen Albert 2022">{{cite book |last1=van der Sleen |first1=Peter |last2=Albert |first2=James S. |title=Encyclopedia of Inland Waters |chapter=Patterns in Freshwater Fish Diversity |publisher=Elsevier |date=2022 |isbn=978-0-12-822041-2 |doi=10.1016/b978-0-12-819166-8.00056-6 |pages=243–255}}</ref> More than 5,600 fish species inhabit [[Neotropic]]al freshwaters alone, such that [[Neotropical fish]]es represent about 10% of all [[vertebrate]] species on the Earth.<ref name="Albert Carvalho Petry 2011">{{cite journal |last1=Albert |first1=James S. |last2=Carvalho |first2=Tiago P. |last3=Petry |first3=Paulo |title=Aquatic Biodiversity in the Amazon: Habitat Specialization and Geographic Isolation Promote Species Richness |journal=Animals |date=June 2011 |volume=1 |issue=2 |pages=205–241 |doi=10.3390/ani1020205 |doi-access=free |pmid=26486313|pmc=4513461 }}</ref>

Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high [[mountain stream]]s (e.g., [[Salvelinus|char]] and [[gudgeon (fish)|gudgeon]]) to the [[abyssal zone|abyssal]] and even [[hadal zone|hadal]] depths of the deepest oceans (e.g., [[cusk-eels]] and [[snailfish]]), although none have been found in the deepest 25% of the ocean.<ref>{{cite journal|last1=Yancey |first1=P.H. |last2=Gerringer |first2=M.E. |last3=Drazen |first3=J.C. |last4=Rowden |first4=A.A. |last5=Jamieson |first5=A. |title=Marine fish may be biochemically constrained from inhabiting the deepest ocean depths |year=2014 |journal=PNAS |pmc=3970477 |pmid=24591588 |doi=10.1073/pnas.1322003111 |volume=111 |issue=12 |pages=4461–4465 |bibcode=2014PNAS..111.4461Y |doi-access=free }}</ref> The deepest living fish in the ocean so far found is a cusk-eel, ''[[Abyssobrotula galatheae]]'', recorded at the bottom of the [[Puerto Rico Trench]] at {{convert|8370|m|ft|abbr=on}}.<!--<ref name="EoF">{{cite book |editor1=Paxton, J.R. |editor2=Eschmeyer, W.N. |last=Nielsen |first=Jørgen G. |year=1998 |title=Encyclopedia of Fishes |publisher=[[Academic Press]] |location=San Diego |page=134 |isbn=0-12-547665-5}}</ref>--><ref>{{cite web |title=What is the deepest-living fish? |url=http://australianmuseum.net.au/what-is-the-deepest-living-fish |work=[[Australian Museum]] |date=23 December 2014 |access-date=18 September 2015 }}</ref>

In terms of temperature, [[Jonah's icefish]] live in cold{{efn|The temperature is often around 0 C. The freezing point of seawater at the surface is -1.85 C, falling to -2.62 C at a depth of 1000 metres. However, the water can be [[supercooled]] somewhat below these temperatures.<ref name="Haumann Moorman Riser Smedsrud 2020">{{cite journal |last1=Haumann |first1=F. Alexander |last2=Moorman |first2=Ruth |last3=Riser |first3=Stephen C. |last4=Smedsrud |first4=Lars H. |last5=Maksym |first5=Ted |last6=Wong |first6=Annie P.S. |last7=Wilson |first7=Earle A. |last8=Drucker |first8=Robert |last9=Talley |first9=Lynne D. |last10=Johnson |first10=Kenneth S. |last11=Key |first11=Robert M. |last12=Sarmiento |first12=Jorge L. |display-authors=5 |title=Supercooled Southern Ocean Waters |journal=Geophysical Research Letters |volume=47 |issue=20 |date=2020-10-28 |doi=10.1029/2020GL090242 |bibcode=2020GeoRL..4790242H |hdl=1912/26495 |hdl-access=free }}</ref>}} waters of the Southern Ocean, including under the [[Filchner–Ronne Ice Shelf]] at a latitude of 79°S,<ref name="Purser Hehemann 2022">{{Cite journal |last1=Purser |first1=Autun |last2=Hehemann |first2=Laura |last3=Boehringer |first3=Lilian |last4=Tippenhauer |first4=Sandra |last5=Wege |first5=Mia |last6=Bornemann |first6=Horst |last7=Pineda-Metz |first7=Santiago E. A. |last8=Flintrop |first8=Clara M. |last9=Koch |first9=Florian |last10=Hellmer |first10=Hartmut H. |last11=Burkhardt-Holm |first11=Patricia |last12=Janout |first12=Markus |last13=Werner |first13=Ellen |last14=Glemser |first14=Barbara |last15=Balaguer |first15=Jenna |display-authors=5 |date=2022 |title=A vast icefish breeding colony discovered in the Antarctic |journal=Current Biology |volume=32 |issue=4 |pages=842–850.e4 |doi=10.1016/j.cub.2021.12.022 |pmid=35030328 |s2cid=245936769 |doi-access=free |bibcode=2022CBio...32E.842P |hdl=2263/90796 |hdl-access=free}}</ref> while [[desert pupfish]] live in desert springs, streams, and marshes, sometimes highly saline, with water temperatures as high as 36 C.<ref name="USFWS 1993">{{cite web |publisher=[[United States Fish and Wildlife Service]] |year=1993 |title=Desert Pupfish (Cyprinodon macularius) Recovery Plan |first1=Paul C. |last1=Marsh |first2=Donald W |last2=Sada |archive-url=https://web.archive.org/web/20111017200006/https://ecos.fws.gov/docs/recovery_plan/931208b.pdf |archive-date=2011-10-17 |url=https://ecos.fws.gov/docs/recovery_plan/931208b.pdf |url-status=live}}</ref><ref name="Shrode Gerking 1977">{{Cite journal |last1=Shrode |first1=Joy B. |last2=Gerking |first2=Shelby D. |year=1977 |title=Effects of Constant and Fluctuating Temperatures on Reproductive Performance of a Desert Pupfish, Cyprinodon n. nevadensis |url=http://dx.doi.org/10.1086/physzool.50.1.30155710 |journal=Physiological Zoology |volume=50 |issue=1 |pages=1–10 |doi=10.1086/physzool.50.1.30155710 |s2cid=82166135 |issn=0031-935X|url-access=subscription }}</ref>

A few fish live mostly on land or lay their eggs on land near water.<ref>{{cite book |last=Martin |first=K.L.M. |title=Beach-Spawning Fishes: Reproduction in an Endangered Ecosystem |year=2014 |publisher=[[CRC Press]] |isbn=978-1-4822-0797-2}}</ref> [[Mudskipper]]s feed and interact with one another on mudflats and go underwater to hide in their burrows.<ref>{{FishBase |genus=Periophthalmus |species=barbarus |year=2006 |month=November |id=12803}}</ref> A single [[undescribed species]] of ''[[Phreatobius]]'' has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged [[leaf litter]].<ref>{{cite web |url=http://www.planetcatfish.com/catelog/species.php?species_id=646 |title=Cat-eLog: Heptapteridae: ''Phreatobius'': ''Phreatobius'' sp. (1) |website=Planet Catfish |access-date=26 November 2006 |url-status=live |archive-url=https://web.archive.org/web/20061023092123/http://www.planetcatfish.com/catelog/species.php?species_id=646 |archive-date=23 October 2006}}</ref><ref>{{cite journal |last1=Henderson |first1=P.A. |last2=Walker |first2=I. |year=1990 |title=Spatial organization and population density of the fish community of the litter banks within a central Amazonian blackwater stream |journal=Journal of Fish Biology |volume=37 |issue=3 |pages=401–411 |doi=10.1111/j.1095-8649.1990.tb05871.x |bibcode=1990JFBio..37..401H }}</ref> [[Cavefish]] of multiple families live in [[underground lake]]s, [[underground river]]s or [[aquifer]]s.<ref name="Helfman 2007">{{cite book |last=Helfman |first=G.S. |year=2007 |title=Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources |pages=41–42 |publisher=[[Island Press]] |isbn=978-1-55963-595-0}}</ref>

=== Parasites and predators ===

{{further|Fish diseases and parasites|Predatory fish}}

Like other animals, fish suffer from [[parasitism]]. Some species use [[cleaner fish]] to remove external parasites. The best known of these are the [[bluestreak cleaner wrasse]]s of [[coral reef]]s in the [[Indian Ocean|Indian]] and [[Pacific Ocean|Pacific]] oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract the attention of the cleaners.{{sfn|Helfman|Collette|Facey|1997|p=380}} Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, ''[[Orange chromide|Etroplus maculatus]]'', the cleaner, and the much larger ''[[Etroplus suratensis|E. suratensis]]''.<ref>{{cite journal |last1=Wyman |first1=Richard L. |last2=Ward |first2=Jack A. |year=1972 |title=A Cleaning Symbiosis between the Cichlid Fishes Etroplus maculatus and Etroplus suratensis. I. Description and Possible Evolution |journal=Copeia |volume=1972 |issue=4 |pages=834–838 |doi=10.2307/1442742 |jstor=1442742 }}</ref>

Fish occupy many [[trophic level]]s in freshwater and marine [[food web]]s. Fish at the higher levels [[Predatory fish|are predatory]], and a substantial part of their prey consists of other fish.<ref name="Myers Worm 2003">{{cite journal |last1=Myers |first1=Ransom A. |last2=Worm |first2=Boris |title=Rapid worldwide depletion of predatory fish communities |journal=Nature |publisher=[[Springer Science and Business Media]] |volume=423 |issue=6937 |year=2003 |issn=0028-0836 |doi=10.1038/nature01610 |pages=280–283|pmid=12748640 |bibcode=2003Natur.423..280M }}</ref> In addition, mammals such as [[dolphin]]s and [[Pinniped|seal]]s feed on fish, alongside birds such as [[gannet]]s and [[cormorant]]s.<ref>{{cite web |title=Predation |url=https://www.nwcouncil.org/fish-and-wildlife/fw-topics/predation/ |publisher=[[Northwest Power and Conservation Council]] |access-date=10 February 2024}}</ref>

<gallery mode=packed>
File:Initial phase parrotfish feeding at Shaab Marsa Alam, Red Sea, Egypt -SCUBA (6336981391).jpg|A [[parrotfish]] feeding on [[algae]] on a [[coral reef]]<!-- Red Sea-->
File:Arothron hispidus is being cleaned by Hawaiian cleaner wrasses, Labroides phthirophagus 1.jpg|A [[cleaner fish]] removing [[Parasitism|parasite]]s from its client, a [[Tetraodontidae|pufferfish]]
File:Barracuda with prey.jpg|A [[barracuda]] preying on a smaller fish
File:1031 california sealion wright odfw (35281910502).jpg|[[Sea lion]], a predatory mammal, eating a large [[Salmonidae|salmonid]]
File:Cormorant with fish (cropped).jpg|[[Cormorant]] with fish prey
</gallery>

== Anatomy and physiology ==

{{main|Fish anatomy|Fish physiology}}

=== Locomotion ===

{{Main|Fish locomotion}}

The body of a typical fish is adapted for efficient swimming by alternately contracting paired sets of [[muscle]]s on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as [[Flight control surfaces|control surfaces]] like an aircraft's flaps, enabling the fish to steer in any direction.<ref name=Sfakiotakis>{{cite journal |last1=Sfakiotakis |first1=M. |last2=Lane |first2=D. M. |last3=Davies |first3=J. B. C. |date=1999 |url=http://www.mor-fin.com/Science-related-links_files/http___www.ece.eps.hw.ac.uk_Research_oceans_people_Michael_Sfakiotakis_IEEEJOE_99.pdf |title=Review of Fish Swimming Modes for Aquatic Locomotion |journal=IEEE Journal of Oceanic Engineering |volume=24 |issue=2 |pages=237–252 |doi=10.1109/48.757275 |bibcode=1999IJOE...24..237S |s2cid=17226211 |url-status=dead |archive-url=https://web.archive.org/web/20131224091124/http://www.mor-fin.com/Science-related-links_files/http___www.ece.eps.hw.ac.uk_Research_oceans_people_Michael_Sfakiotakis_IEEEJOE_99.pdf |archive-date=2013-12-24 }}</ref>

<gallery class=center mode=nolines widths=300px>
File:Lampanyctodes hectoris (Hector's lanternfish).svg|Anatomy of a typical fish ([[lanternfish]] shown):<br />1) [[gill cover]] 2) [[lateral line]] 3) dorsal fin 4) fat fin<br />5) caudal peduncle 6) caudal fin 7) anal fin 8)&nbsp;[[photophore]]s 9)&nbsp;pelvic fins 10) pectoral fins
</gallery>

Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a [[swim bladder]] that allows them to adjust their [[buoyancy]] by increasing or decreasing the amount of gas it contains.<ref>{{cite web |title=Actinopterygii: More on Morphology |url=https://ucmp.berkeley.edu/vertebrates/actinopterygii/actinomm.html |publisher=[[University of California Museum of Paleontology]] |access-date=10 February 2024}}</ref>

The [[fish scale|scales of fish]] provide protection from [[Predation|predator]]s at the cost of adding stiffness and weight.<ref name="Quan Yang Lapeyriere Schaible 2020">{{cite journal |last1=Quan |first1=Haocheng |last2=Yang |first2=Wen |last3=Lapeyriere |first3=Marine |last4=Schaible |first4=Eric |last5=Ritchie |first5=Robert O. |last6=Meyers |first6=Marc A. |title=Structure and Mechanical Adaptability of a Modern Elasmoid Fish Scale from the Common Carp |journal=Matter |volume=3 |issue=3 |date=2020 |doi=10.1016/j.matt.2020.05.011 |pages=842–863}}</ref> Fish scales are often highly reflective; this [[Camouflage#Silvering|silvering provides camouflage]] in the open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility.<ref name="Herring 2002">{{cite book |last=Herring |first=Peter |author-link=Peter Herring |year=2002 |title=The Biology of the Deep Ocean |publisher=[[Oxford University Press]] |isbn=978-0-19-854956-7 |pages=192–195}}</ref>

<gallery class=center mode=nolines widths=200px>
File:Swim bladder.jpg|Gas-filled [[swim bladder]] of a [[Scardinius erythrophthalmus|rudd]] helps maintain neutral [[buoyancy]].
File:Fish scales.jpg|Silvered [[fish scale|scales]] of a [[rohu]] provide protection and camouflage.
</gallery>

=== Circulation ===<!--first, because Respiration section demands knowledge of the heart and circulation-->

[[File:Fish gill respiration.jpg|thumb|upright=1.5|The fish heart pumps blood to the gills, where it picks up oxygen. The blood then flows without further pumping to the body, from where it returns to the heart.]]

Fish have a [[circulatory system|closed-loop circulatory system]]. The [[heart]] pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through the gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart.<ref>{{cite web |title=Animal Circulatory Systems |url=https://organismalbio.biosci.gatech.edu/nutrition-transport-and-homeostasis/animal-circulatory-systems/ |publisher=[[Georgia Tech]] |access-date=10 February 2024}}</ref>

=== Respiration ===

==== Gills ====

{{main|Fish gill}}

Fish exchange gases using [[gill]]s on either side of the [[pharynx]]. Gills consist of comblike structures called filaments. Each filament contains a [[capillary]] network that provides a large [[surface area]] for exchanging [[oxygen]] and [[carbon dioxide]]. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in the gills flows in the opposite direction to the water, resulting in efficient [[countercurrent exchange]]. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath a protective bony cover or [[operculum (fish)|operculum]]. They are able to oxygenate their gills using muscles in the head.<ref name="Romer 1977">{{cite book |last1=Romer |first1=Alfred Sherwood |author1-link=Alfred Romer |last2=Parsons |first2=Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location=Philadelphia |pages=316–327 |isbn=0-03-910284-X}}</ref>

==== Air breathing ====

{{further|amphibious fish}}

Some 400 species of fish in 50 families<!--and 17 orders--> can breathe air, enabling them to live in oxygen-poor water or to emerge on to land.<ref name="Graham Wegner 2010">{{cite book |last1=Graham |first1=Jeffrey B. |last2=Wegner |first2=N.C. |chapter=6. Breathing air in water and in air: the air-breathing fishes |editor-last=Nilsson |editor-first=Göran E. |title=Respiratory Physiology of Vertebrates |publisher=[[Cambridge University Press]] |publication-place=New York |year=2010 |isbn=978-0-521-87854-8 |pages=174–221 |doi=10.1017/CBO9780511845178.007 |url=https://www.researchgate.net/publication/263929585}}</ref> The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to the heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of [[rock pool]]s and the [[intertidal zone]], are facultative air breathers, able to breathe air when out of water, as may occur daily at [[low tide]], and to use their gills when in water. Some coastal fish like [[Dialommus macrocephalus|rockskippers]] and [[mudskipper]]s choose to leave the water to feed in habitats temporarily exposed to the air.<ref name="Graham Wegner 2010"/> Some catfish <!--[[Loricariidae]], [[Callichthyidae]], and [[Scoloplacidae]]--> absorb air through their digestive tracts.<ref name="Armbruster1998">{{cite journal |last=Armbruster |first=Jonathan W. |year=1998 |title=Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes |url=http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf |url-status=dead |journal=[[Copeia]] |volume=1998 |issue=3 |pages=663–675 |doi=10.2307/1447796 |jstor=1447796 |archive-url=https://web.archive.org/web/20090326130527/http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf |archive-date=26 March 2009 |access-date=25 June 2009}}</ref>

=== Digestion ===

The digestive system consists of a tube, the gut, leading from the mouth to the anus. The mouth of most fishes contains teeth to grip prey, bite off or scrape plant material, or crush the food. An [[esophagus]] carries food to the stomach where it may be stored and partially digested. A sphincter, the pylorus, releases food to the intestine at intervals. Many fish have finger-shaped pouches, [[pyloric caeca]], around the pylorus, of doubtful function. The [[pancreas]] secretes enzymes into the intestine to digest the food; other enzymes are secreted directly by the intestine itself. The [[liver]] produces [[bile]] which helps to break up fat into an emulsion which can be absorbed in the intestine.<ref>{{cite web |title=Digestive System |url=http://web.utk.edu/~rstrange/wfs550/html-con-pages/v-digest-sys.html |publisher=[[University of Tennessee]] |access-date=10 February 2024}}</ref>

=== Excretion ===

Most fish release their nitrogenous wastes as [[ammonia]]. This may be excreted through the gills or [[filter (chemistry)|filtered]] by the [[kidney]]s. Salt is excreted by the rectal gland.<ref name="Burton Burton 2017">{{cite book |last1=Burton |first1=Derek |last2=Burton |first2=Margaret |title=Oxford Scholarship Online |chapter=Excretion |publisher=[[Oxford University Press]] |date=2017-12-21 |volume=1 |doi=10.1093/oso/9780198785552.003.0008}}</ref> Saltwater fish tend to lose water by [[osmosis]]; their kidneys return water to the body, and produce a concentrated urine. The reverse happens in [[freshwater fish]]: they tend to gain water osmotically, and produce a dilute urine. Some fish have kidneys able to operate in both freshwater and saltwater.<ref name="The Royal Society 1971">{{cite journal | title=Fish gills: mechanisms of salt transfer in fresh water and sea water | journal=Philosophical Transactions of the Royal Society of London B | volume=262 | issue=842 | date=1971-08-20 | issn=0080-4622 | doi=10.1098/rstb.1971.0090 | pages=209–249| bibcode=1971RSPTB.262..209M | last1=Maetz | first1=J. }}</ref>

=== Brain ===

[[File:Fish brain.png|thumb|upright=0.5|Diagram of [[rainbow trout]] brain, from above|alt=Diagram showing the pairs of olfactory, telencephalon, and optic lobes, followed by the cerebellum and the mylencephalon]]

Fish have small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} However, some fish have relatively large brains, notably [[Mormyridae|mormyrids]] and [[shark]]s, which have brains about as large for their body weight as birds and [[marsupial]]s.{{sfn|Helfman|Collette|Facey|1997|p=191}} At the front of the brain are the [[olfactory bulb|olfactory lobes]], a pair of structures that receive and process signals from the [[nostril]]s via the two [[olfactory nerve]]s. Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the [[telencephalon]], which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to the midbrain is the [[diencephalon]]; it works with [[hormone]]s and [[homeostasis]]. The [[pineal body]] is just above the diencephalon; it detects light, maintains [[circadian]] rhythms, and controls color changes. The [[midbrain]] contains the two [[Midbrain#Corpora quadrigemina|optic lobes]]. These are very large in species that hunt by sight, such as [[rainbow trout]] and [[cichlid]]s. The [[metencephalon|hindbrain]] controls swimming and balance.The single-lobed cerebellum is the biggest part of the brain; it is small in hagfish and [[lamprey]]s, but very large in [[mormyrid]]s, processing their [[electric fish|electrical sense]]. The brain stem or [[myelencephalon]] controls some muscles and body organs, and governs respiration and [[osmoregulation]].{{sfn|Helfman|Collette|Facey|1997|pp=48–49}}

=== Sensory systems ===

{{main|Sensory systems in fish}}

The [[lateral line]] system is a network of sensors in the skin which detects gentle currents and vibrations, and senses the motion of nearby fish, whether predators or prey.<ref name="Bleckmann Zelick 2009">{{Cite journal |last1=Bleckmann |first1=Horst |last2=Zelick |first2=Randy |date=2009-03-01 |title=Lateral line system of fish |journal=[[Integrative Zoology]] |volume=4 |issue=1 |pages=13–25 |doi=10.1111/j.1749-4877.2008.00131.x |pmid=21392273 |doi-access=free}}</ref> This can be considered both a sense of [[touch]] and of [[hearing]]. [[Blind cave fish]] navigate almost entirely through the sensations from their lateral line system.<ref>{{cite book |last=Godfrey-Smith |first=Peter |author-link=Peter Godfrey-Smith |title=Metazoa |publisher=[[Farrar, Straus and Giroux]] |location=New York |isbn=9780374207946 |chapter=Kingfish |date=2020 }}</ref> Some fish, such as catfish and sharks, have the [[ampullae of Lorenzini]], [[electroreceptor]]s that detect weak electric currents on the order of millivolt.<ref name="Albert Crampton 2006">{{cite book |last1=Albert |first1=J. S. |last2=Crampton |first2=W. G. |year=2006 |chapter=Electroreception and Electrogenesis |editor=Lutz, P. L. |title=The Physiology of Fishes |publisher=[[CRC Press]] |location=Boca Raton, Florida |isbn=978-0-8493-2022-4 |pages=429–470}}</ref>

[[Vision in fish|Vision]] is an important [[sensory system]] in fish.<ref name="Guthrie 1986">{{cite book |last=Guthrie |first=D. M. |title=The Behaviour of Teleost Fishes |chapter=Role of Vision in Fish Behaviour |publisher=[[Springer Publishing|Springer]] |publication-place=Boston, Massachusetts |date=1986 |isbn=978-1-4684-8263-8 |doi=10.1007/978-1-4684-8261-4_4 |pages=75–113}}</ref> Fish eyes are similar to those of [[terrestrial animal|terrestrial]] [[vertebrate]]s like [[bird vision|birds]] and mammals, but have a more [[spherical]] [[lens (anatomy)|lens]].<ref name="Guthrie 1986"/> Their [[retina]]s generally have both [[rod cell|rods]] and [[cone cell|cones]] (for [[scotopic vision|scotopic]] and [[photopic vision]]); many species have [[colour vision]], often with three types of cone.<ref name="Guthrie 1986"/> Teleosts can see [[polarized light]];<ref name="Hawryshyn 2010">{{cite journal |last=Hawryshyn |first=Craig W. |title=Ultraviolet Polarization Vision and Visually Guided Behavior in Fishes |journal=Brain, Behavior and Evolution |volume=75 |issue=3 |date=2010 |issn=0006-8977 |doi=10.1159/000314275 |pages=186–194 |pmid=20733294 }}</ref> some such as cyprinids have a fourth type of cone that detects [[ultraviolet]].<ref name="Guthrie 1986"/> Amongst [[jawless fish]], the [[lamprey]] has well-developed eyes,<ref name="Meyer-Rochow 1996">{{cite journal |last1=Meyer-Rochow |first1=V. Benno |last2=Stewart |first2=Duncan |title=Review of larval and postlarval eye ultrastructure in the lamprey (cyclostomata) with special emphasis on Geotria australis (gray) |journal=Microscopy Research and Technique |date=1996 |volume=35 |issue=6 |pages=431–444 |doi=10.1002/(SICI)1097-0029(19961215)35:6<431::AID-JEMT3>3.0.CO;2-L |pmid=9016447 |s2cid=22940203 }}</ref> while the [[hagfish]] has only primitive eye<!--don't wikilink, wrong subject-->spots.<ref name="Lamb Collin Pugh 2007">{{cite journal |last1=Lamb |first1=Trevor D. |last2=Collin |first2=Shaun P. |last3=Pugh |first3=Edward N. |title=Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup |journal=Nature Reviews Neuroscience |volume=8 |issue=12 |date=2007 |issn=1471-003X |pmid=18026166 |pmc=3143066 |doi=10.1038/nrn2283 |pages=960–976}} See also [https://link.springer.com/content/pdf/10.1007/s12052-008-0091-2.pdf Lamb et al.'s "The origin of the Vertebrate Eye", 2008.]</ref>

[[Hearing in fish|Hearing]] too is an important sensory system in fish. Fish sense sound using their lateral lines and [[otolith]]s in their ears, inside their heads. Some can detect sound through the swim bladder.<ref name="Hawkins 1981">{{cite book |last=Hawkins |first=A. D. |editor1-last=Tavolga |editor1-first=William N. |editor2-last=Popper |editor2-first=Arthur N. |editor3-last=Fay |editor3-first=Richard R. |title=Hearing and Sound Communication in Fishes |date=1981 |publisher=[[Springer Publishing|Springer]] |isbn=978-1-4615-7188-9 |pages=109–138 |chapter=6. The Hearing Abilities of Fish |url=https://books.google.com/books?id=egP3BwAAQBAJ}}</ref>

Some fish, including salmon, are capable of [[magnetoreception]]; when the axis of a magnetic field is changed around a circular tank of young fish, they reorient themselves in line with the field.<ref name="Quinn 1980">{{cite journal |last=Quinn |first=Thomas P. |date=1980 |title=Evidence for celestial and magnetic compass orientation in lake migrating sockeye salmon fry |journal=[[Journal of Comparative Physiology A]] |volume=137 |issue=3 |pages=243–248 |doi=10.1007/bf00657119 |s2cid=44036559 }}</ref><ref name="Taylor 1986">{{Cite journal |last=Taylor |first=P. B. |date=May 1986 |title=Experimental evidence for geomagnetic orientation in juvenile salmon, Oncorhynchus tschawytscha Walbaum |journal=[[Journal of Fish Biology]] |volume=28 |issue=5 |pages=607–623 |doi=10.1111/j.1095-8649.1986.tb05196.x |bibcode=1986JFBio..28..607T }}</ref> The mechanism of fish magnetoreception remains unknown;<ref name="Formicki Korzelecka‐Orkisz Tański 2019">{{cite journal |last1=Formicki |first1=Krzysztof |last2=Korzelecka-Orkisz |first2=Agata |last3=Tański |first3=Adam |title=Magnetoreception in fish |journal=Journal of Fish Biology |volume=95 |issue=1 |date=2019 |issn=0022-1112 |doi=10.1111/jfb.13998 |pages=73–91|pmid=31054161 |bibcode=2019JFBio..95...73F }}</ref> experiments in birds imply a quantum [[radical pair mechanism]].<ref name="Hore Mouritsen 2022">{{cite journal |last1=Hore |first1=Peter J. |author1-link=Peter Hore (chemist) |last2=Mouritsen |first2=Henrik |title=The Quantum Nature of Bird Migration |journal=[[Scientific American]] |url=https://www.scientificamerican.com/article/how-migrating-birds-use-quantum-effects-to-navigate/ |date=April 2022 |pages=24–29}}</ref>

=== Cognition ===

{{further|Fish intelligence}}

The cognitive capacities of fish include [[self-awareness]], as seen in [[mirror test]]s. [[Manta rays]] and [[wrasse]]s placed in front of a mirror repeatedly check whether their reflection's behavior mimics their body movement.<ref>{{Cite journal |last1=Ari |first1=Csilla |last2=D'Agostino |first2=Dominic P. |date=2016-05-01 |title=Contingency checking and self-directed behaviors in giant manta rays: Do elasmobranchs have self-awareness? |url=https://doi.org/10.1007/s10164-016-0462-z |journal=Journal of Ethology |volume=34 |issue=2 |pages=167–174 |doi=10.1007/s10164-016-0462-z |s2cid=254134775 |access-date=21 January 2023 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010603/https://link.springer.com/article/10.1007/s10164-016-0462-z |url-status=live|url-access=subscription }}</ref><ref>{{Cite journal |last1=Kohda |first1=Masanori |last2=Hotta |first2=Takashi |last3=Takeyama |first3=Tomohiro |last4=Awata |first4=Satoshi |last5=Tanaka |first5=Hirokazu |last6=Asai |first6=Jun-ya |last7=Jordan |first7=L. Alex |date=2018-08-21 |journal=[[PLOS Biology]] |title=Cleaner wrasse pass the mark test. What are the implications for consciousness and self-awareness testing in animals? |volume=17 |issue=2 |pages=397067 |doi=10.1371/journal.pbio.3000021|doi-access=free |pmid=30730878 |pmc=6366756 |biorxiv=10.1101/397067 |s2cid=91375693}}</ref> ''[[Choerodon]]'' wrasse, [[archerfish]], and [[Atlantic cod]] can solve problems and invent tools.<ref>{{cite magazine |last=Balcombe |first=Jonathan |title=Fishes Use Problem-Solving and Invent Tools |magazine=[[Scientific American]] |date=1 May 2017 |url=https://www.scientificamerican.com/article/fishes-use-problem-solving-and-invent-tools1/ |archive-url=https://web.archive.org/web/20230317010617/https://www.scientificamerican.com/article/fishes-use-problem-solving-and-invent-tools1/ |url-status=live |archive-date=17 March 2023}}</ref> The [[monogamy in animals|monogamous]] cichlid ''[[Amatitlania siquia]]'' exhibits pessimistic behavior when prevented from being with its partner.<ref>{{cite journal |first1=Chloé |last1=Laubu |first2=Philippe |last2=Louâpre |first3=François-Xavier |last3=Dechaume-Moncharmont |date=2019 |title=Pair-bonding influences affective state in a monogamous fish species |journal=Proceedings of the Royal Society B |volume=286 |issue=1904 |at=20190760 |doi=10.1098/rspb.2019.0760 |doi-access=free |pmid=31185864 |pmc=6571461}}</ref> Fish orient themselves using landmarks; they may use mental maps based on multiple landmarks. Fish are able to learn to traverse mazes, showing that they possess spatial memory and visual discrimination.<ref>{{cite web |last=Sciences |first=Journal of Undergraduate Life |url=http://juls.sa.utoronto.ca/Issues/JULS-Vol2Iss1/JULS-Vol2Iss1-Review3.pdf |title=Appropriate maze methodology to study learning in fish |access-date=28 May 2009 |url-status=dead |archive-url=https://web.archive.org/web/20110706211428/http://juls.sa.utoronto.ca/Issues/JULS-Vol2Iss1/JULS-Vol2Iss1-Review3.pdf |archive-date=6 July 2011}}</ref> Behavioral research suggests that fish are [[Sentience|sentient]], capable of experiencing [[Pain in fish|pain]].<ref>{{Cite web |last=Woodruff |first=Michael |date=3 July 2020 |title=The face of the fish |url=https://aeon.co/essays/fish-are-nothing-like-us-except-that-they-are-sentient-beings |access-date=28 July 2024 |website=[[Aeon (magazine)|Aeon]]}}</ref>

=== Electrogenesis ===

[[File:Electroreception system in Elephantfish.svg|thumb|upright=1.5|The [[elephantnose fish]] is a weakly electric fish which generates an [[electric field]] with its [[Electric organ (biology)|electric organ]] and then uses its [[Electroreception and electrogenesis|electroreceptive organs]] to locate objects by the distortions they cause in its electric field.<ref name="von der Emde 1999">{{cite journal |last=von der Emde |first=G. |title=Active electrolocation of objects in weakly electric fish |journal=[[Journal of Experimental Biology]] |volume=202 |issue=10 |date=15 May 1999 |doi=10.1242/jeb.202.10.1205 |pages=1205–1215|pmid=10210662 |bibcode=1999JExpB.202.1205V }}</ref>]]

{{further|Electroreception and electrogenesis}}

[[Electric fish]] such as [[Mormyridae|elephantfishes]], the [[Gymnarchus|African knifefish]], and [[Electrophorus (fish)|electric eels]] have some of their muscles adapted to [[Electroreception and electrogenesis|generate electric fields]]. They use the field to locate and identify objects such as prey in the waters around them, which may be turbid or dark.<ref name="Albert Crampton 2006"/> Strongly electric fish like the electric eel can in addition use their [[Electric organ (fish)|electric organs]] to generate shocks powerful enough to stun their prey.<ref name="Catania 2015 high-voltage">{{cite journal |last1=Catania |first1=Kenneth C. |title=Electric eels use high-voltage to track fast-moving prey |journal=Nature Communications |date=20 October 2015 |volume=6 |page=8638 |doi=10.1038/ncomms9638 |pmid=26485580 |pmc=4667699 |bibcode=2015NatCo...6.8638C}}</ref>

=== Endothermy ===

Most fish are exclusively cold-blooded or [[ectothermic]]. However, the [[Scombroidei]] are [[warm-blooded]] (endothermic), including the [[billfish]]es and tunas.<ref name="Block1993">{{cite journal |last1=Block |first1=B.A. |last2=Finnerty |first2=JR |year=1993 |title=Endothermy in fishes: a phylogenetic analysis of constraints, predispositions, and selection pressures |url=https://www.researchgate.net/publication/226138605 |format=PDF |journal=Environmental Biology of Fishes |volume=40 |issue=3 |pages=283–302 |doi=10.1007/BF00002518 |s2cid=28644501 |access-date=1 October 2018 |archive-date=6 November 2020 |archive-url=https://web.archive.org/web/20201106144640/https://www.researchgate.net/publication/226138605 |url-status=live }}</ref> The [[Lampris guttatus|opah]], a [[lampriform]], uses whole-body endothermy, generating heat with its swimming muscles to warm its body while countercurrent exchange minimizes heat loss.<ref>{{Cite journal |last1=Wegner |first1=Nicholas C. |last2=Snodgrass |first2=Owyn E. |last3=Dewar |first3=Heidi |last4=Hyde |first4=John R. |date=2015-05-15 |title=Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus |journal=Science |volume=348 |issue=6236 |pages=786–789 |doi=10.1126/science.aaa8902 |issn=0036-8075 |pmid=25977549 |bibcode=2015Sci...348..786W |s2cid=17412022 }}</ref> Among the cartilaginous fishes, sharks of the families [[Lamnidae]] (such as the great white shark) and [[Alopiidae]] (thresher sharks) are endothermic. The degree of endothermy varies from the billfishes, which warm only their eyes and brain, to the [[bluefin tuna]] and the [[porbeagle shark]], which maintain body temperatures more than {{convert|20|C}} above the ambient water.<ref name="Block1993"/><ref>{{cite journal |last=Goldman |first=K.J. |title=Regulation of body temperature in the white shark, ''Carcharodon carcharias'' |journal=Journal of Comparative Physiology |year=1997 |volume=167 |series=B Biochemical Systemic and Environmental Physiology |issue=6 |pages=423–429 |doi=10.1007/s003600050092 |s2cid=28082417 |url=http://www.mendeley.com/research/temperature-and-activities-of-a-white-shark-carcharodon-carcharias/ |access-date=12 October 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120406114844/http://www.mendeley.com/research/temperature-and-activities-of-a-white-shark-carcharodon-carcharias/ |archive-date=6 April 2012|url-access=subscription }}</ref><ref>{{cite journal |last1=Carey |first1=F.G. |last2=Lawson |first2=K.D. |title=Temperature regulation in free-swimming bluefin tuna |journal=Comparative Biochemistry and Physiology A |date=February 1973 |volume=44 |issue=2 |pages=375–392 |doi=10.1016/0300-9629(73)90490-8 |pmid=4145757 }}</ref>

{{Anchor|Reproductive method}}

=== Reproduction and life-cycle ===

{{main|Fish reproduction}}

[[File:Salmonlarvakils.jpg|thumb|upright|[[Salmon]] fry hatching from the egg, keeping its [[yolk sac]] ]]

The primary reproductive organs are paired [[testicle]]s and [[ovary|ovaries]].<ref name="Guimaraes">{{cite journal |last1=Guimaraes-Cruz |first1=Rodrigo J. |author2=dos Santos, José E. |author3=Santos, Gilmar B. |title=Gonadal structure and gametogenesis of ''Loricaria lentiginosa'' Isbrücker (Pisces, Teleostei, Siluriformes) |journal=Rev. Bras. Zool. |date=July–September 2005 |volume=22 |issue=3 |pages=556–564 |issn=0101-8175 |doi=10.1590/S0101-81752005000300005 |doi-access=free }}</ref> Eggs are released from the ovary<!--directly or indirectly--> to the [[oviduct]]s.<ref name="Brito">{{cite journal |title=Reproduction of the surubim catfish (Pisces, Pimelodidae) in the São Francisco River, Pirapora Region, Minas Gerais, Brazil |last=Brito |first=M.F.G. |author2=Bazzoli, N. |journal=Arquivo Brasileiro de Medicina Veterinária e Zootecnia |volume=55 |issue=5 |year=2003 |doi=10.1590/S0102-09352003000500018 |issn=0102-0935 |pages=624–633 |doi-access=free }}</ref> Over 97% of fish, including salmon and goldfish, are [[oviparous]], meaning that the eggs are shed into the water and develop outside the mother's body.<ref name="Scott">{{cite book |last=Scott |first=Peter |title=Livebearing Fishes |page=13 |publisher=Tetra Press |year=1997 |isbn=1-56465-193-2}}</ref> The eggs are usually fertilized outside the mother's body, with the male and female fish shedding their [[gamete]]s into the surrounding water. In a few oviparous fish, such as the [[Skate (fish)|skates]], fertilization is internal: the male uses an [[Ichthyology terms#I|intromittent organ]] to deliver sperm into the female's genital opening of the female.<ref name="Miller Kendall 2009"/> Marine fish release large numbers of small eggs into the open water column. Newly hatched young of oviparous fish are [[Ichthyoplankton|planktonic larvae]]. They have a large [[yolk sac]] and do not resemble juvenile or adult fish. The larval period in oviparous fish is usually only some weeks, and larvae rapidly grow and [[metamorphosis|change in structure]] to become juveniles. During this transition, larvae must switch from their yolk sac to feeding on [[zooplankton]] prey.<ref name="Miller Kendall 2009"/> Some fish such as [[Embiotocidae|surf-perches]], [[Goodeidae|splitfins]], and [[lemon shark]]s are [[viviparous]] or live-bearing, meaning that the mother retains the eggs and nourishes the embryos via a structure analogous to the [[placenta]] to connect the mother's blood supply with the embryo's.<ref name="Miller Kendall 2009">{{cite book |last1=Miller |first1=Bruce |last2=Kendall |first2=Arthur W. |title=Early Life History of Marine Fishes |date=2009 |publisher=[[University of California Press]] |isbn=9780520249721 |pages=11–37 |chapter-url=https://content.ucpress.edu/chapters/9317.ch01.pdf |url-status=live |archive-url=https://web.archive.org/web/20160307042241/https://content.ucpress.edu/chapters/9317.ch01.pdf |archive-date=7 March 2016 |chapter=1. Fish Reproduction}}</ref>

=== DNA repair ===

Embryos of externally fertilized fish species are directly exposed during their development to environmental conditions that may [[DNA damage (naturally occurring)|damage their DNA]], such as pollutants, [[ultraviolet|UV light]] and [[reactive oxygen species]].<ref name="Dey2023">Dey A, Flajšhans M, Pšenička M, Gazo I. DNA repair genes play a variety of roles in the development of fish embryos. Front Cell Dev Biol. 2023 Mar 1;11:1119229. doi: 10.3389/fcell.2023.1119229. PMID 36936683; PMCID: PMC10014602</ref> To deal with such DNA damages, a variety of different [[DNA repair]] pathways are employed by fish embryos during their development.<ref name = Dey2023/> In recent years [[zebrafish]] have become a useful model for assessing environmental pollutants that might be genotoxic, i.e. cause DNA damage.<ref>Canedo A, Rocha TL. Zebrafish (Danio rerio) using as model for genotoxicity and DNA repair assessments: Historical review, current status and trends. Sci Total Environ. 2021 Mar 25;762:144084. doi: 10.1016/j.scitotenv.2020.144084. Epub 2020 Dec 14. PMID 33383303</ref>

=== Defenses against disease ===

{{further|Immune system}}

Fish have both non-specific and immune defenses against disease. Non-specific defenses include the skin and scales, as well as the mucus layer secreted by the [[Epidermis (skin)|epidermis]] that traps and inhibits the growth of [[microorganism]]s. If [[pathogen]]s breach these defenses, the [[innate immune system]] can mount an [[inflammation|inflammatory response]] that increases blood flow to the infected region and delivers [[white blood cells]] that attempt to destroy pathogens, non-specifically. Specific defenses respond to particular antigens, such as [[protein]]s on the surfaces of [[pathogenic bacteria]], recognised by the [[adaptive immune system]].{{sfn |Helfman |Collette |Facey |1997 |pp=95–96}} Immune systems evolved in [[deuterostome]]s as shown in the cladogram.<ref name="Flajnik Kasahara 2010"/>

{{clade|style=font-size:95%;line-height:110%;
|label1=[[Deuterostome]]s
|sublabel1='''[[innate immunity]]'''
|1={{clade
   |1=[[Echinoderm]]s, [[hemichordate]]s, [[cephalochordate]]s, [[urochordate]]s
   |label2=[[Vertebrates]]
   |2={{clade
      |sublabel1='''[[variable lymphocyte receptor|VLR adaptive immunity]]'''
      |1=[[Agnatha|Jawless fishes]]
      |sublabel2='''[[V(D)J recombination|V(D)J adaptive immunity]]'''
      |2=[[Osteichthyes|Jawed fishes and tetrapods]]
      }}
   }}
}}

Immune organs vary by type of fish. The jawless fish have [[lymphoid tissue]] within the [[pronephros|anterior kidney]], and [[granulocyte]]s in the gut. They have [[Adaptive immunity in jawless fish|their own type of adaptive immune system]]; it makes use of [[variable lymphocyte receptor]]s (VLR) to generate immunity to a wide range of antigens, The result is much like that of jawed fishes and tetrapods, but it may have [[Convergent evolution|evolved separately]].<ref name="Flajnik Kasahara 2010"/> All jawed fishes have an [[adaptive immune system]] with B and T [[lymphocytes]] bearing [[immunoglobulin]]s and [[T cell receptor]]s respectively. This makes use of [[V(D)J recombination|Variable–Diversity–Joining rearrangement]] (V(D)J) to create immunity to a wide range of antigens. This system evolved once and is basal to the jawed vertebrate clade.<ref name="Flajnik Kasahara 2010">{{cite journal |last1=Flajnik |first1=M. F. |last2=Kasahara |first2=M. |year=2010 |title=Origin and evolution of the adaptive immune system: genetic events and selective pressures |journal=Nature Reviews Genetics |volume=11 |issue=1 |pages=47–59 |doi=10.1038/nrg2703 |pmid=19997068 |pmc=3805090}}</ref> Cartilaginous fish have three specialized organs that contain immune system cells: the epigonal organs around the gonads, [[Leydig's organ]] within the esophagus, and a [[spiral valve]] in their intestine, while their [[thymus]] and [[spleen]] have similar functions to those of the same organs in the immune systems of tetrapods.<ref>{{cite book |last1=Zapata |first1=A.G. |last2=Chiba |first2=A. |last3=Vara |first3=A. |chapter=Cells and tissues of the immune system of fish |title=The Fish Immune System: Organism, Pathogen and Environment |series=Fish Immunology |editor1=Iwama, G. Iwama |editor2=Nakanishi, T. |location=New York |publisher=Academic Press |year=1996 |pages=1–55}}</ref> Teleosts have lymphocytes in the thymus, and other immune cells in the spleen and other organs.<ref>{{cite journal |last=Chilmonczyk |first=S. |title=''The thymus in fish: development and possible function in the immune response'' |journal=[[Annual Review of Fish Diseases]] |volume=2 |year=1992 |pages=181–200 |doi=10.1016/0959-8030(92)90063-4}}</ref><ref>{{cite journal |last1=Hansen |first1=J.D. |last2=Zapata |first2=A.G. |year=1998 |title=''Lymphocyte development in fish and amphibians'' |journal=Immunological Reviews |volume=166 |pages=199–220 |doi=10.1111/j.1600-065x.1998.tb01264.x |pmid=9914914 |s2cid=7965762 }}</ref>

== Behavior ==

=== Shoaling and schooling ===

{{main|Shoaling and schooling}}

[[File:Banco de peces trompeta (Macroramphosus scolopax), islas Azores, Portugal, 2020-07-27, DD 40.jpg|thumb|Fish such as these [[snipefish]]es [[Shoaling and schooling|school]] for safety from predators, and to spawn.<ref name="Pitcher 1986"/>]]

A ''shoal'' is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. A ''school'' is a much more tightly organised group, synchronising its swimming so that all fish move at the same speed and in the same direction.{{sfn|Helfman|Collette|Facey|1997|p=375}} Schooling is sometimes an [[antipredator adaptation]], offering improved vigilance against predators. It is often more efficient to gather food by working as a group, and individual fish optimise their strategies by choosing to join or leave a shoal. When a predator has been noticed, prey fish respond defensively, resulting in collective shoal behaviours such as synchronised movements. Responses do not consist only of attempting to hide or flee; antipredator tactics include for example scattering and reassembling. Fish also aggregate in shoals to spawn.<ref name="Pitcher 1986">{{cite book |last=Pitcher |first=Tony J. |chapter=12. Functions of Shoaling Behaviour in Teleosts |title=The Behaviour of Teleost Fishes |publisher=Springer |year=1986 |pages=294–337 |doi=10.1007/978-1-4684-8261-4_12 |isbn=978-1-4684-8263-8}}</ref> The [[capelin]] migrates annually in large schools between its feeding areas and its spawning grounds.<ref name="Gjøsæter 1998">{{cite journal |last=Gjøsæter |first=H. |title=The population biology and exploitation of capelin (''Mallotus villosus'') in the Barents Sea |journal=Sarsia |year=1998 |volume=83 |issue=6 |pages=453–496 |doi=10.1080/00364827.1998.10420445 |url=https://www.researchgate.net/publication/230660010}}</ref>

=== Communication ===

{{see also|Acoustic communication in aquatic animals}}

Fish communicate by transmitting acoustic signals (sounds) to each other. This is most often in the context of feeding, aggression or courtship.<ref name="Bioacoustics">{{cite journal |last1=Weinmann |first1=S.R. |last2=Black |first2=A.N. |last3=Richter |first3=M.L. |last4=Itzkowitz |first4=M. |last5=Burger |first5=R.M. |title=Territorial vocalization in sympatric damselfish: acoustic characteristics and intruder discrimination |journal=Bioacoustics |date=February 2017 |volume=27 |issue=1 |pages=87–102 |doi=10.1080/09524622.2017.1286263 |s2cid=89625932}}</ref> The sounds emitted vary with the species and stimulus involved. Fish can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.<ref name="Journal">{{cite journal |last1=Bertucci |first1=F. |last2=Ruppé |first2=L. |last3=Wassenbergh |first3=S.V. |last4=Compère |first4=P. |last5=Parmentier |first5=E. |date=29 October 2014 |title=New Insights into the Role of the Pharyngeal Jaw Apparatus in the Sound-Producing Mechanism of Haemulon Flavolineatum (Haemulidae) |journal=Journal of Experimental Biology |volume=217 |issue=21 |pages=3862–3869 |doi=10.1242/jeb.109025 |pmid=25355850 |doi-access=free |bibcode=2014JExpB.217.3862B |hdl=10067/1197840151162165141|hdl-access=free}}</ref>

[[File:French grunts - Haemulon flavolineatum.jpg|thumb|[[Haemulon flavolineatum|French grunt fish]] makes sounds by grinding its teeth. ]]

Some fish produce sounds by rubbing or grinding their bones together. These sounds are stridulatory. In ''[[Haemulon flavolineatum]]'', the French grunt fish, as it produces a grunting noise by grinding its teeth together, especially when in distress. The grunts are at a frequency of around 700&nbsp;Hz, and last approximately 47 milliseconds.<ref name="Journal"/> The longsnout seahorse, ''[[Hippocampus reidi]]'' produces two categories of sounds, 'clicks' and 'growls', by rubbing their coronet bone across the grooved section of their neurocranium.<ref>{{cite journal |last1=Colson |first1=D.J. |last2=Patek |first2=S.N. |last3=Brainerd |first3=E.L. |last4=Lewis |first4=S.M. |title=Sound production during feeding in Hippocampus seahorses (Syngnathidae) |journal=Environmental Biology of Fishes |date=February 1998 |volume=51 |issue=2 |pages=221–229 |doi=10.1023/A:1007434714122 |bibcode=1998EnvBF..51..221C |s2cid=207648816}}</ref> Clicks are produced during courtship and feeding, and the frequencies of clicks were within the range of 50&nbsp;Hz-800&nbsp;Hz. The frequencies are at the higher end of the range during spawning, when the female and male fishes were less than fifteen centimeters apart. Growls are produced when the ''H. reidi'' are stressed. The 'growl' sounds consist of a series of sound pulses and are emitted simultaneously with body vibrations.<ref name="Zoology">{{cite journal |last1=Oliveira |first1=T.P.R. |last2=Ladich |first2=F. |last3=Abed-Navandi |first3=D. |last4=Souto |first4=A.S. |last5=Rosa |first5=I.L. |title=Sounds produced by the longsnout seahorse: a study of their structure and functions|journal=Journal of Zoology|date=26 June 2014|volume=294 |issue=2 |pages=114–121 |doi=10.1111/jzo.12160}}</ref>

Some fish species create noise by engaging specialized muscles that contract and cause swimbladder vibrations. [[Oyster toadfish]] produce loud grunts by contracting sonic muscles along the sides of the swim bladder.<ref name="Experimental">{{cite journal|last1=Fine |first1=L.F. |last2=King |first2=C.B. |last3=Cameron |first3=T.M. |title=Acoustical properties of the swimbladder in the oyster toadfish ''Opsanus tau'' |journal=Journal of Experimental Biology|date=16 October 2009 |volume=212 |issue=21 |pages=3542–3552|doi=10.1242/jeb.033423|pmid=19837896|pmc=2762879|bibcode=2009JExpB.212.3542F }}</ref> Female and male toadfishes emit short-duration grunts, often as a fright response.<ref>{{cite journal |last1=Fine |first1=M.L. |last2=Waybright |first2=T.D. |title=Grunt variation in the oyster toadfish Opsanus tau:effect of size and sex |journal=PeerJ |date=15 October 2015 |volume=3 |issue=1330 |pages=e1330 |doi=10.7717/peerj.1330 |pmid=26623178 |pmc=4662586 |doi-access=free }}</ref> In addition to short-duration grunts, male toadfishes produce "boat whistle calls".<ref name="plos" /> These calls are longer in duration, lower in frequency, and are primarily used to attract mates.<ref name="plos" /> The various sounds have frequency range of 140&nbsp;Hz to 260&nbsp;Hz.<ref name="plos">{{cite journal|last1=Ricci |first1=S.W. |last2=Bohnenstiehl |first2=D. R. |last3=Eggleston |first3=D.B. |last4=Kellogg |first4=M.L. |last5=Lyon |first5=R.P. |title=Oyster toadfish (Opsanus tau) boatwhistle call detection and patterns within a large-scale oyster restoration site |journal=PLOS ONE |date=8 August 2017 |volume=12 |issue=8 |pages=e0182757 |doi=10.1371/journal.pone.0182757|pmid=28792543|pmc=5549733 |bibcode=2017PLoSO..1282757R |doi-access=free}}</ref> The frequencies of the calls depend on the rate at which the sonic muscles contract.<ref>{{cite journal |last1=Skoglund |first1=C.R. |title=Functional analysis of swimbladder muscles engaged in sound productivity of the toadfish |journal=Journal of Cell Biology |date=1 August 1961 |volume=10 |issue=4 |pages=187–200 |doi=10.1083/jcb.10.4.187 |pmid=19866593 |pmc=2225107}}</ref><ref name="Experimental"/>

The red drum, ''[[Sciaenops ocellatus]]'', produces drumming sounds by vibrating its swimbladder. Vibrations are caused by the rapid contraction of sonic muscles that surround the dorsal aspect of the swimbladder. These vibrations result in repeated sounds with frequencies from 100 to >200&nbsp;Hz. ''S. ocellatus'' produces different calls depending on the stimuli involved, such as courtship or a predator's attack. Females do not produce sounds, and lack sound-producing (sonic) muscles.<ref name="Elsevier">{{cite journal|last1=Parmentier|first1=E. |last2=Tock|first2=J. |last3=Falguière|first3=J.C. |last4=Beauchaud|first4=M.|title=Sound production in Sciaenops ocellatus: Preliminary study for the development of acoustic cues in aquaculture |journal=Aquaculture |date=22 May 2014|volume=432|pages=204–211|doi=10.1016/j.aquaculture.2014.05.017|bibcode=2014Aquac.432..204P |url=https://archimer.ifremer.fr/doc/00190/30127/28589.pdf|access-date=21 January 2019|archive-date=3 June 2021|archive-url=https://web.archive.org/web/20210603172200/https://archimer.ifremer.fr/doc/00190/30127/28589.pdf|url-status=live}}</ref>

== Conservation ==

The 2024 [[International Union for Conservation of Nature|IUCN]] [[IUCN Red List|Red List]] names 2,168 fish species that are endangered or critically endangered.<ref>{{cite web |url=https://www.iucnredlist.org/search?query=Fishes&searchType=species |title=Search for 'Fishes' (Global, CR-Critically Endangered, En-Endangered, Species) |access-date=27 February 2024}}</ref> Included are species such as [[Gadus morhua|Atlantic cod]],<ref>{{cite iucn |last=Sobel |first=J. |date=1996 |title=''Gadus morhua'' |volume=1996 |page=e.T8784A12931575 |doi=10.2305/IUCN.UK.1996.RLTS.T8784A12931575.en |access-date=11 November 2021}}</ref> [[Cyprinodon diabolis|Devil's Hole pupfish]],<ref>{{cite iucn |date=2014 |title=''Cyprinodon diabolis'' |volume=2014 |page=e.T6149A15362335 |doi=10.2305/IUCN.UK.2014-3.RLTS.T6149A15362335.en |access-date=11 November 2021}}</ref> [[coelacanth]]s,<ref>{{cite iucn |last=Musick |first=J.A. |date=2000 |title=''Latimeria chalumnae'' |volume=2000 |page=e.T11375A3274618 |doi=10.2305/IUCN.UK.2000.RLTS.T11375A3274618.en |access-date=11 November 2021}}</ref> and [[great white shark]]s.<ref>{{cite iucn |last1=Rigby |first1=C.L. |last2=Barreto |first2=R. |last3=Carlson |first3=J. |last4=Fernando |first4=D. |last5=Fordham |first5=S. |last6=Francis |first6=M.P. |last7=Herman |first7=K. |last8=Jabado |first8=R.W. |last9=Liu |first9=K.M. |last10=Lowe |first10=C.G. |last11=Marshall |first11=A. |last12=Pacoureau |first12=N. |last13=Romanov |first13=E. |last14=Sherley |first14=R.B. |last15=Winker |first15=H. |display-authors=6 |year=2019 |title=''Carcharodon carcharias'' |page=e.T3855A2878674 |access-date=19 December 2019}}</ref> Because fish live underwater they are more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the Devil's Hole pupfish occupies only a single {{convert|3|by|6|m|0}} pool.{{sfn|Helfman|Collette|Facey|1997|pp=449–450}}

=== Overfishing ===

{{main|Overfishing}}

[[File:Time series for collapse of Atlantic northwest cod.png|thumb|upright=1.5|[[Collapse of the Atlantic northwest cod fishery]]<ref name="Hamilton 2001"/> ]]

The [[Food and Agriculture Organization]] reports that "in 2017, 34 percent of the fish stocks of the world's marine fisheries were classified as overfished".<ref name="FAO 2020">{{Cite book |url=http://www.fao.org/documents/card/en/c/ca9229en |title=The State of World Fisheries and Aquaculture 2020 |date=2020 |publisher=[[Food and Agriculture Organization]] |isbn=978-92-5-132692-3 |doi=10.4060/ca9229en |hdl=10535/3776 |s2cid=242949831 |page=54}}</ref> Overfishing is a major threat to edible fish such as cod and [[tuna]].<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/6234881.stm |title=Call to halt cod 'over-fishing' |access-date=18 January 2006 |work=BBC News |date=5 January 2007 |url-status=live |archive-url=https://web.archive.org/web/20070117142906/http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/6234881.stm |archive-date=17 January 2007}}</ref><ref>{{cite news |url=http://news.bbc.co.uk/1/hi/world/asia-pacific/6301187.stm |title=Tuna groups tackle overfishing |access-date=18 January 2006 |work=BBC News |date=26 January 2007 |url-status=live |archive-url=https://web.archive.org/web/20090121111204/http://news.bbc.co.uk/1/hi/world/asia-pacific/6301187.stm |archive-date=21 January 2009}}</ref> Overfishing eventually causes [[fish stocks]] to collapse, because the survivors cannot produce enough young to replace those removed. Such commercial extinction does not mean that the species is extinct, merely that it can no longer sustain a fishery. In the case of the [[Pacific sardine]] fishery off the California coast, the catch steadily declined from a 1937 peak of 800,000 tonnes to an economically inviable 24,000 tonnes in 1968.{{sfn|Helfman|Collette|Facey|1997|p=462}} In the [[Collapse of the Atlantic northwest cod fishery|case of the Atlantic northwest cod fishery]], overfishing reduced the fish population to 1% of its historical level by 1992.<ref name="Hamilton 2001">{{cite journal |last1=Hamilton |first1=Lawrence C. |last2=Butler |first2=M. J. |title=Outport adaptations: Social indicators through Newfoundland's Cod crisis |journal=Human Ecology Review |date=January 2001 |volume=8 |issue=2|pages=1–11 |url=https://www.researchgate.net/publication/247089244}}</ref>
[[Fisheries science|Fisheries scientists]] and the [[fishing industry]] have sharply differing views on the resiliency of fisheries to intensive fishing. In many coastal regions the fishing industry is a major employer, so governments are predisposed to support it.<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/uk/6112352.stm |title=UK 'must shield fishing industry' |access-date=18 January 2006 |work=BBC News |date=3 November 2006 |url-status=live |archive-url=https://web.archive.org/web/20061130055349/http://news.bbc.co.uk/1/hi/uk/6112352.stm |archive-date=30 November 2006}}</ref><ref>{{cite news |url=http://news.bbc.co.uk/1/hi/world/europe/6197433.stm |title=EU fish quota deal hammered out |access-date=18 January 2006 |work=BBC News |date=21 December 2006 |url-status=live |archive-url=https://web.archive.org/web/20061226234953/http://news.bbc.co.uk/1/hi/world/europe/6197433.stm |archive-date=26 December 2006}}</ref> On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be destroyed within fifty years.<ref>{{cite web|title=Ocean study predicts the collapse of all seafood fisheries by 2050|url=http://www.physorg.com/news81778444.html|url-status=live|archive-url=https://web.archive.org/web/20070315014325/http://www.physorg.com/news81778444.html|archive-date=15 March 2007|access-date=13 January 2006|website=phys.org}}</ref><ref>{{cite web |url=http://www.panda.org/about_wwf/where_we_work/europe/what_we_do/mediterranean/about/marine/bluefin_tuna/tuna_at_risk/index.cfm |title=Atlantic bluefin tuna could soon be commercially extinct |publisher=WWF |access-date=18 January 2006 |archive-url=https://web.archive.org/web/20070430205610/http://www.panda.org/about_wwf/where_we_work/europe/what_we_do/mediterranean/about/marine/bluefin_tuna/tuna_at_risk/index.cfm |archive-date=30 April 2007 |url-status=dead}}</ref>

=== Other threats ===

A key stress on both freshwater and marine ecosystems is [[habitat degradation]] including [[water pollution]], the building of dams, removal of water for use by humans, and the introduction of [[invasive species|exotic]] species including predators.{{sfn|Helfman|Collette|Facey|1997|p=463}} Freshwater fish, especially if [[Endemism|endemic]] to a region (occurring nowhere else), may be threatened with extinction for all these reasons, as is the case for three of Spain's ten endemic freshwater fishes.<ref name="Elvira 1995">{{cite journal |last=Elvira |first=Benigno |title=Conservation status of endemic freshwater fish in Spain |journal=Biological Conservation |publisher=Elsevier |volume=72 |issue=2 |year=1995 |issn=0006-3207 |doi=10.1016/0006-3207(94)00076-3 |pages=129–136|bibcode=1995BCons..72..129E }}</ref> River dams, especially major schemes like the [[Kariba Dam]] (Zambezi river) and the [[Aswan Dam]] ([[River Nile]]) on rivers with economically important fisheries, have caused large reductions in fish catch.<ref>{{cite book |last1=Jackson |first1=Donald C. |last2=Marmulla |first2=Gerd |title=The influence of dams on river fisheries |publisher=FAO Fisheries |volume=Technical paper 419 |date=2001 |pages=1–44 |url=http://www.friendsofmerrymeetingbay.org/cybrary/pages/20010000_UN_FAO_Dams,%20fish%20and%20fisheries.pdf#page=6}}</ref> Industrial bottom trawling [[Environmental impact of fishing|can damage seabed habitats]], as has occurred on the [[Georges Bank]] in the North Atlantic.<ref name="Duplisea Frisk Trenkel 2016">{{cite journal |last1=Duplisea |first1=Daniel E. |last2=Frisk |first2=Michael G. |last3=Trenkel |first3=Verena M. |title=Extinction Debt and Colonizer Credit on a Habitat Perturbed Fishing Bank |journal=PLOS ONE |publisher=Public Library of Science (PLoS) |volume=11 |issue=11 |date=2016-11-28 |issn=1932-6203 |doi=10.1371/journal.pone.0166409 |doi-access=free |page=e0166409 |pmid=27893775 |pmc=5125594 |bibcode=2016PLoSO..1166409D }}</ref> Introduction of aquatic [[invasive species]] is widespread. It modifies ecosystems, causing biodiversity loss, and can harm fisheries. Harmful species include fish but are not limited to them;<ref name="Lovell Stone 2006">{{cite journal |last1=Lovell |first1=Sabrina J. |last2=Stone |first2=Susan F. |last3=Fernandez |first3=Linda |title=The economic impacts of aquatic invasive species: a review of the literature |journal=Agricultural and Resource Economics Review |volume=35 |issue=1 |year=2006 |pages=195–208 |doi=10.1017/S1068280500010157|url=https://ageconsearch.umn.edu/record/280839/files/NCEE2005-02.pdf }}</ref> the arrival of a [[comb jelly]] in the Black Sea damaged the [[anchovy]] fishery there.<ref>{{cite book |last1=Knowler |first1=D. |last2=Barbier |first2=E.B. |chapter=The Economics of an Invading Species: a Theoretical Model and Case Study Application |title=The Economics of Biological Invasions |editor1=Perrings, C. |editor2=Williamson, M. |editor3=Dalmazzone, S. |year=2000 |publisher=Edward Elgar |location=Cheltenham |pages=70–93}}</ref><ref name="Lovell Stone 2006"/> The opening of the [[Suez Canal]] in 1869 made possible [[Lessepsian migration]], facilitating the arrival of hundreds of Indo-Pacific marine species of fish, algae and invertebrates in the [[Mediterranean Sea]], deeply impacting its overall biodiversity <ref>Atlas of Exotic Fishes in the Mediterranean Sea. 2nd Edition. 2021. (F. Briand Ed.) CIESM Publishers, Paris, Monaco 366 p.[https://ciesm.org/catalog/index.php?article=2021]</ref> and ecology.<ref name="Golani 1998">{{cite journal |last=Golani |first=Daniel |title=Impact of Red Sea fish migrants through the Suez Canal on the aquatic environment of the Eastern Mediterranean |journal=Bulletin Series Yale School of Forestry and Environmental Studies |issue=103 |year=1998 |pages=375–387}}</ref> The predatory [[Nile perch]] was deliberately introduced to [[Lake Victoria]] in the 1960s as a commercial and sports fish. The lake had high biodiversity, with some 500 [[Endemism|endemic]] species of [[cichlid]] fish. It drastically altered the lake's ecology, and [[Fishing on Lake Victoria|simplified the fishery]] from multi-species to just three: the Nile perch, the [[silver cyprinid]], and another introduced fish, the [[Nile tilapia]]. The [[haplochromine]] cichlid populations have collapsed.<ref name="Coulter Allanson Bruton 1986">{{cite journal |last1=Coulter |first1=George W. |last2=Allanson |first2=Brian R. |last3=Bruton |first3=Michael N. |last4=Greenwood |first4=P. Humphry |last5=Hart |first5=Robert C. |last6=Jackson |first6=Peter B. N. |last7=Ribbink |first7=Anthony J. |title=Unique qualities and special problems of the African Great Lakes |journal=Environmental Biology of Fishes |publisher=[[Springer Science and Business Media]] |volume=17 |issue=3 |year=1986 |issn=0378-1909 |doi=10.1007/bf00698196 |pages=161–183|bibcode=1986EnvBF..17..161C }}</ref><ref name="Achieng 1990">{{cite journal |last=Achieng |first=A. P. |year=1990 |title=The impact of the introduction of the Nile Perch, ''Lates niloticus'' (L.), on the fisheries of Lake Victoria |journal=[[Journal of Fish Biology]] |volume=37, Suppl. A |pages=17–23 |doi=10.1111/j.1095-8649.1990.tb05016.x |bibcode=1990JFBio..37S..17A }}</ref>

== Importance to humans ==

=== Economic ===

{{main|Commercial fishing|Fish farming}}

[[File:Trawlers overfishing cod.jpg|thumb|upright=1.25|A [[Trawler (fishing)|trawler]] hauling in a large catch of [[cod]], 2016 ]]

Throughout history, humans have used [[fish as food|fish as a food source]] for [[dietary protein]]. Historically and today, most fish harvested for human consumption has come by means of catching wild fish. However, fish farming, which has been practiced since about 3,500 BCE in ancient China,<ref>{{cite magazine |last1=Spalding |first1=Mark |title=Sustainable Ancient Aquaculture |url=http://voices.nationalgeographic.com/2013/07/11/sustainable-ancient-aquaculture/ |magazine=National Geographic |access-date=13 August 2015 |date=11 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150518210243/http://voices.nationalgeographic.com/2013/07/11/sustainable-ancient-aquaculture/ |archive-date=18 May 2015}}</ref> is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish.<ref name="Helfman">{{cite book |last1=Helfman |first1=Gene S. |title=Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources |date=2007 |publisher=[[Island Press]] |isbn=978-1-59726-760-1 |pages=11}}</ref> [[Fishing]] is accordingly a large global business which provides income for millions of people.<ref name="Helfman"/> The [[Environmental Defense Fund]] has a guide on which fish are safe to eat, given the state of pollution in today's world, and which fish are obtained in a sustainable way.<ref name="EDF fish">{{cite web |url=https://seafood.edf.org/ |title=EDF Seafood Selector: Fish Choices that are Good for You and the Oceans |publisher=[[Environmental Defense Fund]] |access-date=21 January 2024}}</ref> As of 2020, over 65 million tonnes (Mt) of marine fish and 10 Mt of freshwater fish were captured, while some 50 Mt of fish, mainly freshwater, were farmed. Of the marine species captured in 2020, [[anchoveta]] represented 4.9 Mt, [[Alaska pollock]] 3.5 Mt, [[skipjack tuna]] 2.8 Mt, and [[Atlantic herring]] and [[yellowfin tuna]] 1.6 Mt each; eight more species had catches over 1 Mt.<ref>{{cite book |year=2022 |title=The State of World Fisheries and Aquaculture 2022. Towards Blue Transformation |location=Rome |publisher=[[Food and Agriculture Organization]] |doi=10.4060/cc0461en |hdl=10535/3776 |isbn=978-92-5-136364-5 |url=https://doi.org/10.4060/cc0461en}}</ref>

=== Recreation ===

{{further|Fishkeeping|Recreational fishing}}

Fish have been recognized as a source of beauty for almost as long as used for food, appearing in [[cave art]], being raised as [[ornamental fish]] in ponds, and displayed in [[aquarium]]s in homes, offices, or public settings. Recreational fishing is fishing primarily for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit, or [[artisanal fishing]], which is fishing primarily for food. The most common form of recreational fishing employs a [[fishing rod|rod]], [[fishing reel|reel]], [[fishing line|line]], [[fish hook|hooks]], and a wide range of [[bait (luring substance)|baits]]. Recreational fishing is particularly popular in North America and Europe; government agencies often actively manage target fish species.<ref>{{cite book |title=The Angler in the Environment: Social, Economic, Biological, and Ethical Dimensions |publisher=[[American Fisheries Society]] |year=2011 |isbn=978-1-934874-24-0 |editor-last=Beard |editor-first=T. Douglas|location=Bethesda, Maryland |pages=365}}</ref><ref>{{cite book |title=Recreational Fisheries: Social, Economic and Management Aspects |publisher=[[Wiley-Blackwell]] |year=1998 |isbn=978-0-852-38248-6 |editor-last=Hickley |editor-first=Phil |pages=328 |editor-last2=Tompkins |editor-first2=Helena}}</ref>

=== Culture ===

{{main|Fish in culture}}

Fish themes have symbolic significance in many religions. In ancient [[Mesopotamia]], fish offerings were made to the gods from the very earliest times.<ref name="BlackGreen1992">{{cite book |last1=Black |first1=Jeremy |first2=Anthony |last2=Green |title=Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary |url=https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana |publisher=[[The British Museum Press]] |year=1992 |isbn=978-0-7141-1705-8 |pages=82–83 |url-status=live |archive-url=https://web.archive.org/web/20180220073403/https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana |archive-date=20 February 2018}}</ref> Fish were also a major symbol of [[Enki]], the god of water.<ref name="BlackGreen1992"/> Fish frequently appear as filling motifs in [[cylinder seal]]s from the [[First Babylonian dynasty|Old Babylonian]] ({{circa}} 1830 BC – {{circa}} 1531 BC) and [[Neo-Assyrian Empire|Neo-Assyrian]] (911–609 BC) periods.<ref name="BlackGreen1992"/> Starting during the [[Kassites|Kassite Period]] ({{circa}} 1600 BC – {{circa}} 1155 BC) and lasting until the early [[Achaemenid Empire|Persian Period]] (550–30 BC), healers and exorcists dressed in ritual garb resembling the bodies of fish.<ref name="BlackGreen1992"/> During the [[Seleucid Empire|Seleucid Period]] (312–63 BC), the legendary Babylonian [[culture hero]] [[Oannes (mythology)|Oannes]] was said to have dressed in the skin of a fish.<ref name="BlackGreen1992"/> Fish were sacred to the Syrian goddess [[Atargatis]]<ref name="Hyde2008">{{cite book |last=Hyde |first=Walter Woodburn |date=2008 |orig-year=1946 |title=Paganism to Christianity in the Roman Empire |url=https://books.google.com/books?id=H_VLAwAAQBAJ&q=Atargatis |location=Eugene, Oregon |publisher=[[Wipf and Stock Publishers]] |isbn=978-1-60608-349-9 |pages=57–58 |access-date=12 December 2020 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010624/https://books.google.com/books?id=H_VLAwAAQBAJ&q=Atargatis |url-status=live}}</ref> and, during her festivals, only her priests were permitted to eat them.<ref name="Hyde2008"/> In the [[Book of Jonah]], the central figure, a [[prophet]] named [[Jonah]], is swallowed by a giant fish after being thrown overboard by the crew of the ship he is travelling on.<ref name="Sherwood">{{cite book |last=Sherwood |first=Yvonne |date=2000 |title=A Biblical Text and Its Afterlives: The Survival of Jonah in Western Culture |url=https://books.google.com/books?id=Kivijc6ZX5QC&q=Jonah |publisher=[[Cambridge University Press]] |isbn=978-0-521-79561-6 |pages=1–8 |access-date=12 December 2020 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010619/https://books.google.com/books?id=Kivijc6ZX5QC&q=Jonah |url-status=live}}</ref> [[Early Christianity|Early Christians]] used the ''[[ichthys]]'', a symbol of a fish, to represent Jesus.<ref name="Hyde2008"/><ref name="Coffman2008">{{cite magazine |url=http://www.christianitytoday.com/ch/asktheexpert/oct26.html |first=Elesha |last=Coffman |title=What is the origin of the Christian fish symbol? |magazine=[[Christianity Today]] |date=8 August 2008 |access-date=13 August 2015 |archive-date=30 January 2016 |archive-url=https://web.archive.org/web/20160130092733/http://www.christianitytoday.com/ch/asktheexpert/oct26.html |url-status=live }}</ref> Among the [[deity|deities]] said to take the form of a fish are [[Ikatere]] of the [[Polynesians]],<ref name="TeAra">{{cite web| url=https://teara.govt.nz/en/ngarara-reptiles/page-2| title='Ngārara – reptiles, Page 2. From sea to land', Te Ara - the Encyclopedia of New Zealand| publisher=Bradford Haami | accessdate=4 May 2018}}</ref>
the shark-god [[Kāmohoaliʻi]] of [[Hawaii|Hawai{{okina}}i]],<ref>{{cite book |last=Thrum |first=Thomas |title=Hawaiian Folk Tales |url=https://www.sacred-texts.com/pac/hft/hft27.htm |publisher=[[A. C. McClurg]] |year=1907 |page=86}}</ref>
and [[Matsya]] of the Hindus.<ref>{{Cite book |last=Bandyopadhyaya |first=Jayantanuja |url=https://books.google.com/books?id=gwUF11NRyT4C&dq=matsya+avatar+Purana&pg=PA136 |title=Class and Religion in Ancient India |date=2007 |publisher=Anthem Press |isbn=978-1-84331-332-8 |page=136 |access-date=10 July 2022 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008120302/https://books.google.com/books?id=gwUF11NRyT4C&dq=matsya+avatar+Purana&pg=PA136 |url-status=live }}</ref> The constellation [[Pisces (constellation)|Pisces]] ("The Fishes") is associated with a legend from Ancient Rome that [[Venus (mythology)|Venus]] and her son [[Cupid]] were rescued by two fishes.<ref>[[Ovid]] ''[[Fasti]]'' 2.457ff</ref>

Fish feature prominently in art,<ref name=Moyle>{{cite journal |last1=Moyle |first1=Peter B. |author1link=Peter B. Moyle |last2=Moyle |first2=Marilyn A. |title=Introduction to fish imagery in art |journal=Environmental Biology of Fishes |date=May 1991 |volume=31 |issue=1 |pages=5–23 |doi=10.1007/bf00002153 |bibcode=1991EnvBF..31....5M |s2cid=33458630 }}</ref> in films such as ''[[Finding Nemo]]''<ref>{{cite journal |last=Tidwell |first=Christy |title='Fish Are Just like People, Only Flakier': Environmental Practice and Theory in ''Finding Nemo'' |journal=Americana: The Journal of American Popular Culture |issue=8 |year=2009 |url=https://www.americanpopularculture.com/journal/articles/spring_2009/tidwell.htm}}</ref> and books such as ''[[The Old Man and the Sea]]''.<ref>{{cite journal |last1=Durga |first1=P. |last2=Sai |first2=Kanaka |title=Nature of Existential Struggle in ''The Old Man and the Sea'' |journal=Journal of English Language and Literature JOELL |volume=4 |issue=4 |year=2017 |pages=19–21}}</ref> Large fish, particularly sharks, have frequently been the subject of [[Horror film|horror movies]] and [[Thriller (genre)|thrillers]], notably the novel ''[[Jaws (novel)|Jaws]]'', made into a film which in turn has been parodied and imitated many times.<ref>{{cite book |last1=Alabaster |first1=Jay |editor1-last=Jackson |editor1-first=Kathy Merlock |editor2-last=Simpson |editor2-first=Philip L. |title=This shark, swallow you whole": Essays on the Cultural Influence of Jaws |date=2023 |publisher=[[McFarland & Company|McFarland]] |isbn=978-1-4766-7745-3 |pages=124– |chapter=The Goofy Great White: Jaws and Our Love for an Apex Predator}}</ref> Piranhas are shown in a similar light to sharks in films such as ''[[Piranha (1978 film)|Piranha]]''.<ref>{{cite web |last1=Zollinger |first1=Sue Anne |title=Piranha – Ferocious Fighter or Scavenging Softie? |url=http://indianapublicmedia.org/amomentofscience/piranhaferocious-fighter-scavenging-softie/ |website=A Moment of Science |publisher=[[Indiana Public Media]] |access-date=1 November 2015 |date=3 July 2009 |url-status=live |archive-url=https://web.archive.org/web/20151017010303/http://indianapublicmedia.org/amomentofscience/piranhaferocious-fighter-scavenging-softie/ |archive-date=17 October 2015}}</ref>

<gallery mode=packed widths=150 heights=180>
File:Matsya painting.jpg|[[Avatar]] of [[Vishnu]] as a [[Matsya]], India
File:Bartolomeo Passerotti - The Fishmonger's Shop - WGA17072.jpg|''The Fishmonger's Shop'', [[Bartolomeo Passerotti]], 1580s
File:Goldfish Matisse.jpg|''[[Goldfish (Matisse)|Goldfish]]'' by [[Henri Matisse]], 1912
</gallery>

== See also ==
{{main|Outline of fish}}
<!-- Please keep alphabetical -->
{{columns-list|colwidth=22em|
* [[Deep sea fish]]
* [[Fish acute toxicity syndrome]]
* [[Fish development]]
* [[Forage fish]]
* [[List of fish common names]]
* [[List of fish families]]
* [[Mercury in fish]]
* [[Otolith]] – bone used for determining the age of a fish
* [[Pregnancy (fish)]]
* [[Walking fish]]
}}

== Notes ==

{{notelist}}

== References ==

{{reflist}}

== Sources ==
* {{cite book |last=Benton |first=M. J. |date=2005 |url=https://books.google.com/books?id=VThUUUtM8A4C |title=Vertebrate Palaeontology |publisher=[[John Wiley & Sons]] |edition=3rd |isbn=978-140514449-0 }}
* {{cite book |last1=Helfman |first1=G. |last2=Collette |first2=B. |last3=Facey |first3=D. |title=The Diversity of Fishes |publisher=[[Wiley-Blackwell]] |year=1997 |isbn=978-0-86542-256-8 |edition=1st}}
* {{cite book |last=Nelson |first=Joseph S. |chapter=Taxonomic Diversity |title=Fishes of the World |year=2016 |publisher=[[John Wiley & Sons]] |isbn=978-1-118-34233-6}}

== Further reading ==
* {{cite web |ref=none |last1=Eschmeyer |first1=William N. |last2=Fong |first2=Jon David |year=2013 |url=http://researcharchive.calacademy.org/research/ichthyology/ |title=Catalog of Fishes |publisher=[[California Academy of Sciences]] |access-date=28 February 2013 |archive-date=21 November 2018 |archive-url=https://web.archive.org/web/20181121203659/http://researcharchive.calacademy.org/research/ichthyology/ |url-status=live}}
* {{cite book |ref=none |last1=Helfman |first1=G. |last2=Collette |first2=B. |last3=Facey |first3=D. |last4=Bowen |first4=B. |year=2009  |url=http://www.blackwellpublishing.com/helfman/ |title=The Diversity of Fishes: Biology, Evolution, and Ecology |publisher=[[Wiley-Blackwell]] |isbn=978-1-4051-2494-2 |edition=2nd |access-date=26 January 2010 |archive-date=26 August 2021 |archive-url=https://web.archive.org/web/20210826212914/http://www.blackwellpublishing.com/helfman/ |url-status=live}}
* Moyle, Peter B. (1993) [https://books.google.com/books?id=cu-J9tqy4IQC&q=Fish%3A+An+Enthusiast%27s+Guide ''Fish: An Enthusiast's Guide''] {{Webarchive |url=https://web.archive.org/web/20230317010703/https://books.google.com/books?id=cu-J9tqy4IQC&q=Fish%3A+An+Enthusiast%27s+Guide |date=17 March 2023 }} University of California Press. {{ISBN |978-0-520-91665-4}} – good lay text.
* {{cite book |ref=none |last1=Moyle |first1=Peter B. |last2=Cech |first2=Joseph J. |year=2003 |title=Fishes, An Introduction to Ichthyology |edition=5th |publisher=Benjamin Cummings |isbn=978-0-13-100847-2}}
* {{cite book |ref=none |author-link=Helen Scales |last=Scales |first=Helen |year=2018 |title=Eye of the shoal: A Fishwatcher's Guide to Life, the Ocean and Everything |publisher=Bloomsbury Sigma |isbn=978-1-4729-3684-4}}
* {{cite book |ref=none |author-link=Neil Shubin |last=Shubin |first=Neil |year=2009 |url=https://books.google.com/books?id=c008kdNwR1cC |title=Your inner fish: A journey into the 3.5 billion year history of the human body |publisher=[[Vintage Books]] |isbn=978-0-307-27745-9 |access-date=15 December 2015 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010712/https://books.google.com/books?id=c008kdNwR1cC |url-status=live}} [http://www.uctv.tv/search-details.aspx?showID=16412 ''UCTV'' interview] {{Webarchive |url=https://web.archive.org/web/20210114070358/https://books.google.com/books?id=c008kdNwR1cC |date=14 January 2021}}

== External links ==
{{Commons category multi|Fish|Actinopterygii|align=right|Marine aquarium fish|Freshwater aquarium fish}}
{{Wikiquote}}
* [https://db.angfa.org.au/ ANGFA] – Illustrated database of freshwater fishes of Australia and New Guinea
* [https://www.fishbase.se/search.php FishBase online] – Comprehensive database with information on over 29,000 fish species
* {{Webarchive |url=https://archive.today/20121215023321/http://fishdata.siu.edu/ |date=15 December 2012 |title=Fisheries and Illinois Aquaculture Center – Data outlet for fisheries and aquaculture research center in the central US}}
* {{Webarchive |url=https://web.archive.org/web/20080312223952/http://www.nativefish.org/ |date=12 March 2008 |title=The Native Fish Conservancy – Conservation and study of North American freshwater fishes}}
* [https://www.fao.org/fishery/en/topic/2888 United Nation] – Fisheries and Aquaculture Department: Fish and seafood utilization

{{diversity of fish|state=expanded}}
{{Aquatic ecosystems}}
{{Marine aquatic ecosystems}}
{{Evolution of fish}}
{{Toxins}}
{{Brood parasite}}
{{Taxonbar|from=Q152}}
{{Authority control}}

[[Category:Fish| ]]
[[Category:Aquatic ecology]]
[[Category:Fishing|Fish]]
[[Category:Ichthyology]]
[[Category:Obsolete vertebrate taxa<!--Class:Pisces is mentioned in article-->]]
[[Category:Seafood]]
[[Category:Paraphyletic groups]]