A fish is a poikilothermic water-dwelling vertebrate with gills. Fishes (over 27,315 species) are a paraphyletic group and are divided into the bony fishes (class Osteichthyes, 22,000 species); cartilaginous fishes (class Chondrichthyes, 800 species); and various groups of jawless fishes (class Agnatha, 75 species), including lampreys and hagfish. Fishes come in different sizes, from a 45 foot whale shark to a 8 mm long dwarf goby. Other aquatic animals, such as jellyfish and cuttlefish, are not true fish.
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 and 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, and Pacific sardines. The authors recommended immediate, drastic cutbacks in fish catches and reservation of ocean habitats worldwide.
- Deep sea fishes
- Fish anatomy
- Fish and chips
- Fish farming
- Fish migration
- Fish (singer)
- Ichthyology (the study of fish)
- List of fish families
- List of fish common names
- List of freshwater aquarium fish species
- Marine aquarium fish species
- Fishing Light Attractors
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