- This page is about the animals. For other meanings, please see Fish (disambiguation).
A fish is a poikilothermic (cold-blooded) * water-dwelling vertebrate with gills. There are over 27,000 species of fish, making them the most diverse group of vertebrates. Taxonomically, fish are a paraphyletic group whose exact relationships are much debated; a common division is into the jawless fishes (class Agnatha, 75 species including lampreys and hagfish), the cartilaginous fishes (class Chondrichthyes, 800 species including sharks and rays), with the remainder classed as bony fishes (class Osteichthyes).
Fishes come in different sizes, from the 14-meter (45 ft) whale shark to a 7-mm (just over 1/4 of an inch) long stout infantfish. Many types of aquatic animals named "fish", such as jellyfish and cuttlefish, are not true fish.
* Certain species of fishes maintain elevated body temperatures and may be considered endothermic to varying degrees. The trait is believed to have evolved separately at least five different times as evidenced by differences in morphology. Endothermic teleosts (bony fishes) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of ‘primitive’ mackerel (Gasterochisma melampus). All sharks in the family Lamnidae – shortfin mako , longfin mako, white, porbeagle, and salmon shark – are known to have the capacity for endothermy, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of endothermy varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 deg. C above ambient water temperatures. Endothermy, though metabolically costly, is thought to provide advantages such as increased contractile force of muscles, higher rates of central nervous system processing, and higher rates of digestion.
Note on usage: "fish" vs. "fishes"
"Fishes" is the proper English plural form of "fish" that biologists use when speaking about two or more fish species, as in "There are over 25,000 fishes in the world" (meaning that there are over 25,000 fish species in the world). When speaking of two or more individual fish organisms, then the word "fish" is used, as in "There are several million fish in the species Gadus morhua" (meaning that G. morhua comprises several million individuals). To see both in action, consider the statement "There are twelve fish in this aquarium, representing five fishes" (meaning that the aquarium contains twelve individuals, some of the same species and some of different species, for a total of five species).
Fishes can be found in almost all large bodies of water in either salt, brackish or fresh water, at depths ranging from just below the surface to several thousand meters. However, hyper-saline lakes like the Great Salt Lake do not support fish. Some species of fish have been specially bred to be kept and displayed in an aquarium.
Fish are an important source of food. Other water-dwelling animals such as mollusks and crustaceans (commonly called shellfish) are often considered as fish when used as food. Catching fish for the purpose of food or sport is known as fishing. The annual yield from all fisheries worldwide is about 100 million tonnes.
Overfishing is a threat to many species of fish. On May 15 2003, the journal Nature reported that all large oceanic fish species worldwide had been so systematically overcaught that fewer than 10% of 1950 levels remained.  Particularly imperilled were sharks, Atlantic cod, Bluefin tuna, and Pacific sardines. The authors recommended immediate, drastic cutbacks in fish catches and reservation of ocean habitats worldwide.