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{{Short description|Gill-bearing non-tetrapod aquatic vertebrates}}
{{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]]a
}}
A '''fish''' 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 from the long tradition of grouping all fish into a single [[Class (biology)|class]] (<nowiki>''Pisces''</nowiki>), modern [[phylogenetics]] views fish as a [[paraphyletic]] group.
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Most fish are [[ectotherm|cold-blooded]], their body temperature varying with the surrounding water, though some large, [[nekton|active swimmers]] like the [[white shark]] and [[tuna]] can maintain 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]].
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There are over 33,000 extant species of fish, which is more than all species of amphibians, reptiles, birds, and mammals combined. Most fish belong to the class ''[[Actinopterygii]]'', which accounts for approximately half of all living vertebrates. This makes fish easily the largest group of vertebrates by number of species.
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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".
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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 [[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 fish.
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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 [[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.
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== Etymology ==
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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]].
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{{anchor|Fish or fishes}}
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Though often used interchangeably, in biology ''fish'' and ''fishes'' have different meanings. ''Fish'' is used as a singular noun, or as a plural to describe multiple individuals from a single species. ''Fishes'' is used to describe different species or species groups.
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== Evolution ==
{{main|Evolution of fish}}
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=== Fossil history ===
{{further|Timeline of fish evolution}}
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[[File:Dunkleosteus profile.jpg|thumb|''[[Dunkleosteus]]'' was a giant [[Devonian]] armoured [[Placodermi|placoderm]], c. 400 [[myr|mya]].]]
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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]].During the late [[Cambrian]], other jawless forms such as [[conodont]]s appear.
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[[Gnathostomata|Jawed vertebrates]] appear in the [[Silurian]], with giant armoured [[Placodermi|placoderms]] such as ''[[Dunkleosteus]]''. Jawed fish, too, appeared during the Silurian: the cartilaginous [[Chondrichthyes]] and the bony [[Osteichthyes]].
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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".
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=== Phylogeny ===
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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. Some tetrapods, such as [[cetacea]]ns and [[Ichthyopterygia|ichthyosaurs]], have [[secondary aquatic adaptation|secondarily acquired]] a fish-like body shape through [[convergent evolution]]. 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".
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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. The [[cladogram]] shows the [[phylogeny|evolutionary relationships]] of all groups of living fishes (with their respective diversity) and the tetrapods. [[Extinct]] groups are marked with a [[dagger (mark)|dagger]] (†); groups of uncertain placement are labelled with a question mark (?) and dashed lines (- - - - -). Groups with over 25,000 species are in '''boldface'''.
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{{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= ? |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 = ? |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]]
                     |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 terrestrial species, not considered fish) [[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]]
                                       }}
                                    }}
                                 }}
                              }}
                           }}
                        }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}
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=== Taxonomy ===
{{main|Taxonomy of fish}}
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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.
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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.
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=== Diversity ===
{{main|Diversity of fish}}
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Fish range in size from the huge {{convert|16|m|adj=on}} [[whale shark]] to some tiny teleosts only {{convert|8|mm|sigfig=1|adj=on}} long, such as the cyprinid ''[[Paedocypris progenetica]]'' and the [[stout infantfish]].
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File:Rhincodon typus fgbnms (cropped).jpg|Largest: [[whale shark]]
File:Paedocypris progenetica 001.jpg|Smallest: e.g. ''[[Paedocypris progenetica]]''
</gallery>
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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.
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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>
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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.
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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>
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== 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.]]
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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. [[Coral reef]]s in the [[Indo-Pacific]] constitute the center of diversity for marine fishes, 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. 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.
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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. 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}}.
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In terms of temperature, [[Jonah's icefish]] live in cold waters of the Southern Ocean, including under the [[Filchner–Ronne Ice Shelf]] at a latitude of 79°S, while [[desert pupfish]] live in desert springs, streams, and marshes, sometimes highly saline, with water temperatures as high as 36 C.
A few fish live mostly on land or lay their eggs on land near water. [[Mudskipper]]s feed and interact with one another on mudflats and go underwater to hide in their burrows. A single [[undescribed species]] of ''[[Phreatobius]]'' has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged [[leaf litter]]. [[Cavefish]] of multiple families live in [[underground lake]]s, [[underground river]]s or [[aquifer]]s.
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=== Parasites and predators ===
{{further|Fish diseases and parasites|Predatory fish}}
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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. 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]]''.
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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. In addition, mammals such as [[dolphin]]s and [[Pinniped|seal]]s feed on fish, alongside birds such as [[gannet]]s and [[cormorant]]s.
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File:Initial phase parrotfish feeding at Shaab Marsa Alam, Red Sea, Egypt -SCUBA (6336981391).jpg|A [[parrotfish]] feeding on [[algae]] on a [[coral reef]]
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
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== Anatomy and physiology ==
{{main|Fish anatomy|Fish physiology}}
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=== Locomotion ===
{{Main|Fish locomotion}}
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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.
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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
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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.
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The [[fish scale|scales of fish]] provide protection from [[Predation|predator]]s at the cost of adding stiffness and weight. 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.
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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>
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=== Circulation ===
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[[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.]]
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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.
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=== Respiration ===
==== Gills ====
{{main|Fish gill}}
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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.
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==== Air breathing ====
{{further|amphibious fish}}
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Some 400 species of fish in 50 families can breathe air, enabling them to live in oxygen-poor water or to emerge on to land. 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. Some catfish  absorb air through their digestive tracts.
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=== Digestion ===
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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.
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=== Excretion ===
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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. 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.
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=== Brain ===
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[[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]]
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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. 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. 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]].
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=== Sensory systems ===
{{main|Sensory systems in fish}}
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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. 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. Some fish, such as catfish and sharks, have the [[ampullae of Lorenzini]], [[electroreceptor]]s that detect weak electric currents on the order of millivolt.
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[[Vision in fish|Vision]] is an important [[sensory system]] in fish. 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]]. 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. Teleosts can see [[polarized light]]; some such as cyprinids have a fourth type of cone that detects [[ultraviolet]]. Amongst [[jawless fish]], the [[lamprey]] has well-developed eyes,while the [[hagfish]] has only primitive eye spots.
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[[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.
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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. The mechanism of fish magnetoreception remains unknown; experiments in birds imply a quantum [[radical pair mechanism]].
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=== Cognition ===
{{further|Fish intelligence}}
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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. ''[[Choerodon]]'' wrasse, [[archerfish]], and [[Atlantic cod]] can solve problems and invent tools. The [[monogamy in animals|monogamous]] cichlid ''[[Amatitlania siquia]]'' exhibits pessimistic behavior when prevented from being with its partner. 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. Behavioral research suggests that fish are [[Sentience|sentient]], capable of experiencing [[Pain in fish|pain]].
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=== Electrogenesis ===
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[[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.]]
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{{further|Electroreception and electrogenesis}}
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[[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. 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.
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=== Endothermy ===
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Most fish are exclusively cold-blooded or [[ectothermic]]. However, the [[Scombroidei]] are [[warm-blooded]] (endothermic), including the [[billfish]]es and tunas. 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. 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.
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{{Anchor|Reproductive method}}
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=== Reproduction and life-cycle ===
{{main|Fish reproduction}}
[[File:Salmonlarvakils.jpg|thumb|upright|[[Salmon]] fry hatching from the egg, keeping its [[yolk sac]] ]]
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The primary reproductive organs are paired [[testicle]]s and [[ovary|ovaries]]. Eggs are released from the ovary to the [[oviduct]]s. 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. 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. 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. 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.
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=== DNA repair ===
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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]]. To deal with such DNA damages, a variety of different [[DNA repair]] pathways are employed by fish embryos during their development. In recent years [[zebrafish]] have become a useful model for assessing environmental pollutants that might be genotoxic, i.e. cause DNA damage.
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=== Defenses against disease ===
{{further|Immune system}}
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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]]. Immune systems evolved in [[deuterostome]]s as shown in the cladogram.
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{{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]]
      }}
   }}
}}
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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]]. 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. 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. Teleosts have lymphocytes in the thymus, and other immune cells in the spleen and other organs.
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== Behavior ==
=== Shoaling and schooling ===
{{main|Shoaling and schooling}}
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[[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.]]
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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. 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. The [[capelin]] migrates annually in large schools between its feeding areas and its spawning grounds.
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=== Communication ===
{{see also|Acoustic communication in aquatic animals}}
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Fish communicate by transmitting acoustic signals (sounds) to each other. This is most often in the context of feeding, aggression or courtship. 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.
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[[File:French grunts - Haemulon flavolineatum.jpg|thumb|[[Haemulon flavolineatum|French grunt fish]] makes sounds by grinding its teeth. ]]
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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. 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. 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.
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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. Female and male toadfishes emit short-duration grunts, often as a fright response. In addition to short-duration grunts, male toadfishes produce "boat whistle calls". These calls are longer in duration, lower in frequency, and are primarily used to attract mates. The various sounds have frequency range of 140&nbsp;Hz to 260&nbsp;Hz. The frequencies of the calls depend on the rate at which the sonic muscles contract.
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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.
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== Conservation ==
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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. Included are species such as [[Gadus morhua|Atlantic cod]], [[coelacanth]]s, and [[great white shark]]s. 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.
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=== Overfishing ===
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{{main|Overfishing}}
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[[File:Time series for collapse of Atlantic northwest cod.png|thumb|upright=1.5|[[Collapse of the Atlantic northwest cod fishery]]]]
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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". Overfishing is a major threat to edible fish such as cod and [[tuna]]. 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. 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.
[[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. On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be destroyed within fifty years.
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=== Other threats ===
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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. 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. 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. Industrial bottom trawling [[Environmental impact of fishing|can damage seabed habitats]], as has occurred on the [[Georges Bank]] in the North Atlantic. 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; the arrival of a [[comb jelly]] in the Black Sea damaged the [[anchovy]] fishery there. 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  and ecology. 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.
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== Importance to humans ==
=== Economic ===
{{main|Commercial fishing|Fish farming}}
[[File:Trawlers overfishing cod.jpg|thumb|upright=1.25|A [[purse seiner]] hauling in hundreds of tons of [[Chilean jack mackerel]], 2016]]
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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, is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish. [[Fishing]] is accordingly a large global business which provides income for millions of people. 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. 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.
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=== Recreation ===
{{further|Fishkeeping|Recreational fishing}}
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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.
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=== Culture ===
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{{main|Fish in culture}}
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Fish themes have symbolic significance in many religions. In ancient [[Mesopotamia]], fish offerings were made to the gods from the very earliest times. Fish were also a major symbol of [[Enki]], the god of water. 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. 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. 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. Fish were sacred to the Syrian goddess [[Atargatis]] and, during her festivals, only her priests were permitted to eat them. 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. [[Early Christianity|Early Christians]] used the ''[[ichthys]]'', a symbol of a fish, to represent Jesus. Among the [[deity|deities]] said to take the form of a fish are [[Ikatere]] of the [[Polynesians]],
the shark-god [[Kāmohoaliʻi]] of [[Hawaii|Hawai{{okina}}i]],
and [[Matsya]] of the Hindus. 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.
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Fish feature prominently in art, in films such as ''[[Finding Nemo]]'' and books such as ''[[The Old Man and the Sea]]''. 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. Piranhas are shown in a similar light to sharks in films such as ''[[Piranha (1978 film)|Piranha]]''.
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<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>
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== See also ==
{{main|Outline of fish}}

{{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]]
}}
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== External links ==
{{Commons category multi|Fish|Actinopterygii|align=right|Marine aquarium fish|Freshwater aquarium fish}}
{{Wikiquote|Fish}}
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{{diversity of fish|state=expanded}}
{{Aquatic ecosystems}}
{{Marine aquatic ecosystems}}
{{Evolution of fish}}
{{Toxins}}
{{Brood parasite}}
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[[Category:Fish| ]]
[[Category:Aquatic ecology]]
[[Category:Fishing|Fish]]
[[Category:Ichthyology]]
[[Category:Obsolete vertebrate taxa]]
[[Category:Seafood]]
[[Category:Paraphyletic groups]]
{{二次利用|date=18 August 2025, at 17:43}}
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